United States
Environmental Protection
Agency
Office of
Radiation Program*
Washington DC 20460
Technical Note
ORP/TAD80 7
Radiation
xvEPA
Technical Assessment
of Radon-222
Control Technology
for Underground
Uranium Mines
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Technical Note
ORP/TAD-80-7
Technical Assessment of Radon-222
Control Technology for
Underground Uranium Mines
B. T. Kown
V. C. Van der Mast
K. L Ludwig
Prepared under Contract No. 68-02-2616
Task No. 9
April 1980
Project Officer
M. M. Gottlieb
Office of Radiation Programs (ANR-458)
U.S. Environmental Protection Agency
Washington, D.C. 20460
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DISCLAIMER
This report has been reviewed by the Office of Radiation
Programs, U.S. Environmental Protection Agency, and approved for
publication.. Approval does not signify that the contents neces-
sarily reflect the views and policies of the U.S. Environmental
Protection Agency, nor does mention of trade names or commercial
products constitute endorsement or recommendation for use.
ii
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Foreword
The Office of Radiation Programs, EPA, is developing
standards for radioactive air pollutants under the authority of
the Clean Air Act, as amended in 1977. Technically enhanced
sources of naturally occurring radioactivity, such as under-
ground uranium mines, may release large quantities of radon-222
into the atmosphere. Because of the potential adverse health
effects to population groups, underground uranium mines warrant
investigation as to the feasibility of reducing the radon-222
releases. This study addresses various control options for a
hypothetical mine.
In sponsoring this study, we have worked closely with the
U.S. Bureau of Mines, Department of the Interior, with a common
objective of protecting both the underground worker and the
surrounding population by reducing the amount of radon-222
released into fresh air pathways, and hence to the environ-
ment. Readers of this report are encouraged to comment on its
technical merits and conclusions. Additional information is
welcome.
Office of Radiation Programs (ANR-458)
U. S. Environmental Protection Agency
Washington, D.C. 20460
111
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ABSTRACT
This report presents the results of a preliminary evalua-
tion of potential radon-222 control technologies for underground
uranium mines. The evaluated technologies are (1) use of a
sealant coating on exposed ore surfaces; (2) bulkheading of
worked-out areas; (3) activated carbon adsorption of radon from
contaminated mine air; (4) mine pressurization; and (5) miscel-
laneous technology (chemical reaction of radon in contaminated
mine air).
Underground uranium mines vary widely in size, shape, depth,
ore grade, lithology, layout, and mining method. Accordingly,
the radon sources and their emission rates also vary widely from
mine to mine. A hypothetical mine was used to estimate the radon
emission rates from various sources and to assess the radon
control technologies. The hypothetical mine, which has the
capacity of 1,000 tons of ore per day and has produced 480,000
tons of uranium ore over two years' operation, has 8.86 Ci/day
radon emission into the underground mine air. This includes 4.51
Ci/day from worked-out areas and 4.35 Ci/day from working areas.
The five radon control technologies are evaluated for their
application to the hypothetical mine. This includes evaluation
of their effectiveness in controlling radon emission, cost,
potential problems, safety considerations, and equipment avail-
ability. Sealant coating may be applied to the 2.54 Ci/day radon
sources and reduce 1.01 Ci/day at a cost of $1.45 per ton of
produced ore. Bulkheading of worked-out areas may be applied to
the 4.51 Ci/day radon sources and divert all 4.51 Ci/day radon
emission at a cost of $0.34 per ton of ore produced; 3.25 Ci/day
to the exhaust ventilation system, while 1.26 Ci/day is decayed
in the bulkheaded areas. (More recent information indicates that
as much as 2.95 Ci/day to 3.56 Ci/day of the 4.51 Ci/day of radon
from the worked-out areas may decay within the bulkheaded
areas.) Activated carbon adsorption, used in conjunction with
bulkheading, may be applied to 3.25 Ci/day radon sources and
reduce 3.09 Ci/day, which otherwise would be discharged to the
surface atmosphere, at a cost of $4.32 per ton of produced ore.
The concept of mine pressurization to reduce radon emission
into the mine air appears to be promising; however, further tests
are needed to verify the concept. Use of highly reactive chemi-
cal oxidants to react with radon in underground uranium mine air
appears possible, but tested chemicals are very corrosive in the
presence of humidity, extremely toxic, and not commercially
available.
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CONTENTS
Disclaimer ii
Foreword iii
Abstract iv
Figures vi
Tables vii
Acknowledgment viii
1. Introduction 1
2. Summary, Conclusions, and Recommendations .... 5
3. Radon Sources and Emission Rates in Underground
Uranium Mines 12
4. Underground Uranium Mines and the Case Mine ... 19
5. Radon Control Technology 29
References 55
Appendix
A. Design Criteria of Activated Carbon System
for Radon Removal 58
Glossary 60
v
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FIGURES
Number Page
1 Growth of working level (WL) in pure radon
(100 pCi/L) 17
2 Schematic diagram of an example underground
uranium mine 20
3 Modified room-and-pillar stope 24
4 Schematic diagram of a bulkhead 35
5 Effects of temperature on activated carbon
adsorption of radon 38
6 Radon removal from mine air by carbon
adsorption - System 1 42
7 Radon removal system from mine air by
carbon adsorption - System 2 43
8 Radon removal from mine air by carbon
adsorption - System 3 44
9 Schematic diagram of mine pressurization. ... 52
VI
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TABLES
Number Page
1 Assessment of Radon Control Technologies
Summary. ..............8
2 Radon Sources and Emission Rates from Case Mine . . 27
3 Radon Adsorption on Various Carbons 39
4 Major Equipment List of Case Mine
Carbon System 47
VI1
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ACKNOWLEDGMENTS
The authors wish to express their thanks to the Project
Officer, Mark M. Gottlieb of the Office of Radiation Programs,
U. S. Environmental Protection Agency, for his support and
guidance throughout the study. We also wish to acknowledge
R. C. Bates and J. C. Franklin of the U. S. Bureau of Mines,
Spokane Mining Research Center, for their assistance to the
project.
The authors also wish to thank those who reviewed the
draft final report and provided useful comments and suggestions
P. J. Davies
A. B. Dory
R. W. Englehart
Aurel Goodwin
J. M. Hughes
D. B. Lindsay
L. H. Norris
P. F. Pullen
W. J. Shelley
Paul B. Smith
J. W. Thomas
Bechtel Corporation
Atomic Energy Control Board,
Canada
NUS Corporation
Mine Safety & Health Admin.,
U.S. Department of Labor
Ontario Mining Association
Arthur D. Little, Inc.
Dewey, Oklahoma
RioAlgom Limited, Canada
Kerr-McGee Nuclear Corp.
Region 8 Radiation Program
EPA, Denver, Colorado
Chatham, New Jersey
Vlll
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SECTION 1
INTRODUCTION
This is the final report on the study, "Technical
Assessment of Radon-222 Control Technology for Underground
Uranium Mines," conducted for the Office of Radiation Programs,
U. S. Environmental Protection Agency (EPA)f under Contract
No. 68-02-2616, Task No. 9.
The health hazards of breathing air contaminated with
radon-222 and its daughter products have been recognized since
the 1940's.(6) The concentration of radon daughters in the
air of working areas in underground uranium mines is regulated
by the Mine Safety and Health Administration (MSHA). The
commonly used technique for controlling the concentration of
radon daughters in the mine air is forced-air ventilation which
dilutes and removes the contaminated air from the mine to the
surface atmosphere. At present there is no EPA standard for
the concentration of radon and its daughter products in the
exhaust ventilation air from an underground mine, or the
surface atmospheric air. The U. S. Nuclear Regulatory
Commission (NRC) regulations for radon concentrations in air
are 30 pCi/L (10 CFR 20.103) and 3 pCi/L (10 CFR 20.106) for
restricted and unrestricted areas, respectively.
The Office of Radiation Programs of EPA has the responsi-
bility for setting standards for the airborne emissions of
radioactive nuclides under the Clean Air Act as amended in
1977. This includes radon-222 and its daughter products from
underground uranium mines. If, in the judgement of EPA, it is
not feasible to prescribe an emission standard for controlling
radon, EPA may instead promulgate a design, equipment, work
practice, operational standard, or a comination thereof, which
is adequate to protect the public health.
Because of program requirements, EPA contracted for a two-
month quick response task for a preliminary technical
evaluation of the potential options for controlling radon-222
released to the surface environment from underground uranium
mines based on information and literature supplied by EPA and
obtained from other readily available sources.
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OBJECTIVE OF THE STUDY
The purpose of this study is to provide the EPA with some of
the necessary information to enable them to make a sound decision
on future activities leading to the setting of emission standards
or other regulations for the control of radon emissions from
underground mines.
This study provides EPA with a first-cut technical assess-
ment of various potential methods of controlling radon from
underground uranium mines. The study, based on presently avail-
able literature, attempts to characterize the major sources of
radon and to review promising methods of controlling radon
emissions from those sources. The radon control technologies
evaluated were:
Use of a sealant coating on exposed ore surface
Systematic bulkheading of worked out areas
Activated carbon adsorption of radon from highly radon-
contaminated air
Mine pressurization to suppress radon emission
Use of chemical oxidants to react with radon.
This study is solely aimed at means of preventing radon
release to the surface atmosphere. It considers only active
underground uranium mines and does not consider completed or
inactive mines. This study also does not address open pit min-
ing, subsequent milling and tailings, atmospheric diffusion, the
ultimate health effect of diffused radon, or the question of
mine workers' protection, except where such means of preventing
radon release impinges upon the subject.
PROJECT METHODOLOGY
To fulfill the objectives of this technical assessment,
four major tasks were performed:
Characterization of radon source
Definition of a typical mine, "the case mine"
Conceptual design of radon control systems
Technical and cost evaluation of control systems.
Characterization of Radon Sources
The available literature pertaining to the problem, sources,
2
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and emission rate of radon in underground uranium mines, was
reviewed. Technical experts in the field were also contacted for
consultation^1). It became apparent that information on the radon
sources is limited, and that radon emission is a very complex
subject which depends on many variables such as the ore charac-
teristics, mining method, climate, and age of mine.
Since this study is a preliminary assessment, it was decided
to use a simplified version of the radon source which would allow
a generalized assessment of the problem. A discussion of the
radon problem and a simplified source is presented in Section 3.
Definition of Case Mine
The assessment of radon control technology required a repre-
sentative underground uranium mine to be used as a model mine
(the Case Mine). However, it was quickly realized that all
underground uranium mines are different in their size, shape,
mining methods, or radon emission. It was not possible to define
a typical underground uranium mine. It was decided, with EPA
approval, that a simple underground uranium mine of 1,000 tons
per day using the modified room-and-pillar mining method would
be hypothesized for the purpose of this study. The case mine and
its radon source are presented in Section 4.
Conceptual Design of Radon Control Systems
Conceptual design of the radon control systems applied to
the case mine was necessary for the assessment of these tech-
nologies. Available literature on these technologies was re-
viewed to develop design criteria. There is very little actual
experience of these technologies, except for the limited practice
of bulkheading and limited test of sealant coating. Each control
technology was applied in a manner that was effective for the
hypothetical case mine. The conceptual design is presented in
Section 5.
Technical and Cost Evaluation
Each control system applied to the case mine was evaluated
for:
Effectiveness in radon control
Capital and operating costs
Potential problems
Assessment of operational requirements
Design and safety consideration
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Availability of material and equipment.
The cost is an approximation based on the conceptual design
of the radon control systems applied to the case mine. An esti-
mating method consistent with the conceptual nature of the design
was employed for this study. All cost data represent 1978 dollars.
The technical evaluation is based on the application of
these technologies to the case mine. Wherever possible, the
application to real mines is also discussed. The technical and
cost evaluation is presented in Section 5.
