>•-- of Radiation Programs

                      Jerry J. Swift
                     James M. Hardin
                      Harry W. Galley
                      January 1976
                 Office of Radiation Programs
                Environmental Analysis Division
                  Washington, D.C. 20460

     The Office of Radiation Programs carries out a National Program
designed to evaluate the exposure of man to ionizing and nonionizing
radiation and to promote development of controls necessary to protect
the public health and safety and assure environmental quality.

     As man's appreciation of the risks of exposure to radiation has
developed, an awareness has developed that the tailings piles of
inactive uranium mills may be causing a risk to the public from radi-
ation of sufficient magnitude to warrant remedial action and develop-
ment of. necessary controls.  As part of a cooperative program with the
Energy Research and Development Administration to assess the risk and
provide remedial action if needed, this report has been prepared to
provide a basis for an initial assessment of the potential radiation
doses from the tailings piles, and to point out the kinds of informa-
tion needed to prepare a thorough evaluation of the local radiation
risks from individual tailings piles.

     Comments on this analysis would be appreciated.   These should be
sent to the Director, Environmental Analysis Division,  of the Office
of Radiation Programs.
                                    W.  D.  Rowe,  Ph.D.
                             Deputy MAiAtan
                                     Radiation ?JWQ>iamt>

        Potential Radiological Impact of Airborne Releases
         and Direct Gamma Radiation to Individuals Living
             Near Inactive Uranium Mill Tailings Piles
     As part of a program to determine which measures to use to
control radioactivity from tailings piles at inactive uranium
mills, the U.S. Environmental Protection Agency has prepared
estimates of the potential for exposure to radioactivity pre-
sented by those piles.  The gamma radiation field and the radon-
222 release rate are estimated on the basis of the radium-226
concentration in each pile.  Potential exposures to airborne
particulate uranium, thorium-230 and radium-226 are estimated
from field measurements.  Dispersion of airborne material was
calculated with adaptations of computer codes which use common
models for annual-average and single-plume dispersion.

     The estimated potential annual dose from radioactive radon
decay products to individuals in dwellings in the vicinity of an
average inactive pile is approximately 8 rem to the tracheobronchial
region of the lungs at about 50 meters from the pile, 0.3 rem at 1
kilometer, and 0.1 rem at about 2.2 kilometers.  The corresponding
doses to the pulmonary region of the lungs from airborne uranium
thorium-230, and radium-226 are estimated to be about one-third as
large, within 1 kilometer of the pile.  Gamma exposure rates on the
tailings are up to 1 mR/hr.  Estimated exposure rates are in reason-
able agreement with the limited data from field measurements.

     Should an individual be exposed continuously to a dose equiva-
lent of 8 rem/yr to the tracheobronchial region of the lung, it
would require 100 years of exposure to double his risk of bronchial
cancer.  Also, this level of exposure is considered equivalent to
0.5 Working Level Months per year, which is approximately 1/10 of
the radiation protection guidance for the protection of underground
uranium miners issued by the Environmental Protection Agency.
Average individuals exposed over a lifetime to a dose equivalent
of 0.3 rem/yr and 0.1 rem/yr would increase their risk of bronchial
cancer by about 3 percent and 1 percent respectively.

     Although there are uncertainties in these estimates, the
indicated conclusions are that the radon releases, which are
difficult to control, are the hazard of greatest significance,
that the radioactive particulate matter releases may sometimes
be significant also, and that thorough measurements are needed
for assessment of individual piles.

     The potential national and worldwide radiological impact of
radon-222 was not evaluated.