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SECTION 2
SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS
The results of this study are summarized in this section.
Conclusions and recommendations for future EPA efforts in con-
trolling radon release to the surface environment are also
included.
RADON SOURCE IN UNDERGROUND URANIUM MINES
The radon source and radon emission rate in underground
uranium mines varies considerably depending on the geological
characteristics of the ore deposit, ore grade, mining
technique, and mine atmosphere. Areas specially noted for
radon emission in underground uranium mines are:
Surface of drifts driven through the ore body
Exposed surface in extracted area
Raises or drifts near the ore body
Muck piles in working areas
Ore spills and ore cars in the haulage ways
Ground water entering mine workings after passing a
uranium deposit.
Radon emission rate from an exposed mine surface varies in
a range of 10 to 100 pCi/ft2-sec depending on the ore grade,
mine environment, rock characteristics, and mining activity.
For this study, the following approximate emission rates are
assumed:
55 pCi/ft2-sec (4.8 x 10~6 Ci/ft2-day) for a
medium grade ore surface (1)
28 pCi/ft2_Sec (2.4 x 10"6 Ci/ft2-day) for a low
grade ore area*
2.4 x 10-3 ci/ton-day for ore muck piles.
This is in good agreement with a more recent value of 22.4 pCi/ft2-
sec calculated from information in reference 33.
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Groundwater entering a mine working after passing through a
uranium deposit, may be one of the major radon sources. However,
it is not possible to estimate, based on presently available
information, the rate of radon emission from the mine water into
the mine air. This study did not include the mine water as a
source of radon emission into the mine air.
UNDERGROUND URANIUM MINES AND CASE MINE
Because uranium ore deposits are erratic in size, shape,
and lithology, every mine has a different layout and a different
mining method. Defining a typical underground uranium mine was
not possible. For this study, a hypothetical mine, defined only
by major radon sources, is selected and used for evaluation of
the radon control technology.
The Case Mine
The Case Mine has a large uniform tabular ore body which is
mined by modified room-and-pillar stoping method. The case mine
has:
Eight developing stopes at various stages of development
(250 TPD ore from eight developing stopes)
Two developed stopes ready for extraction
Five extracting stopes at various stages of extraction
(750 TPD ore from five extracting stopes)
Twenty-five completed stopes
A total of 240,000 tons per year from 12.5 stopes per
year
Each completed stope has 77,000 ft^ of exposed surface
and 300,000 ft3 of air space
Ventilation rate of 240,000 CFM.
The Radon Emission Rates in the Case Mine
In the case mine, 51 percent of the radon is emitted from
25 extracted stopes (mined out area), while the remaining 49
percent is emitted from active working areas (see Table 2). Not
all of the radon emitted into the underground mine space is dis-
charged to the surface environment. Some of the radon, espe-
cially that emitted into an isolated space such as a bulkheaded
mined out stope, will be trapped and subsequently decayed.
Radon sources and emission rates in the case mine are sum-
marized as follows:
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Emission Rate
Radon Sources Ci/day
8 Developing Stopes
Drifts 1.43
Muck piles 0.38
2 Developed Stopes
Drifts 0.71
5 Extracting Stopes
Drifts & extracted
areas 1.35
Muck piles 0.48
25 Extracted Stopes
Extracted areas 4.51
8.86
ASSESSMENT OF RADON CONTROL TECHNOLOGIES
Five radon control technologies are applied to the Case
Mine and evaluated for their cost, effectiveness, operational
requirements, and potential problems. An assessment of radon
control technologies is summarized in Table 1.
Use of Sealant Coating
In the Case Mine, only drifts of developing stopes are
coated. Annually, 530,000 ft2 of drifts are coated using
2,400 cubic yards of Shotcrete, 69,600 gal of HydroEpoxy 156,
and 17,000 gal of'HydroEpoxy 300, at a total cost of $344,300
(see Section 5 for details). The cost per ton of ore is
$1.45. The sealant coating reduces radon emission from drifts
of the developing stopes by 1.01 Ci/day; 11 percent reduction
of the total radon source.
Bulkheading of Extracted Stopes
Every extracted stope in the case mine is sealed using
eight bulkheads as soon as it is completed. All of the bulk-
headed stopes are connected to the exhaust ventilation system
using bleeder pipes. The systematic bulkheading of the 12.5
stopes per year, with 100 CFM* bleeding from each stope, will
divert all 2.25 Ci/day radon source in 12.5 completed stopes
and discharge 1.62 Ci/day to the surface. The bulkheading cost
of 12.5 worked-out stopes is $80,000 per year ($0.34 per ton of
ore) .
*More recent information indicates that flow rates of 10-20 CFM are
sufficient to maintain the bulkheads at negative pressure'32).
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00
TABLE 1. ASSESSMENT OF RADON CONTROL TECHNOLOGIES SUMMARY
Applicability Radon Redaction cost
Ci/Day % of Total(U Ci/Day Ii of Total<*T $/Ton of Ore
Sealant Coating 2.54 29 1.01 11 1.45
Bulkheading 4.51 51 1.26<2> 14 0.34
(4.51) (51)
Activated Carbon 3.25(2) 37 3.09 35 4.32
Mine Pressurization(3)
Miscellaneous Technology(3)
(Use of chemical oxidants)
NOTES: (1) Based on 8.86 Ci/day radon emission from the case mine.
(2) Diversion of 4.51 Ci/day from 25 worked out stopes, 1.26 Ci/day
decay in bulkheaded worked out stopes, and emission of 3.25 Ci/day
into the exhaust ventilation system.
(3) Available information was not enough to evaluate the effectiveness
and cost.
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Activated Carbon Adsorption of Radon from
Contaminated Mine Air
The systematic bulkheading of all extracted stopes with 100
CFM bleeding from each stope, diverts all radon emitted in the
extracted stope (0.18 Ci/day for each stope) from the mine air,
but discharges 72 percent of it (0.13 Ci/day) to the exhaust
ventilation system and eventually to the surface environment.
The difference (0.05 Ci/day) decays in the bulkheaded stope.
In the case mine application of the activated carbon system,
one carbon system is provided to each bleeder pipe of the bulk-
headed stopes. The carbon systems remove 95 percent of the radon
from the bleeder pipes, which otherwise will be discharged to the
exhaust ventilation air.
Based on five years average life of the carbon systems, each
system costs $83,000 to install and operate for five years; this
represents $4.32 per ton of ore. Applied to all 25 bulkheaded
stopes in the case mine, the carbon systems will remove 35 per-
cent (3.1 Ci/day) of the radon emitted from the entire mine at
the capital cost of $825,000 and an annual operating cost of
$250,000 (amortization is not included).
Mine Pressurization
Mine pressurization has been found to be effective in some
cases in controlling radon emissions into the mine atmosphere.
However, its effectiveness in controlling radon emissions into
the surface environment, which is the primary concern of this
study, has not been proven.
Effectiveness of mine pressurization in reducing the radon
emission rate depends on the flow of gas (gas in the intersti-
tial pores and mine air) from the exposed ore surface into the
ore body, caused by the positive pressure (the mine pressure is
higher than the surface atmospheric pressure). However, the gas
flow has to be slow enough to allow radon in the gas to decay
before reaching the ultimate sink the surface environment.
Further tests are required to confirm the effectiveness of mine
pressurization.
A mine could be pressurized using readily available equip-
ment. The cost for mine pressurization, which would vary widely
for different mines, is not determined in this study.
Miscellaneous Radon Control Technology
(Use of Chemical Oxidants)
The concept of removing radon from a contaminated uranium
mine air by reacting it with liquid and solid chemical agents,
does not appear feasible at this time. Tested chemicals are
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extremely corrosive in the presence of moisture, toxic, and not
commercially available. Use of these chemicals in an under-
ground uranium mine would be difficult because of their toxic
nature and hazardous by-products.
CONCLUSIONS AND RECOMMENDATIONS
An underground uranium mine of 1,000 tons per day has radon
sources emitting approximately 9 Ci/day. If all radon emitted
into the mine air is discharged to the surface atmosphere via the
exhaust ventilation air, this would represent a discharge of
240,000 CFM of the exhaust ventilation air having 920 pCi/L.
Whether the radon emission of 9 Ci/day from an underground
uranium mine represents a public health hazard is not known at
this time. Presently, there is no EPA ambient air quality
standard or emission standard for radon concentration. The NRC
regulation for radon concentration in unrestricted areas is
3 pCi/L (10 CFR 20.106). The hazardous exposure limit and
atmospheric dispersion of radon must be determined before
concluding whether or not the radon emission from underground
uranium mines is a public health hazard.
Limited reduction of the radon emission is possible using
currently available technology. Sealant coating applied to the
case mine reduces 11 percent of the total radon emission at the
cost of $1.45 per ton of ore. Bulkheading of extracted stopes is
cheap ($0.34 per ton of ore) and very effective in preventing
radon contamination of the mine air (diverts 51 percent of the
total radon emission), but less effective in reducing the radon
emission to the surface environment (reduces 15 percent* of the
total radon emission by decay). Activated carbon system applied
to bleeder pipes of the bulkheaded stopes is more effective
(removes 35 percent of the total radon emission), but also more
expensive ($4.32 per ton of ore).
Mine pressurization may be effective for reducing the radon
emission into the mine air and into the surface environment, but
further tests are required to verify the concept. Removal of
radon from the contaminated air by reacting with chemicals
appears impractical at the present time because of the toxicity
of chemicals and by-products.
This study is based on limited information. If EPA decides
that radon discharge on the order of 10 Ci/day from an under-
ground uranium mine represents a public health hazard, then the
further studies listed below will assist EPA by providing
additional information on the characterization of radon sources
and the applicability of control technology. Further studies may
include:
Characterization of the radon source
Flow rates of 10/20 CFM would result in reductions of 40/33 percent
of the total radon emission to the surface environment (see page 33).
10
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Classification of underground uranium mines into
groups which have similar radon sources and may
also have similar radon control strategies
Development of inexpensive sealant materials that
are effective in an operating mine environment
Development of better bulkhead
Test of mine pressurization
Development of effective activated carbon system.
Characterization of Radon SourcesBetter radon emission
rates for various sources are required for a meaningful assess-
ment of the radon problem and control requirement. A source
characterization study should be conducted to develop a list of
radon sources and radon emission rates as a function of the
mineral property, mining technique, and mine environment.
Classification of Underground Uranium Mines--Radon emission
rates and applicable radon control technology vary from mine to
mine. Grouping of the uranium mines into those which have simi-
lar radon sources and which would probably utilize similar radon
control strategies is necessary in order for EPA to promulgate
a fair standard, if one is required.
Development of Inexpensive Sealant MaterialThe cost of
the sealant coating in this study is primarily that of the
material cost. It is very high compared to an asphalt emulsion
sealant tested on uranium tailings. Finding a cheaper and effec-
tive coating material is essential if there is to be wide coating
application.
Development of Better Bulkhead--Bulkheads commonly used by
the present uranium mines are found to be ineffective in forming
an airtight seal. Development of a better bulkhead that is more
durable and airtight is necessary.
Test of Mine PressurizationThe concept of mine pressuriza-
tion to reduce radon emission into the atmosphere of underground
uranium mines is attractive. However, further tests are required
to confirm the effectiveness of a long-term mine pressurization
in reducing the radon emission and to determine its impact on
the mining activity as well as the surface environment.
Development of Effective Activated Carbon System--The acti-
vated carbon systems suggested in this study require a series of
field tests to debug the components and to optimize the overall
systems. Actual demonstration of the activated carbon applica-
tion is essential if the industry is to accept the technology.
11
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SECTION 3
RADON SOURCES AND EMISSION RATES
IN UNDERGROUND URANIUM MINES
All^rocks, sands, and soils containing uranium also contain
some radium. Except for places where chemical leaching has
resulted in depletion of uranium or radium, the two will be found
in radioactive secular equilibrium.
Radium-226 decays to radon-222, which further decays to a
series of daughter products. The radium decay products and their
half-life are as follows:
Element Half-Life Radiation (Mev)
Radium-226 l,620y a-4.7, Y-0.19
Radon-222 3.82 d a-5.48, Y-0.51
Polonium-218 3.05m a-6.00
Lead-214 26.8m 0-0.65, Y-0.29
Bismuth-214 19.7m B-1.5. Y-1.8
Polonium-214 1.6 x 10'4s a-7.68
Lead-210 22 y 0-0.02, Y-0.05
Bismuth-210 5.0d 6-1.16
Polonium-210 138 d a-5.3, y-0.80
Lead-206 Stable
When radium decays in an ore body, it produces radon. Radon
is a gaseous and not too chemically reactive element. Radon will
diffuse through the ore body. If radon diffuses out of the ore
body into the mine atmosphere before it decays, then radon and
its decay products will become airborne radioactivity. Radon
has a half-life of 3.82 days. If radon decays within the ore
body or host rock, its chemically reactive decay products, which
are solids, will be permanently trapped in the ore body.
Uranium mining activity exposes new radium containing ore
surfaces to the mine atmosphere. The mining activity not only
increases new ore surface (new radon sources), but also reduces
the distance for radon to diffuse through before entering the
mine atmosphere. When uranium ore is broken, trapped radon will
12
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escape into the mine atmosphere; then the radon emission will
reach an equilibrium determined by the ore characteristics and
the mine atmospheric condition. Because of the long half-life
of radium (1,620 years), the radon emission from the exposed ore
surface will continue a long time.
The rate of radon production within an ore body is independ-
ent of the mine atmospheric conditions or mining activity. The
radon production rate depends only on the radium concentration
and is nearly constant throughout the mine life. However, the
amount of radon that gets into the interstitial pore space and
diffuses through the ore body into the mine atmosphere depends
on the ore characteristics (porosity, ore fracture, and ore
thickness), moisture content, groundwater flow, and the mine
atmospheric conditions (temperature, pressure, and humidity).
Generally, an increase in the mine atmospheric pressure
decreases radon emission; that is, high atmospheric pressure
creates a pressure gradient which causes flow of mine air
through the pores away from the exposed surface into the ore
body, thereby suppressing diffusion of radon into the mine atmo-
sphere. However, the extent of this effect is greatly dependent
on the porosity of the ore body and host rock. There has to be
an area of sustained low pressure in order to maintain a con-
tinuous flow. There have been a number of theoretical and
experimental investigations ^2j3ป^ on the subject. The effect
of pressure on radon emission is discussed in a later section
on mine pressurization.
Diffusion of radon through the ore body is expected to
increase as the mine temperature increases, as the gas diffusion
coefficient increases with the temperature. However, temperature
variation in an underground uranium mine is generally in the
narrow range of 15 - 30ฐC and the effect of temperature variation
on the radon emission rate is not expected to be significant.
Moisture of mine air and of the ore body appears to have a
considerable effect on the radon emission^5). The radon emission
rate is low at very low moisture, but increases rapidly with
increasing moisture, quickly reaching a plateau. The moisture
of normal mine air is usually high enough that the plateau rate
is reached. Mechanism of the moisture effect is not well under-
stood. It is cited^ that the moisture effect is a surface
absorption phenomenon; that is, the dry mineral grains may ab-
sorb radon thus preventing its movement out into the mine atmo-
sphere. When the mine atmosphere contains high moisture, the
mineral grains are saturated with water molecules, and radon
moves through the media without being absorbed. When the ore
becomes supersaturated, radon has to diffuse through the water
in the pore and the radon emission is decreased. As much as 60
percent difference in the radon emission between a dry and wet
ore has been reported (5'.
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Mining activity, especially blasting, also affects the
radon emission. However, this is a momentary increase and the
peak usually disappears quickly.
Thus, the rate of radon emission from an exposed uranium
ore body varies widely with such variables as radium concentra-
tion in the ore, ore porosity, mine temperature, mine pressure,
mine humidity, mining activity, etc. It also depends on the
nature of the exposed surface; size and shape of broken pieces,
and roughness.
Areas of major radon emission in an underground uranium
mine are:
Stope development areas where ore bodies are initially
exposed by drifting
Stope extraction areas where ore is extracted. These
areas have exposed ore surfaces and muck piles which
have a high surface area per ton of ore.
Completed stope areas where ore has been extracted and
ribs and backs are caved in, exposing low grade ore
(waste ore)
Raises driven to the ore body from haulageways. As
these raises are driven into the ore body, they expose
ores.
Haulageways where broken ores are transferred from an
active stope area to the shaft area. Generally, a
haulageway is located beneath the ore and itself is not
a radon source, but ore is moved through the haulageway
and ore spills will emit radon.
Areas where mined ores are stored temporarily. In con-
tinuous mining operation, mined ores are frequently
stored in drifts and ore passes.
Drain ditches carrying mine water which was in contact
with the ore body may release radon.
It is impossible to accurately predict the radon emission
rate from a given exposed ore surface, or to accurately estimate
the exposed area. As a rule of thumb, an emission rate of 55 pCi
per square foot of exposed surface per sec has been suggested
for a medium grade uranium ore^). Neither the quantity of mine
water per ton of ore nor the radon emitted from it can be deter-
mined at this time. In the estimation of radon emission rates
in the case mine, mine water was not included.
14
-------
It has been reported^ ' that approximately 4 to 9 square
feet of the ore surface is exposed for each ton of ore mined;
the higher figures for a small mine. Assuming an average figure
of 7 square feet of the exposed surface per ton of ore mined, a
uranium mine of 1,000 tons per day, operating 240 days per year,
will create 1.68 million square feet of new exposed ore surface
annually, and new radon sources of 8 Ci/day every year of the
mine operation.
Not all of the radon emitted into the mine air is taken out
of the mine into the surface atmosphere. Some of the worked out
stopes are bulkheaded or backfilled. Some of the worked out
stopes being used as airways are partially blocked using bulk-
heads to isolate parts of the worked out area and reduce the
radon contamination. Radon emitted into these isolated spaces
is trapped until it either decays or leaks out into the mine
atmosphere.
HEALTH HAZARDS OF UNDERGROUND URANIUM MINE AIR
The health hazards of breathing uranium mine air contami-
nated with radon-222 and its daughter products have been recog-
nized as early as the 1940's<6). The concentration of each
daughter product depends on the time that has elapsed after the
radon contamination (age of the air). In general, the mine
atmosphere contains only the first five daughter products:
Element Half-Life Radiation (Mev)
Radon-222 3.82d a-5.48, y-0.51
Polonium-218 (RaA)* 3.05m a-6.00
Lead-214 (RaB)* 26.8m g-0.65, y-0.29
Bismuth-214 (RaC)* 19.7m 0-1.5, y-1-8
Polonium-214 (RaC')* 1.6xlQ-^s ex -7.68
Lead-210 (RaD) 22y B-0.02, Y-0.05
* Health Hazard Elements
When mine air contaiminated by radon and its daughters is
breathed into the lungs, radon, being an inert gaseous element,
is not retained in the lungs. However, the daughter products,
which are highly ionized and attached to airborne dust particles,
may be retained in the lungs.
In terms of health hazards, the alpha emitting daughter
products, polonium-218 and polonium-214 (Radium A and Radium C1),
are of major concern. However, lead-214 and bismuth-214 are
also included as hazardous isotopes because they may also be
retained by the lung tissue during respiration and later decay
to alpha emitting polonium-214. The beta and gamma rays emitted
15
-------
by lead-214 and bismuth-214 make negligible contributions to the
radiation dose in the lung tissue. Because of the long half-life
of lead-210, it is not expected to remain in the lungs and is
excluded from the dose calculation.
Because of the extremely short half life (0.00016 sec) of
polonium-214 (RaCf), bismuth-214 (RaC) and polonium-214 (RaCf)
are considered as a single isotope emitting alpha, beta, and
gamma radiation with the half-life (19.7 min) of bismuth-214
(RaC) .
Each atom of polonium-218 (RaA) retained by the lung tissue
produces 13.68 Mev of alpha energy (6 Mev from polonium-218 and
7.68 Mev from polonium-214). Each atom of lead-214, bismuth-214,
and polonium-214 produces 7.68 Mev. The radiation intensity
associated with an underground uranium mine is measured by a
unit called "Working Level" (WL) <7>.
"One WL is defined as any combination of first four
radon daughters RaA, RaB, RaC, and RaC' in
one liter of air which ultimately produce 1.3 x 105
Mev of alpha energy."
When radon enters uncontaminated mine air, the air has ini-
tially zero daughter products and zero WL. Radon itself is not
included in the radioactivity included in the WL. As radon
decays, it produces the daughter products and the daughter pro-
ducts contribute to the WL. Starting with 100 picocuries of
radon (3.7 disintegration per sec., or 6.5 x lO"1^ gram), build-
up of the total WL and each daughter's contribution to the WL as
a function of the decay time is shown in Figure 1.
As seen in Figure 1, the health hazards (alpha radiation
dose) of mine air depends not only on the radon radioactivity,
but also on the age of the mine air. Increased ventilation rate
in a given mine reduces the health hazard (working level) first
by diluting the contaminated air, and secondly by shortening of
the air retention time in the mine, hence, resulting in a
"younger" air. However, a high ventilation rate does not reduce
the total radon and its daughter products emitted into the sur-
face environment. A high ventilation rate will probably result
in increased radon and daughter discharge into the surface
environment by not allowing radon to decay in the mine.
Presently, the Federal Regulation, administered by MSHA of
the Department of Labor, limits the radon daughter concentration
in the mine air in the underground working area to 1.0 WL and a
total annual exposure of 4.0 Work Level Month (WLM). One WLM is
defined as 173 hours exposure to a radon daughter concentration
of 1.0 WL.
16
-------
1.0
0.8
0.6
UJ
>
u
10.4
tr
0.2
20
40 60
TIME, MINUTES
Figure 1. Growth of working level (WL) in pure radon (100 pCi/L)
17
-------
It should be recognized that the working level is a measure-
ment for mine worker exposure to alpha emitting particulates, and
not a true measure of the total air contamination. For the
general public safety, radon in the surface ambient air should
be included because radon will eventually decay and produce
hazardous daughter products. The working level does not include
potential exposure of gamma radiation from radon daughter prod-
ucts trapped inside the exposed ore body. The gamma radiation
from the trapped radon daughter products may be a significant
source of radiation exposure; however, quantitative information
on the gamma source is not available at this time. The gamma
radiation source is not included in this study.
18
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SECTION 4
UNDERGROUND URANIUM MINES AND THE CASE MINE
The majority of uranium deposits in the United States are in
New Mexico, Wyoming, Colorado, Utah, and Texas. Nearly 90 per-
cent of the uranium deposits in the United States are sandstone
deposits. Ore deposits are generally tabular layers that lie
nearly parallel to the bedding planes. The flat-lying deposits
usually occur in more than one stratigraphic horizon and they
may occur in clusters. Ore bodies are generally irregular in
shape and size, ranging from small masses of only a few feet in
width and length to those that are tens of feet thick, hundreds
of feet wide, and thousands of feet long. Ore bodies range from
a few hundred to several million tons.
Presently, open pit, in-situ leaching, and underground mining
techniques are used for uranium mining. However, future uranium
production is expected to be more from underground mining.
Because of the extreme variation of uranium ore bodies in
size, shape, depth, continuity, physical properties, geologic
structure, grade, and groundwater condition, each underground
uranium mine is unique in its layout and mining method. Each
mine is a one-of-a-kind. Furthermore, the configuration of a
specific underground uranium mine changes continuously as opera-
tion progresses. Defining a typical underground uranium is
impossible.
Some general features of underground uranium mines are dis-
cussed here by following sequential activities from shaft sink-
ing to ore production. Later in this section, the hypothetical
underground mine used in this study is defined. This hypothet-
ical mine (the case mine) is defined solely for the purpose of
identifying the major radon sources and evaluating the radon
control technologies applied to these radon sources.
GENERAL FEATURES OF UNDERGROUND URANIUM MINES
A schematic section of a hypothetical underground uranium
mine is shown in Figure 2. The sequential events in development
of such a mine would be as follows:
Product shaft sinking
19
-------
SHOP
>
. ORE BODY
CZL
>,
_u *
13 {
HAULAGE DRIFT
\/
CLAMSHELL
SHAFT
CLEAN-UP
LONGHOLE
DRILLING
HAULAGE
DRIFT
DRIVING
SHAFT SUMP
Figure 2.
Schematic diagram of an example underground
uranium mine.
20
-------
Haulage drifting
Ventilation shaft sinking
Longhole exploration
Raising to stope level
Stope development (primary ore production)
Stope extraction (final ore production).
Production Shaft
Depending on depth and geologic conditions, either vertical
or inclined shafts may be used as access to the ore. However,
the trend is toward vertical shafts as deeper deposits are
developed. These range in depth from a few hundred feet to the
present-day maximum of about 3,000 feet currently being estab-
lished in New Mexico.
Modern production shafts are circular and concrete lined,
with internal diameters ranging from 10 to 16 feet depending on
production requirements. Ore and waste ore are hoisted in two
skips operating in balance. Utility lines (electric cables,
water and compressed air pipes) are attached to the shaft wall.
The production shaft will generally extend a hundred feet
or more below the production level in order to accommodate
spillage clean-up and sump capacity. It is generally located
outside the ore perimeter and does not, therefore, present a
radon problem.
Haulage Drifts
Following shaft construction, haulage drifts are extended
out below the anticipated ore horizons. They will usually be
driven about 8 feet by 9 feet, with a 1 percent gradient to
favor both the loaded haulage and mine drainage.
Since the haulage drifts are driven below the ore horizon
in barren or near barren material, they should not pose a radon
problem in themselves. However, unexpected extension to the ore
body and isolated ore masses may occasionally be encountered.
Groundwater seeping into the haulage drift system is con-
fined to a shallow ditch established to one side of the drift.
In some instances, such groundwater may have passed through a
high radon area and may, consequently, contain dissolved radon.
This would then be released into the haulage drift.
21
-------
Longhole Exploration
Following haulage drift development, a series of explora-
tory longholes are drilled upward and outward from the haulage
to better delineate the ore bodies in respect to thickness,
grade, elevation, and dip or roll. The drilling is normally done
in a fan-shaped pattern. The angle, depth, and number of holes
per fan may vary. Hole diameter is generally 1-3/4 inches, and
they may serve to drain water from the ore horizon prior to
mining. They will most likely become plugged during the actual
mining process.
Raisings to Stope Level
Raises are openings established between the haulage and ore
horizons to provide access for men, materials, fresh air, broken
ore, and exhaust air. A stope (a unit working area) will have
two or more raises. They are generally about 4 feet in diameter
and steel lined. They are usually not considered to be major
radon sources.
Stope Development and Extraction
In planning for the extraction of an ore deposit, the ore
bodies are divided into suitably sized blocks that can be mined
conveniently as working units. These are known as stopes. The
size and shape of a stope may vary, depending on the ore body
geometry, its dimensions, and the mining method.
The ore in a stope is usually removed in two stages; stope
development stage and stope extraction stage. The stope develop-
ment stage comprises development of a drift network within the
ore body to provide access to all portions of it. As much as 30
to 35 percent of the total ore may be removed in this development
stage. The stope extraction stage consists of removal of all
remaining mineable ore.
The stoping method varies widely from mine to mine, and
even place to place within the same mine. It depends on the ore
body geometry and geology, the distribution of ore, the nature
of the ground, and the presence of water.
A commonly used stoping method for relatively thin, flat-
lying ore bodies is that known as "modified room-and-pillar".
It provides excellent opportunity for close extraction of a
deposit and for mining to the full extent of the ore. At the
same time, low grade ore or barren material may be left behind
in the worked out areas. Other stoping methods such as sub-level
open stoping or cut-and-fill stoping are also used for mining
thick and massive ore bodies or very unstable ore bodies. These
methods offer less flexibility and less opportunity for effec-
tive ore extraction and ore quality control.
22
-------
A mine based on the modified room-and-pillar method is
selected for this study. There are numerous variations of the
modified room-and-pillar stoping method practiced in uranium
mining. For the purpose of identifying potential radon sources
and their control methods, a simplified version of a modified
room-and-pillar stoping method is presented here.
Modified Room-and-Pillar--The modified room-and-pillar
stoping method is a typical method for a competent ore body of
15 feet or less thickness. It is the most commonly used. In
the modified room-and-pillar stoping method, a network of devel-
opment drifts produces a series of pillars to be mined during
the stope extraction stage. Normally, the drifts are 6 feet by
6 feet and the pillars are 40 feet by 40 feet (Figure 3).
Once the drift network is completed, the stope is ready for
extraction. However, a developed stope may not be extracted for
as much as six months or even a year. The stope development is
also a sort of exploration activity, and often is handled by a
separate crew from the stope extraction. During the stope devel-
opment, and before the extraction starts, the exposed surfaces
of the drifts and ore muck piles are the primary radon sources.
Some drifts will be left intact longer than others, depending on
the mining plan and schedule.
During the stope extraction, pillars are blasted and removed
through drifts to raises using slushers or loaders. The extrac-
tion generally begins away from the ore pass and progresses
toward the ore pass. Some pillars of low grade ore may be left
behind in order to control subsidence. During the stope extrac-
tion, ore muck is piled in the stope and is the major radon
source. In addition, the exposed surface of low grade mineral-
ized rock left behind and ore pillars which have not been
removed also emit radon.
When the extraction is completed, most of the pillars are
gone and the open cavity begins to cave in. Some of the drifts
may have been left intact because of poor quality ore, for future
access, or for other reasons. The exposed surfaces of the mined
area (mineralized rock) and of the unmined drifts will continue
to emit radon and contaminate the air in the mined stope. Unless
the air in the extracted stope is effectively isolated and con-
fined, it may leak into and contaminate ventilation air.
Ventilation System
Adequate ventilation is needed in all underground mining to
supply fresh air to the mine workers and to flush out air con-
taminated with dust and fumes. In underground uranium mining,
the need for adequate ventilation is even more acute because of
the need to dilute and remove mine air contaminated with radon
and its daughter products. Because each underground uranium
23
-------
PILLARS
STORE
160' x 240'
Figure 3. Modified room-and-pillar stope.
-------
mine has a one-of-a-kind layout and mining plan, the ventilation
system is also unique for each.
The ventilation system usually consists of a primary and
several secondary systems. The primary ventilation system
includes the main intake airway, fresh airways, exhaust air
drifts, and exhaust ventilation shaft. Fans are used on either
the intake shaft or exhaust shaft. Positive pressure ventila-
tion in the primary system results from the use of downcast fans
at the ground surface and negative pressure from the use of up-
cast exhaust fans. The production shaft and haulage drifts are
commonly used as fresh air intake and fresh airways. In a large
mine, there may be more than one shaft for fresh air intake and
a combination of downcast and upcast fans at different vent
shafts.
The secondary system, sometimes called "booster" or "auxil-
iary" system, consists of fans and vent tubing to redirect
portions of the primary air supply to specific working areas
that are not on the main ventilation system. These systems
utilize small fans (5 - 25 HP) which usually push air through
flexible tubing (bags) to working areas.
In a modern mine, exhaust air from working areas contami-
nated with radon is collected into exhaust drifts and routed
directly to the exhaust shaft. However, in older mines, radon
contaminated air from a working area is often discharged into
the next working area or into the primary air system, thereby
contaminating it. In older mines operating under positive
ventilation, exhaust air is usually allowed to escape by a con-
venient route which might be the main access shaft. Modern
practice of underground uranium mining, however, dictates that
radon contaminated exhaust air be conducted away through segre-
gated exhaust drifts other than haulage drifts and shafts. More
and more mines are taking great care to separate exhaust airways
and taking steps to prevent exhaust air from contaminating the
fresh air supply.
As a rule of thumb, one cubic foot per minute of fresh air
per annual ton of ore removed from the mine has been suggested^.
An underground uranium mine with 1,000 tons per day capacity
(240,000 tons per year) will require 240,000 CFM ventilation.
The ventilation system is discussed further in a later section
on mine pressurization.
THE CASE MINE
Assessment of radon control technologies for underground
uranium mines requires the definition of a typical underground
uranium mine. The source and quantity of radon have to be
defined to determine the feasibility and effectiveness of the
radon control technologies.
25
-------
However, because of the erratic nature of uranium ore depos-
its, each mine has a unique layout, mining plan, and mining
method, each different from other mines. The characteristics of
the radon source and the radon emission rates in one mine are
also different from another. The application of radon control
technology has to consider each mine separately. The time and
manpower limits of this study do not allow for the study of many
mines, or the details of any one mine.
The radon control technologies evaluated in this study are
still in an early development phase. There are uncertainties in
their applicability and effectiveness. Additional development
work is required before these technologies become ready for an
actual application.
For the purpose of this study, simplification of the usually
complex underground uranium mine was necessary. A 1,000 tons
per day mine of an ideally uniform ore body was selected for the
study. The selected mine (the case mine) is based on one mining
method modified room-and-pillar stoping. All stopes are the
same size and height.
The radon emission rates from various radon sources are
approximated based on uniform emission rates for the drift sur-
faces in the ore (55 pCi/ft2-sec) , surfaces of waste ore (28 pCi/
and ore muck piles (2.4xlO"3 Ci/ton-day) .
In view of this study being a preliminary evaluation of the
technologies yet to be developed, the simplification of the mine
and the approximation of the radon emission rates were necessary
and appear justified. The radon sources and emission rates of
the selected mine are summarized in Table 2.
The design and operating conditions of the case mine are
summarized as follows:
Case Mine
Ore Deposit: Medium grade, large flat ore
body of uniform 10 feet mineable
ore thickness.
Mining Method: Modified room-and-pillars , haul-
ageways under the ore body.
Drifts in the ore body are 6 ft
by 6 ft on average.
Capacity: 240,000 tons per year ore produc-
tion (1,000 tons per day).
Stope Size: On average, each stope is 160 ft
by 240 ft and 12.5 new stopes are
26
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TABLE 2. RADON SOURCES AND EMISSION RATES FROM CASE MINE
to
-j
Radon Sources
Unit Emission Rate
Total Internal
Emission
Developing Stopes (8)
Exposed ore surface
Muck piles
Developed Stopes (2)
Exposed ore surface
Muck piles
Sources
12,800 ft. drifts
(8 stopes)
drift headings
6,400 ft. drifts
(2 stopes)
2 2
Area, ft Weight, tons pCl/ฃt -sec Ci/ton-day
300,000
150,000
Extracting Stopes (5)
Exposed ore surface half extracted stopes 190,000
190,000
Muck piles
8,000 ft. drfts
(5 stopes)
working areas
(5 stopes)
Extracted Stopes (25)
Exposed ore surface 25 extracted stopes 1,900,000
Muck piles
160
200
55
55
28
55
28
2.4 x 10-3
00
Ci/day
1.43
0.38
0.71
0.45
0.90
1.35
2.4 x 10~3 0.48
4.51
-------
Stope Development:
Stope Extraction:
Completed Stope:
Ventilation:
developed and extracted per year.
On the average, there are eight
stopes at various stages of de-
velopment and two developed
stopes ready for extraction. A
developed stope has 3,200 feet of
drifts (77,000 ft2 of exposed ore
surface and 115,000 ft3 (7,200
tons) of ore removed. Drifts are
of 6 ft by 6 ft on the average.
Each stope being developed has,
on the average, 1600 feet of
drifts, 20 tons of unremoved muck
piles, and 3,600 tons of ore re-
moved. A total of 375 tons of ore
are removed from eight developing
stopes per day.
On average, there are five stopes
at various stages of extraction.
A total of 625 tons of ore are
extracted from five extracting
stopes per day. At completion,
190,000 ft3 (12,000 tons) of ore
has been removed from each stope
during the extraction. Each ex-
tracting stope, on the average,
has 40 tons of unremoved muck
piles and 1600 ft of drifts left.
On average, 12.5 stopes are com-
pletely extracted per year. There
are 25 completed stopes. Each ex-
tracted stope has 76,000 ft2 of
exposed low grade ore surface
primarily sills (floor) and backs
(ceiling), and it has 300,000 ft3
of void space before cavein.
240,000 CFM ventilation - 1 CFM
of ventilation air per ton of ore
removed per year.
It must be recognized that the case mine is not a typical
underground uranium mine, and its total radon emission rate is a
crude approximation based on many assumptions and guestimates of
radon emission rates from various sources. For a more accurate
estimate of the total radon emission from an underground uranium
mine, more accurate radon emission rates and more exact mine
modeling are needed.
28
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SECTION 5
RADON CONTROL TECHNOLOGY
The main objective of this study is to assess the potential
options for controlling radon emission to the surface environment
from underground uranium mines. Five potential radon control
technologies, which have been subjected to limited experimental
tests, are applied to the case mine and evaluated for their
effectiveness, cost, potential problems, reliability, and equip-
ment availability. These technologies are:
Sealant coating on exposed ore surfaces
Bulkheading of worked out areas (extracted stopes)
Activated carbon adsorption of radon from highly
contaminated air
Mine pressurization
Chemical scrubbing of highly contaminated mine air.
It should be noted that these technologies are presently
in an early development phase. The assessment is based on
limited publications and a great deal of engineering judgement.
SEALANT COATING FOR RADON CONTROL
The best method for controlling radon in an underground
uranium mine is to prevent radon from entering the mine air.
The application of a gas-tight coating over the exposed ore
surface has been tested in recent years to evaluate various
potential sealants.
U.S. Bureau of Mines, Spokane Mining Research Center (9~12)
has screened over 65 coating materials for potential use in
underground uranium mines. These materials are also screened
for toxicity, flammability, and applicability in the underground
mine condition. The Bureau of Mines conducted several field
tests^10' n) of the sealant application in the Dakota mine in
Ambrosia Lake, New Mexico; Twilight mine in Colorado; and other
mines in New Mexico. These tests involved HydroEpoxy coating
of the exposed ore surface of drifts and chambers.
29
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Lawrence Livermore Laboratory(13^ has evaluated many poten-
tial sealants for possible use in underground mines. Their
study involved measurement of permeation coefficients of the
films and coatings and evaluation for their toxicity. Battelle
Northwest(14^ has also investigated the use of an asphalt emul-
sion sealant to contain radon from uranium tailings.
Findings of these investigations are summarized as follows:
Under carefully controlled laboratory conditions, many
sealants have extremely low permeation coefficients
which will theoretically provide a better than 100:1
attenuation of radon emissions. However, the presence
of so-called pinholes and the difficulty of applying
a perfect coating on an ore surface reduces the effec-
tiveness of the sealants considerably.
Field tests suggest that water-based epoxies such as
HydroEpoxy 156 and HydroEpoxy 300 are well suited for
the underground mine application. A three coat system
HydroEpoxy 156, HydroEpoxy 300, preceded by Shotcrete
base coating, was found to be effective in the range of
50 to 75 percent radon stoppage. Shotcrete is needed
for eliminating cracks and to provide a better base for
the sealants.
The amount of sealants used varies considerably for
different mines <15).
Shotcrete - $0.12 to 0.72/ft2 @ $0.25/gal
HydroEpoxy 156 - $0.06 to 0.19/ft2 @ $7.36/gal
HydroEpoxy 300 - $0.12 to 0.30/ft2 @ $6.40/gal
The exposed ore surface (radon source) which can be
coated with a sealant is limited. Drifts through the
ore body are major radon sources and best suited for
the sealant coating. However, most of the drifts in a
modern uranium mine are destroyed as the mining prog-
resses. In a room-and-piliar stope mine, most drifts
driven during the stope development stage are mined
out (destroyed) during pillar extraction. The sealant
coating applied to these drifts will thus have a
limited life.
Sealant based on an asphalt emulsion is found to be
effective for stopping radon emanation from uranium
tailings. Asphalt emulsion is cheap (3.5.
However, its applicability in an underground mine has
not been tested.
30
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Sealant Application in the Case Mine
In the case mine, the drift network in a developing stope
is the only area that can be coated. Workers are not permitted
to go into an extracted stope, except drifts leading to the
extracted area. Haulage drifts are driven under the ore body
where there is usually little or no radon emission.
Two-man coating crews will coat all the drifts in develop-
ing stopes during the third shift. Only ribs and backs (ceiling)
are coated. The floor is not coated because it is covered with
semi-consolidated muck. The conditions of sealant application
in the case mine are as follows:
Drifts are coated during the stope development stage.
Every year 12.5 new stopes are developed and extracted.
There are, on average, eight developing stopes, two
developed stopes, and five extracting stopes at any
time of the year.
On the average, the coating stops 60 percent of radon
emission and a coated surface has an 8-month life
before being destroyed.
Only 75 percent of the drifts in a developing stope
are coated. Drifts which will be extracted shortly
are not coated. Only 75 percent of the exposed sur-
face of the drifts (ribs and backs) are coated. The
floors are not coated because use of slusher will
destroy the coating on the semi-consolidated muck.
A two-man crew (third shift) uses one Shotcrete
sprayer and one sealant sprayer. The crew first
prepares the surface and applies Shotcrete followed
with HydroEpoxy 156 and HydroEpoxy 300.
Amounts of sealants used are:
Shotcrete - 909 gal per 1000 ft2
HydroEpoxy 156 - 18 gal per 1000 ft2
HydroEpoxy 300 - 32 gal per 1000 ft2
Cost of the Sealant Coating
Annually, 530,000 ft2 of drift surfaces are coated using
480,000 gal Shotcrete, 69,600 gal of HydroEpoxy 156, and 17,000
gal HydroEpoxy 300. A two-month supply of sealants is stored
in the mine site.
31
-------
Capital Costs
Shotcrete Machine $15,000
Sealant Sprayer 3,000
Stored Sealant 50,000
$68,000
Operating Costs
Materials $298,000
Labor - Two man-years 32,000
Maintenance (equipment) 4,500
Amortization (5 years, 1070 interest) 9 .800
$344,300
Sealant Effectiveness in the Case Mine Application
The sealant coating of 530,000 ft2 per year at $344,300
($1.45 per ton of ore removed) reduces 369 Ci of radon emitted
into the mine air. This is based on average life of eight months
for the coating, 60 percent reduction of radon emission by the
coating, and 55 pCi/ft2 - sec (4.75 x 10~6 Ci/ft2 - day) radon
emission from an uncoated ore surface.
The coating reduces 23 percent (1.01 Ci/day) of the radon
emanation from total active stopes (4.35 Ci/day) and 11 percent
of the radon from the entire mine (8.86 Ci-/day), which has 25
completely extracted stopes (600,000 tons ore mined). These
figures are approximate estimates of the sealant effectiveness
based on many assumptions and approximations.
Use of the sealant coating as a radon control method would
require a careful mining plan so that the coating activity does
not interfere with the mining activity. Effects of the presence
of coating materials in the mined ore on the subsequent ore
processing have not been evaluated. However, no major adverse
effects are expected. All necessary equipment for application of
the sealant is readily available.
BULKHEADING
Bulkheading of mined out areas such as extracted stopes is
the only radon control method currently practiced in some under-
ground uranium mines. Bulkheading is used to isolate the worked
out areas and to prevent the contaminated air from these aban-
doned areas from mixing with fresh ventilation air and also to
control the direction of air flow to the working areas.
The U.S. Bureau of Mines, Spokane Mining Research Center
has recently conducted a field test of bulkheading to determine
the effectiveness of various types of bulkheadings and the
32
-------
effectiveness of a bleeder pipe used with a bulkhead. Bulkheads
commonly used in the present uranium mine industry were not air-
tight. The test indicated that an inorganic sealant commonly
used for fire proofing (Mine-Guard) provided a better sealing.
Even with the new bulkhead based on Mine-Guard, the contaminated
air still leaked out around the bulkhead. Bleeding a small
amount of the contaminated air from the enclosed space using a
bleeder pipe stopped the leakage. The contaminated air that is
bled off must be piped to a convenient lower pressure area in
the return airways or an exhaust fan installed on the pipe.
When a mined out space such as an extracted stope is bulk-
headed, the exposed ore surface inside the bulkheaded space
continues to emit radon. The radon concentration inside the
space will build up until the radon emission equals the radon
decay and leakage:
aA = A CV + CL
where a = radon emanation per unit area Ci/ft^/day
A = exposed surface, ft^
X = decay constant, 0.181 day"!
V = bulkheaded space, ft 3
C = radon concentration, Ci/ft-^
L = leakage rate, ft^/day
For an extracted stope in the case mine with 300,000 ft^ of
extracted space, and 77,000 ft 2 of the exposed ore surface emit-
ting 28 pCi/ft^ - sec radon, the relationship between the leakage
(or bleeding rate) and the radon concentration is as follows:
Leakage Turnover Time Radon Cone. Percent Decayed
SCFM Days pCi/a %
0 0 118,000 100.0
30 7 66,000 57.3
60 3.6 46,000 39.2
100 2.1 33,000 28.2
180 1.2 21,000 17.7
360 0.6 11,000 9.9
If the bulkheading seals off the worked out space air-tight,
then all radon will decay within the space; this will provide
the most effective means of radon control. However, bulkheading
alone generally does not provide air-tight seals. As the mine
barometric pressure changes, the bulkheads and cracks will
breathe, leaking highly contaminated air. Sometimes the leakage
of the highly contaminated air can be more hazardous to the mine
worker than if the bulkheading wasn't there.
33
-------
Use of a bleeder pipe connecting the bulkheaded area to the
exhaust ventilation system will prevent the leakage of the highly
contaminated air. The bleeding creates slightly negative pres-
sure inside the bulkheaded area which eliminates the leakage.
In a later section, use of an activated carbon to treat the air
that is bled off, instead of discharging into the exhaust venti-
lation system, will be evaluated.
Bulkhead Design
In an actual mine, a worker often has to have access to the
other side of a bulkhead. A common practice is to construct a
bulkhead as a temporary structure using a few pieces of lumber
posts, brattice cloth, and urethane foam spray. A bulkhead used
in this study has been improved over the bulkheads currently
used in the mining industry, with more layers of impermeable
sheets and coating.
The bulkhead shown in Figure 4 is constructed with succes-
sive layers of plywood, polyethylene sheet, brattice cloth, and
coating (HydroEpoxy 300 or Mine Guard) over timber posts. The
cost of the bulkhead shown in Figure 4 is estimated to be $550
each; $300 for the labor and $250 for the materials.
Bulkhead Application in the Case Mine
Sealing off a worked out area in an underground uranium
mine to control radon depends on many factors which are unique
to each individual mine. Some of these factors are:
Worked out areas in some mines are used as an airway
or an access to other areas of the mine. Sealing
off such an area with bulkheads may require an expen-
sive alternative airway or access route.
In some mines, especially older ones, some worked out
areas may not be accessible for bulkhead installation.
Installation of a bleeder pipe connected to an exhaust
ventilation system may be very costly.
For bulkheading to be effective, an ore barrier may
have to be left permanently between the stopes. This
will reduce the ore production and, therefore, increase
the ore cost per ton.
The applicability of bulkheading as a radon control tech-
nology has to be evaluated for each mine separately. Also, for
a given mine, there may be many alternative bulkheading strat-
egies. The bulkheading strategy for this study is the simplest
plan in which every stope, as it is completed, is sealed in the
same systematic manner.
34
-------
TIMBER
EPOXY
COATING
1RATTICE
COATED
W/EPOXY
LAGGING
POLYETHYLENE
Figure 4. Schematic diagram of a bulkhead.
35
-------
As each stope (shown in Figure 3) is completed, it is
sealed with eight bulkheads (six for drifts and two for raises).
Each completed stope has 12 drifts connecting to adjoining
stbpes. Each new stope is connected to one completed stope.
This is based on an assumption that stopes are lined one after
the other, and a new stope touches on one side of the completed
stope. Bulkheads for raises are expected to be different from
those used for drifts, but for simplicity, the same costs are
assumed for all bulkheads. The contaminated air inside the bulk-
headed area is bled out at the rate of 100 CFM from each sealed
stope.
Selection of the 100 CFM bleeding rate has no basis other
than that it is close to an upper limit, if the bled air from
each stope is treated by an activated carbon adsorption system
(this is discussed in a later section). Further investigation
of bulkheading is required to determine whether the 100 CFM
bleeding rate is sufficient to create the necessary negative
pressure in a completed stope, or if a lower rate may satisfy
the requirement.
Sealing off one stope of the case mine using eight bulk-
heads and 100 CFM bleeding, will divert 0.18 Ci/day radon from
potential contamination of the mine air and discharge only 0.13
Ci/day radon to the surface environment. The remaining 0.05
Ci/day is decayed in the sealed stope.
Cost and Effectiveness of the Case Mine Bulkheading
Annually, 12.5 stopes will be sealed using 100 bulkheads.
Each sealed stope is connected to the exhaust ventilation system
by 1000 feet of 6-inch PVC pipe (guestimate). It is also assumed
that 100 CFM of the contaminated air in the sealed stope will be
bled out into the exhaust system. The annual costs are summa-
rized as follows:
Material & Labor
Bulkheading $55,000
Piping 12,000
$67,000
Maintenance
@ 20% of Material & Labor 13,400
TOTAL $80,400
The systematic bulkheading of 12.5 stopes per year with
100 CFM bleeding from each stope will divert 2.25 Ci/day radon
from the mine air and discharge only 1.62 Ci/day to the surface
36
-------
environment. The bulkheading eliminates 0.63 Ci/day radon by
letting it decay in the sealed stopes. The bulkheading system
cost is 80,400 per year ($0.34 per ton of ore removed).
The estimated effectiveness of the case mine bulkheading is
an approximation based on many crude assumptions. However, it
is apparent that the bulkheading is a very effective means of
diverting the contaminated air from the underground mine workers,
and it also reduces the radon emission to the surface environ-
ment by letting some of it decay inside the bulkheaded area.
Potential Problems and Solutions
Bulkheading for radon control must consider the danger of
potential exposure of very high radon concentration by the mine
workers. A bleeder pipe connecting the bulkheaded stope to the
exhaust ventilation system or a radon removal system must be
installed.
If a mine worker has to go into a bulkheaded stope, the
stope has to be either decontaminated by venting it, or the
miner must use an approved respirator with charcoal canister.
The mining plan must consider bulkheading at a very early
phase and include design of the ventilation and mining system
to accommodate the bulkheading. The mining system must be
planned to minimize the need for a mine worker to go into a
bulkheaded stope.
RADON ADSORPTION ON ACTIVATED CARBON
There has been limited research and development work on
adsorption of radon by activated carbon.(17 ~22' Findings of
these investigations can be summarized as follows:
Radon gas can be adsorbed from air by various activated
carbons.
The capacity of a given carbon to adsorb radon depends
on volumetric flowrate of air only and not on radon
concent rat ion.
The capacity of a given carbon to adsorb radon depends
strongly on temperature, as shown in Figure 5.
The capacity of a given carbon to adsorb radon is
reduced by the moisture in the air (Table 3).
To maximize carbon bed utilization, air velocities
should be as low as possible. Air velocities between
0.5 and 2.5 Iit/cm2-min have been suggested.
37
-------
10*
_
ฎv
>
\
\
\
\
V
\
\
\
-------
TABLE 3. RADON ADSORPTION ON VARIOUS CARBONS
Adsorption Capacity*
Type of Carbon
KT-2M
ฐ2
KT-1
[SKT
Temp.
ฐC
18
18
18
18
cc of
Dry Air
9650
6300
9200
7400
Air/g Carbon
Humid Air (1007o)
4450
2100
NA
5250
Ref
19
19
19
19
Sutcliffe-Speakman
207 C
Norit RFL 3
Norit RFL 111
Ultrasorb
Pittsburgh PCB
25
3530
NA
15
25
25
25
25
4610
'4660
5000
5690
NA
NA
NA
NA
15
15
15
15
* Adsorption Capacity represents the air volume per gram of
Carbon before radon breakthrough.
39
-------
Concept and Design Basis
Mine air of high radon concentration is extracted from a
closed-off area of the mine, such as a bulkheaded stope, and
passed through an activated carbon system to remove radon. This
creates a negative pressure in the closed-off area and prevents
contamination of the ventilation air by high radon air from the
closed-off area.
Presently available information is not sufficient for the
design of a full-scale carbon facility. The design basis used
in this study is based on available information and engineering
judgement. Some process requirements are discussed briefly as
follows:
The carbon system will be sized for 100 CFM air flow.
The 100 SCFM flowrate requirement approaches the upper
limit for the acceptable size of an underground carbon
adsorption unit, as will be seen later. It also repre-
sents a two-day air turnover time for a typical 10' x
160' x 240' completed stope and approaches "typical"
bleed-off flowrates considered by investigators. The
negative pressure generated in the closed-off area will
depend on various other factors such as bulkhead and
type of rock formation.
The radon contained in the bleed stream is captured by
activated carbon and allowed to decay underground.
Because of the rugged mine environment, automation
should be kept to a minimum. The unit should be able
to operate unattended for 24 hours. Interruption through
utility stoppages should not cause safety problems.
The carbon adsorption unit should be simple to install
and operate. It should be able to withstand rough
handling during installation and operation.
Description of Proposed Carbon System
For this study, each bleeder pipe from a sealed stope is
provided with one modular unit of the activated carbon adsorp-
tion system. There will be 12.5 modules installed per year for
12.5 completed stopes, each treating 100 CFM of the contaminated
air.
Many flow schemes can be considered for the radon adsorption
on carbon. Three schemes are discussed here. One of these
schemes, which appears to have an advantage over others (Carbon
System #1), will be studied in greater detail to identify opera-
tional problems and allow cost estimation.
40
-------
Carbon System //!--
Carbon System #1 (Figure 6) consists of two carbon adsorp-
tion systems in series. A small flow of air (100 SCFM) is bled
off from a bulkheaded area of the mine (completed stope). The
air is filtered in order to remove dust particles and radon
daughter products.
Radon is then adsorbed in a carbon column, sized to allow
at least 24 hours of unattended operation. Once a day, the
carbon column is regenerated using hot air. After cooling and
removing condensed water, the contaminated air from regeneration
is sent through a second carbon column to adsorb, once again,
the radon gas, this time utilizing considerably less carbon.
The second column is designed to allow self-regeneration by
decay of the radon. Moisture build-up may be a problem in the
second column and occasional drying may be required.
Carbon System #2--
If moisture in the air proves to be a major problem, a
system such as shown in Carbon System #2 (Figure 7) may be con-
sidered. Since the first carbon column is regenerated daily,
moisture is also removed from the column. Most of the moisture
from regeneration is removed after cooling in the heat exchanger.
The remaining moisture is then removed in a desiccant column
(lithium chloride, silica gel, activated alumina or molecular
sieves) thereby enhancing the carbon adsorption and preventing
moisture build-up in the second column. The desiccant may be
placed in front of the first carbon bed. This increases the
capacity of the carbon bed, but requires more water to be
removed by the desiccant.
Carbon System #3--
Radon adsorption capacity on carbon is enhanced drastically
by a reduction in air temperature. This principle is used in
Carbon System #3 (Figure 8). Air is dehumidified and cooled to
-40ฐC. This allows operation of a single carbon column only.
The column is designed so that it will autoregenerate through
radioactive decay of radon.
Carbon System on the Surface--
An alternative to installing many underground carbon sys-
tems treating the contaminated air from bleeder pipes connected
to worked out stopes, is to collect all contaminated air from
these bleeder pipes and treat them in a large carbon system
located at the surface. The concept of treating contaminated
air in a large carbon svstem located at the surface was sug-
gested by A. D. Little rzz'.
Carbon Adsorption System for the Case Mine
In addition to the three systems briefly described before,
41
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FILTER
BLOWFR I PRIMARY CARBON BED
BLOWER | (1800 Ibs)
100-300JCFM
HEATER
(50KW)
1 BLOWER
(2HP)
CONDENSED
WATER TO
DRUMS
T
I RADON
MONITOR
i
SECONDARY |
CARBON BEDi
(150lbs) I
SPARE
PRIMARY
ADSORPTION
CARBON
REGENERATION &
SECONDARY
ADSORPTION
Figure
6.
Radon removal from mine air by
carbon adsorption - System 1.
-------
100CFM
PRIMARY
CARBON
COLUMN
FILTER BLOWER
HEATER
_J WATER HEATER
DEHUMIDIFICATION
COLUMN (DESICANT)
SECONDARY
CARBON
COLUMN
PRIMARY
ADSORPTION
CARBON
REGENERATION &
SECONDARY
ADSORPTION
Figure 7. Radon removal system from mine air
by carbon adsorption - System 2.
-------
100CFM
20ฐ C
CLEAN
AIR
AVWWWWtt
-40ฐC
DRY AIR
FILTER BLOWER
DEHUMIDIFICATION &
REFRIGERATION SYSTEM
CARBON
ADSORPTION
COLUMN
Figure 8.
Radon removal from mine air by
carbon adsorption - System 3.
-------
many other flow schemes are possible. The selection of an adsorp-
tion system for a full-scale facility will have to be made
considering a number of criteria such as:
Simplicity and ruggedness - compatible with the
mine environment
Weight and size limitations to satisfy underground
mine installations
System effectiveness
Maintenance requirements and reliability
Utility requirements
Safety
Cost.
Although presently available information is not sufficient
for selection of the most optimum system, Carbon System #1
appears promising, simple, and perhaps least costly (see appen-
dix for design criteria).
As shown in Figure 5, in the selected carbon system, con-
taminated air from a bulkheaded stope is filtered to remove dust
particles and daughter products. It is then sent through carbon
column #1 utilizing a variable speed blower. Carbon column #1
(four feet in diameter and five feet in height) is designed to
operate 24 hours without regeneration. The treated air is moni-
tored continuously for radon concentration. The treated air may
be piped into the returning exhaust air system. Incoming con-
taminated air is analyzed once a day.
After 23 hours operation, the carbon bed #1 is regenerated
utilizing a 75 kW external electrical air heater. A 300 SCFM
nominal flow of air is heated to approximately 250ฐC and sent
through carbon bed #1 counter-currently in a recirculation mode.
Recirculation of the hot air through the carbon bed allows the
bed to heat up to 110 C. When the desired bed temperature has
been reached, the recirculation rate and power input are reduced
as required. Hot air is drawn off from the primary carbon system
in a repeated purge and refill manner and sent through a finned
tube, air-to-air heat exchanger, and water trap. The condensed
water, contaminated with radon, is stored in a drum and allowed
to decay. The cold (30ฐC) bleed stream from the water trap is
directed through a smaller carbon column for adsorption of radon.
Carbon bed -f/2 contains 150 Ibs of carbon. A 55 gallon drum can
be used. Carbon bed '#2 can take radon charges of the 20 regen-
eration air, assuming no radon decay. Since the radon half-life
is 3.8 days, carbon bed #2 will last indefinitely. When all the
45
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radon is desorbed from carbon bed #1, the heater is turned off,
and cold ventilation air is blown through carbon column #1 as
required to lower the bed temperature. The total regeneration
cycle will take approximately one hour. During a week-end, the
carbon system may be shut off.
Since moisture may have an adverse effect on adsorption of
radon, the second bed may have to be "dried" occasionally. A
spare column is, therefore, installed to be used when radon in
column #2 is allowed to decay prior to the "drying" operation.
Carbon beds, especially carbon bed #2, will be loaded with the
radon daughters; primarily lead-210 (weak gamma and beta emitter).
When it is no longer usable, it may be buried in a secured sec-
tion of a completed stope.
Radon release in the exit stream is monitored continuously
by a device such as developed by Franklin et al (23). The
monitor consists of a scintillation chamber, a photo multiplier,
an amplifier, and a window circuit that screens out the low-level
pulses and noise. It provides continuous monitoring and record-
ing. The inlet radon concentration is measured once a day prior
to regeneration operations utilizing the same devices.
Safety precautions should be taken to protect the mine
workers. Additional radon monitoring equipment may need to be
installed nearby the adsorption system to alert the miners in
case of system malfunctions. The miners should also be protected
from yray emissions through proper positioning of the carbon
columns or through shielding. Carbon column #2 could conceivably
be located behind the bulkhead. A list of the major equipment
required with approximate cost is shown in Table A.
Cost and Effectiveness of Activated Carbon System
An average 12.5 activated carbon system, each treating 100
CFM, will be installed per year to treat the contaminated air
from the sealed stopes. The capital and operating costs for
each unit are summarized as follows:
Capital Cost of Each Unit
Major equipment $22,000
Auxiliaries & Installation 11,000
$33,000
Annual Operating Cost of Each Unit
Material (carbon, fitters, piping) $ 1,000
Utilities (25,000 kwh @ Ac/kwh) 1,000
Labor (0.25 man-year) 8.000
$10,000
Amortizing (on avg. 5-yr life, 10% interest) 8,700
TOTAL $18,700
46
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TABLE 4. MAJOR EQUIPMENT LIST OF CASE MINE CARBON SYSTEM
Major Equipment
Carbon Bed #1
Carbon Bed #2
Spare Carbon Bed
Electrical Heater
Heat Exchanger
Radon Monitor
TOTAL
Specifications
4' x 5* Carbon bed/1800 Ibs Carbon
55 gallon drum; 150 Ibs Carbon
55 gallon drum; 150 Ibs Carbon
75 kW; Stage Controller
Finned tube; 555 sq. ft. bare basis
Custom Design
Cost*
$ 4,000
500
500
4,000
9,000
4,000
$22,000
*Approximate purchased equipment cost.
-------
The activated carbon system is used in conjunction with the
bulkheading strategy; that is, the activated carbon system treats
the bled air from the sealed stope. Each carbon system treats
100 CFM of the contaminated air with 0.13 Ci/day radon. The
carbon system is expected to remove 95 percent or more of radon.
On average, 12.5 carbon units are installed per year at the
total cost of $412,500. A total of 563 Ci of radon will be
removed by 12.5 carbon units at the annual operating cost of
$233,750 ($18,700 for each unit).
A carbon system installed in a completed stope has to be
operated continuously until the mine is shut down. Some carbon
systems installed in the early phase of the mine will be operated
for longer than those installed later. If an average operating
life of five years is assumed for all carbon systems, each carbon
system will cost a total of $83,000 to install and operate for
five years. This represents $4.32 per ton of ore produced. Each
carbon system removes 45 Ci of radon per year, and it will remove
225 Ci over the five-year life of the carbon system.
Assessment of the Case Mine Carbon System
Based on experience with krypton and xenon adsorption on
activated carbon and on preliminary radon adsorption tests, radon
adsorption on activated carbon appears technically feasible,
utilizing commercial carbons and standard equipment. Application
of this technology should be considered in combination with other
approaches such as bulkheading. Assessment of the activated
carbon system applied to the case mine is summarized as follows:
The carbon system is an integral part of the bulkhead-
ing strategy. Since the cost of the carbon adsorption
treatment depends strongly on the air flow rate, the
bleed rate should be minimized. This minimum rate
depends on geologic conditions, bulkhead construction,
and atmospheric conditions.
Removal of radon from the contaminated air by activated
carbon adsorption, instead of dilution by forced-air
ventilation, will reduce total ventilation requirements
and, therefore, there may be a net economic benefit,
particularly for deep mines. This reduction in ventila-
tion air cannot be quantified at present.
Operation of a carbon adsorption system is foreign to
uranium mining operations, and skilled operators may,
therefore, have to be hired or trained. Such skills
are often not available in mining types of communities.
While maintaining a negative pressure behind a bulkhead,
the 100 SCFM bleed stream also extracts approximately
48
-------
78 percent of the radon emitted into the completed
stope. Nearly all (>95 percent) radon is adsorbed on
the carbon and allowed to decay while adsorbed.
Potential safety problems associated with the adsorption
system should be given additional attention. All eventu-
alities, such as interruption in electrical service and
any potential malfunctions, should be studied. Adequate
alarm systems should be installed. Workers should be
protected against y-ray emissions from radioactive decay.
Carbon column #2 could be installed behind the bulkhead
or shielding can be used. Fire extinguishers should be
available.
The carbon adsorption system itself requires additional
developmental work prior to design of a commercial unit:
Activated carbon best suited for the radon
adsorption service should be selected. The
radon adsorption capacity, the effect of
moisture, should be determined experimen-
tally.
-- Appropriate regeneration procedures need to
be developed. Heat and temperature require-
ments need to be determined. The volume of
regeneration air should be minimized. Carbon
aging should be studied.
Prototype adsorption units should be tested
under various field conditions.
MINE PRESSURIZATION FOR RADON CONTROL
Radon gas inside the ore body diffuses out into the mine
through an interconnecting pore structure by molecular diffusion,
convective flow, or both. The driving force for molecular dif-
fusion is the radon concentration difference between the rock
interstitial space and the mine atmosphere. The driving force
for convective flow is a pressure gradient. The molecular dif-
fusion is a relatively slow process compared to the convective
flow of radon contaminated air through the pores.
Mine pressurization has been suggested for controlling radon
emissions into the mine air by providing a convective flow into
the ore body. Limited experiments have been conducted to study
the effects of mine pressurization on radon emissions into the
mine. Schroeder, et al. (2^) conducted experiments in 1963 and
1964 at Lake Ambrosia, NM. They found that applying a 1 cm Hg
of overpressure on a section of the mine would reduce the radon
emission approximately ten-fold.
49
-------
Subsequently, the Bureau of Mines (5ป9)has tested the over-
pressurization technique in various mines with mixed results.
Switching from a blowing to an exhausting ventilation system
doubled the radon concentration in one of the stopes in the
Dakota mine, whereas only a 20 percent change was observed in
the Laguna, NM mine. Tests conducted at the Twilight mine<5)
indicate that a differential pressure of 0.02 inch water across
the bulkhead of a stope reduces the radon concentration in the
stope by more than 90 percent. No information is provided with
respect to the ultimate fate of the radon which has been removed
from the ventilation air.
For the mine pressurization to work, an air "sink" into
which the air can flow is required. This may be the surface
atmosphere. If the ore body, however, is surrounded by an
impermeable barrier, convective flow of air into the ore body
will stop in a short time, regardless of the mine pressure, and
the radon emissions into the mine atmosphere will return to the
non-pressurized condition^2).
Some preliminary experiments have been conducted by the
Bureau of Mines in a Kerr-McGee mine for testing mine pumping^
concepts. The intent is to provide a large negative pressure
inside the mine during off-shift hours, utilizing an exhaust fan,
thereby extracting large quantities of radon from the ore bodies
and then releasing the vacuum during on-shift hours. It is
thereby assumed that back-flow of air into the rock during work-
ing hours will significantly reduce radon emissions. Results so
far, however, have been inconclusive. It is unlikely that total
radon emissions to the surface will be reduced by this technique.
Many questions still remain with respect to radon control
by mine pressurization, particularly as relate to the air sink
and permeability of the gas flow.
If the ore body and host rock are impermeable to gas
flow, or if some other flow barrier exists, mine pres-
surization will only have a temporary effect on radon
emissions into the mine.
If the ore formation has an adequate permeability and
extends all the way to the surface, radon control in
the mine by mine pressurization is a distinct possi-
bility. However, the permeability has to be such that
gas flow is slow to allow decay of radon before reach-
ing the surface environment.
Designs of Mine Pressurization
Mine pressurization can be pressurization of the entire
mine or pressurization of a selected area within the mine, or a
combination of the two. An alternate technique, called mine
50
-------
pumping, has also been considered
Total Mine Pressurization--
In pressurization of the entire mine (Figure 9), a downcast
fan located on the surface is employed rather than an upcast
exhaust fan system normally used. The air intake is provided
through a ventilation shaft. The overpressure in the mine
depends on the pressure drop through the mine. If a higher
pressure is needed, a restriction can be placed in the exhaust
vent hole.
To convert a mine from an upcast exhaust ventilation to a
downcast ventilation, the following additional expenditures
would be required:
Reinstallation of fan
Additional vent hole
Air lock
Additional fan
Connecting drifts.
There may also be additional power requirements, depending
on the pressure to be used in the mine. The cost of pressuriz-
ing a mine depends on many variables such as:
Mine ventilation layout
The desired pressure and air flowrates
The desired reduction in radon emissions.
In view of the undefined nature of this technology and the
hypothetical nature of the case mine, it is meaningless to spec-
ulate on the cost for pressurizing the case mine.
Stope Pressurization--
Pressurization of a stope is accomplished using a booster
fan located at one of the raises. The booster fan takes fresh
air from a haulage drift and blows it into the stope at slightly
higher pressure. Pressurization of a working stope might
require:
Bulkheads
Closeable doors
Additional drifting
51
-------
BLOWER
Po
P
t 1 t
1 1 1
I AIR I FLOW I
1 1 1
JP P P
/ 1
/
P
STORE
P>Po
Figure 9. Schematic diagram of mine
pressurization.
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Additional vent tubing
Fan and auxiliaries.
Assessment of Mine Pressurization
Based on the available literature, it appears that mine
pressurization may be a viable method to reduce radon emissions
into the mine atmosphere depending on geological factors. How-
ever, the effectiveness of mine pressurization in reducing radon
emission into the mine air and also to the surface environment,
cannot be quantitated at the present time. More study is neces-
sary to evaluate the effectiveness of mine pressurization for
radon control. The study should include detection of the change
in the radon emission from the surrounding surface environment
in addition to that in the mine atmosphere.
The mine pressurization is expected to have different
results for different mines. An actual test will be the only
means of determining the effects of pressurization for any
particular mine.
The equipment necessary for pressurizing an entire mine or
a working stope is readily available.
Pressurizing a mine will have little effect on working con-
ditions in the mine, except the presence of air locks, which will
require a special evacuation plan for underground workers. /Haul-
age through air lock doors slows down the tramming operation and
increases maintenance costs .
MISCELLANEOUS RADON CONTROL TECHNOLOGY
The concept of removing radon from radon-contaminated mine
air by reacting it with strong oxidizing agents, such as bromine
trifluoride (BrF3) and dioxygenyl hexafluoroantimonate (02SbF6),
have been investigated by Argonne National Laboratory (25-31).
Their findings are summarized as follows:
Liquid oxident bromine trifluoride (BrF3> is very effec-
tive in oxidizing radon from the contaminated mine air.
The reaction product of radon is nonvolatile ionic com-
pound. A liquid scrubber may be used to react radon
with the oxidant. However, the oxidant is very corro-
sive, toxic, and unstable especially in the presence of
water vapor. The scrubber will probably require corro-
sion resistant material and dehumidification of the air
before scrubbing to minimize the oxidant consumption.
Solid oxidant, dioxygenyl hexafluoroantimonate (02SbFg),
reacts rapidly with radon gas, forming a nonvolatile
radon compound; hence, it can be used for purification
53
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of radon-contaminated mine air using an absorption bed
concept. However, in the presence of moisture the
oxidant is highly corrosive, toxic and unstable. The
absorption system will require a pretreatment of
dehumidification and a special corrosion resistant
material.
These concepts are still in a laboratory investigation
stage. Many more laboratory tests and pilot plant
investigations are required to determine chemical con-
sumption, side reactions, reaction products, handling
property of the reactants and product, types of equip-
ment, equipment construction materials, and design
parameters.
Although the concept of radon removal by reacting it with
a strong oxidant appears technically feasible, the corrosive
and toxic nature of the reactants make their applicability in an
underground uranium mine questionable.
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REFERENCES
1. Bates, R.C., and J. C. Franklin. Private communication.
November 1978.
2. Bates, R.C., and J. C. Edwards. "Radon Emanation Relative
to Changing Barometric Pressure and Physical Constraints",
Second Conference on Uranium Mining Technology, Nov. 13-17,
1978, Reno, Nevada.
3. Schroeder, G.L., and R. D. Evans. "Some Basic Concepts in
Uranium Mine Ventilation", Trans. Soc. of Mining Engineers
244, 301 - 8, 1969.
4. Franklin, J.C., and R. F. Marquardt. "Continuous Radon Gas
Survey of Twilight Mine", Bureau of Mines TPR 93, Jan. 1976.
5. Bates, R.C., and J. C. Franklin. "U.S. Bureau of Mines
Radiation Control Research", Proceedings of Uranium Mining
Technology Conference, April 25-29, 1977, Reno, Nevada.
6. A. D. Little, Inc. "Engineering Evaluation of Radon Daughter
Removal Techniques", Bureau of Mines, Dept. of Interior
Contract J0265011, January 1978.
7. Evans, R.D., "Engineers Guide to the Elementary Behavior of
Radon Daughters", Health Physics Pergamon Press, vol. 17,
1969.
8. Stover, C.F., and W. E. Conrad. "Feasibility Studies for
Underground Uranium Properties", Second Conference on
Uranium Mining Technology, November 13-17, 1978, Reno, NV.
9. Bates, R.C., and J. C. Franklin. "Uranium Mine Radon Control
Research", ANS Special Session, San Francisco, Dec. 2, 1977.
10. Bates, R.C., "Rock Sealant Restricts Falling Barometer
Effect", pp. 39, Mining Engineering. December 1977.
11. Franklin, J.C., R. C. Bates, and J. L. Habberstad. "Sealant
May Provide Effective Barriers to Radon Gas in Uranium
Mines", pp. 116, EMJ, September 1975.
55
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References (continued)
12. Franklin, J.C., L. T. Nuzum, and A. L. Hill. "Polymeric
Materials for Sealing Radon Gas into the Walls of Uranium
Mines", Bureau of Mines, RI 8036, 1975.
13. Hammon, E.G., et al. "Development and Evaluation of Radon
Sealant for Uranium Mines", Lawrence Livermore Laboratory,
UCRL - 51818, 1975.
14. Koehmstedt, P.L., J. N. Hartley, and P. K. Davis. "Use of
Asphalt Emulsion Sealants to Contain Radon and Radium in
Uranium Tailings", Battelle, BNWL-2190, UC-20, Jan. 1977.
15. Franklin, J.C., T. 0. Meyer, and R. C. Bates. "Barrier for
Radon in Uranium Mines", U.S. Bureau of Mines, RI8259, 1977.
16. Franklin, J.C., C. S. Musulin, and D. Thebeau. "Research
on Bulkheads for Radon Control in Mines", Second Conference
on Uranium Mining Technology, November 13-17, 1978, Reno,
Nevada.
17. Strong, K.P., and D. M. Levins. "Dynamic Adsorption of
Radon on Activated Carbon", 15th DOE Nuclear Air Cleaning
Conference.
18. Thomas, J.W., "Radon Adsorption by Activated Carbon in
Uranium Mines.
19. Thomas, J.W., "Evaluation of Activated Carbon Canisters for
Radon Protection in Uranium Mines", January 1974.
20. Thomas, J.W., "Noble Gas Absorption Process", U.S. Patent
No. 3,890,121 (June 17, 1975).
21. Kapitanor, Y.T., I. V. Porlow, N. P. Semikin, and
A. S. Serdyukora. "Adsorption of Radon on Activated Carbon"
International Geology Review, vol. 12, no. 7.
22. A. D. Little, Inc. "Advanced Techniques for Radon Gas
Removal". Prepared for Bureau of Mines PB-243898,
May 1975.
23. Franklin, J.C., R. J. Zawdzki, T. 0. Meyer, and A. L. Hill.
"Data Acquisition System for Radon Monitoring", Bureau of
Mines, RI-8100, 1976.
24. Schroeder, G.L., R. D. Evans, and H. W. Kraner. "Effect of
Applied Pressure on the Radon Characteristics of an Under-
ground Mine Environment", Society of Mining Engineers,
Transaction, 235, No. 1, March 1966.
56
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References (continued)
25. Stein, L., "Removal of Radon from Air by Oxidation with
Bromine Trifluoride," Journal of Inorganic Nuclear Chemistry,
vol. 35, pp. 39 - 43, 1973.
26. Hohorst, F. A., L. Stein, and E. Gebert. "Hexafluoroiodine
(VII) Hexafluoroantimonate (V) (IFs + SbFg)," Inorganic
Chemistry, vol. 14, pp. 22 - 23, 1975.
27. Stein, L., and F. A. Hohorst. "Reaction of Dioxygenyl
Hexafluorantimonate with Water, Carbon Monoxide, Methane,
Sulfur Oxide, Nitric Oxide and Nitrogen Dioxide," Journal
of Inorganic Nuclear Chemistry, Supplement, 1976.
28. Stein, L., "Noble Gas Compounds: New Methods for
Chemistry," vol. 47, No. 9, pp. 15 - 20.
29. Stein, L. "Chemical Methods for Removing Radon and Radon
Daughters from Air," Science, March 31, vol. 175, pp. 1463-
1465.
30. Stein, L., "Atmosphere Purification of Xenon, Radon, and
Radon Daughter Elements," U.S. Patent 3,829,551, Aug. 31
1974.
31. Stein, L. "Atmospheric Purification of Radon and Radon
Daughter Elements," U.S. Patent 3,788,499, Dec. 11, 1973.
32. Conversation with John Franklin, U. S. Bureau of Mines,
February 1980.
33. Jackson, P. 0., R. W. Perkins, L. C. Schwendiman, N. A.
Wogman, J. A. Glissmeyer, and W. I. Enderlin. "Radon-222
Emissions in Ventilation Air Exhausted from Underground
Uranium Mines," NUREG/CR-0627, PNL-2888, RE, Pacific North-
west Laboratory, Richland, Washington 99352, March 1979.
(See Table V, p. 26, and Figure 11, p. 38.)
57
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APPENDIX A
DESIGN CRITERIA OF ACTIVATED CARBON SYSTEM
FOR RADON REMOVAL
OVERALL SYSTEM
Flowrate: <100 SCFM
Air Temperature: 20ฐC
Air Relative Humidity: 100 percent
Radon Removal Required: >95 percent
Adsorption - Desorption Cycle for Column #1
Minimum 23 hours adsorption
One hour desorption time
Adsorption Column #2: At least 95 percent decay of
radon
Radon monitoring required for treated air
Radon alarms required
Unattended system operation for at least 24 hours
Provision for y~ray protection for workers
PRIMARY CARBON COLUMN
Design adsorption capacity of carbon: 6,000 cc/g (100
percent humidity,
200C)
Adsorption layer: 10 cm for 0.5 lit/min-cm2
Carbon bed: A feet diameter
Length of bed: 5 feet
Carbon needed for 23 hours: 780 kg
Carbon in bed: 820 kg
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HEAT REQUIREMENTS FOR REGENERATION (PER CYCLE)
Bed heat-up (20ฐC to I10ฐC): 90,000 Btu
Moisture removal: 160,000 Btu (maximum)
Total (maximum): 250,000 Btu/cycle
Heat is supplied by a 75 kW external electrical air heater.
Air is recirculated through the bed until the appropriate temper-
ature level is reached. Heating elements in the bed are not
practical because of the low thermal conductivity of carbon and
the potential for local overheating. Air temperatures should
not exceed 250ฐC.
HEAT EXCHANGER - COOLER
Load (maximum): 250,000 Btu/Hr
Heat Transfer Coefficient, U = 6.5 Btu/hr ft2 ฐF
LMTD = 7OOF
Area: 555 sq. ft. (bare tube basis)
Assume steel, finned tubes
SECONDARY CARBON COLUMN
Design adsorption capacity of carbon: 3,000 cc/g carbon
Purge gas during regeneration: 250 cubic feet
Carbon bed: 150 Ib drum, 20-day life with no decay.
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GLOSSARY
case mine: The hypothetical mine selected for this study.
curie (Ci): A source of radionuclide which undergoes radioactive decay of
3.7 x 1010 disintegration per second.
cut-and-fill Stoping: A stoping method in which the ore is excavated by
successive flat or inclined slices, working upward from the level.
After each slice is blasted, all broken ore is removed, and the stope
is filled with waste before the next slice is taken out.
developing stope: Stope in which development drifts are being driven to
gain access to ore.
drift: A horizontal opening in or near an ore body and parallel to the
course of the vein or the long dimension of the ore body.
drift surface: Exposed surface of drift.
extracting stope: Stope in which the ore is being extracted.
haulage drift: Drift developed for movement of men, supplies, waste, and
ore.
HydroEpoxy 300: Two component, water base epoxy manufactured by ACME
Chemical & Insulation Company.
half-life of radon: Time in which a half of radon will decay.
muck: Ore broken in process of mining.
ore: Mineral of sufficient value as to quality and quantity which may be
mined with profit.
ore body: Mineral deposit that can be worked at a profit.
orepass: Vertical or inclined passage for the downward transfer of ore.
picocurie (pCi): 10~10 curie; 0.037 dis./sec.
raise: Vertical or inclined opening driven upward from a haulage level
to the ore level.
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vtbs of drift: Side of a pillar or the wall.
voom-and-pitlar stoping: Stoping method in which the ore is first mined
in rooms and then ore in the pillars is subsequently mined.
Shotarete: Pneumatically applied portland cement mortar.
slusher: Mechanical dragshovel loader.
stope: Unit excavation from which ore is being, or has been, excavated in
a series of steps.
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