Foreword [[[ iii

Abstract [[[   v

1.0  Summary [[[   1

2.0  Introduction ............................. ....................   2

     2 . 1  Background ..............................................   2
     2.2  Present State of Knowledge ..............................   3

3.0  Radiological Assessment of Emissions from Uranium Mill
     Tailings Piles ...............................................   6

     3 . 1  Radioactivity Emitted from Tailings Piles ...............   7
          3.1.1  Radon ............................................   7
          3.1.2  Radioactive Particulate Material .................  13
          3.1.3  External Gamma Ray Exposures .....................  14
     3.2  Airborne Dispersion of Radionuclides ....................  15
     3.3  Radon-222 Dosimetry .....................................  16
     3.4  Airborne Thorium-230 and Radium-226 Dosimetry ...........  18
     3.5  Gamma Ray Dosimetry .....................................  22

4.0  Results [[[  22

     4. 1  Air Concentrations ......................................  22
     4.2  Radiation Doses .........................................  25

5.0  Discussion [[[  28

     5.1  Results for the Average Inactive Tailings Pile ..........  28
     5.2  Uncertainties ...........................................  30
          5.2.1  Source Term ......................................  31



Section 2

2.2-1  Summary of Onpile and Offpile Stations at the Four Study
       Areas	    5

Section 3

3.1-1  Inactive Uranium Mill Tailings Piles	   11

3.3-1  Calculated Alpha Dose Rates (millirem/yr) from Inhalation
       of Short-Lived 222Rn Daughter Products to the Basal Cell
       Nuclei of Segmental Bronchi	   19

3.3-2  Conversion Factors and Equivalents for Radon-222 and
       Daughters	   20

3.4-1  Dose Conversion Factors for Airborne Thorium-230 and
       Radium-226	   21

3.4-2  Percent Deposition of Particles in the Pulmonary Region of
       the Lung as a Function of Activity Mean Aerodynamic
       Diameter	   23

Section 3

3.0-1  Uranium-238 Natural Radioactive Decay Series	     8

Section 4

4.1-1  Ground-Level Radon-222 Concentrations and Dose Equivalent
       Rates from an Area Source Compared  to a Point  Source (Axis
       of a Single Plume)	     24

4.1-2  The Distribution in the Crosswind Direction of Radon-222
       Concentrations 300 m Downward from  the Center  of  a 200-m-
       Radius Area Source Compared to a Point Source  (Single
       Plume)	     26

4.1-3  Annual Average Concentrations of Radon-222  Around a 0.4 km
       Diameter Area Source Averaged Over  All Directions	     27

1.0  Summary
          This study is part of an evaluation of abandoned uranium
     mill tailings to determine the need for remedial measures to
     reduce their radiological health effects.  The public health
     significance of the mill tailings was estimated on the basis
     of average conditions for 20 inactive mill sites.  While this
     assessment may not typify actual conditions at all uranium
     mill tailings sites, several important observations were made.
     First, both theoretical estimates and experimental evidence
     indicate that individuals in the general population may be
     receiving high radiation doses to the lung as a result of the
     release of radon and its short-lived daughter products.
     Respirable airborne tailings particles bearing radionuclides
     provide another significant mode of potential radiation expo-
     sure at small downwind distances, as does the gairnna radiation
     emitted from the tailings piles.  Secondly, in order to accu-
     rately assess the potential public health significance of
     uranium tailings sites for purposes of determining remedial
     measures, a more comprehensive assessment of the problem using
     actual data applicable to existing sites rather than assumptions
     applicable to a model site is required.
          This report has two main objectives:  (1) to assess
     realistically the potential radiological implications of air-
     borne releases and direct gamma radiation from inactive uranium
     mill tailings sites, and (2) to discuss the uncertainties inherent
     in each aspect of the present assessment methodology.

          The radiological assessment of the radon-222 emissions indi-
     cates that the estimated annual average dose from radon-222 decay
     products to individuals in dwellings in the vicinity of an average
     uranium tailings site is approximately 8 rem to the lungs at about
     50 meters from the pile, 0.3 rem at 1 kilometer,  and 0.1 rem at
     approximately 2.2 kilometers.   The radiological effects of wind-
     blown particulates containing thorium-230 and radium-226 were also
     evaluated and were determined to be less of a problem.   While there
     is the possibility of doses to nearby individuals from these
     radionuclides fully as large as those from decay products of radon-
     222, the annual dose to the lungs of an individual within 1 kilo-
     meter of the pile from uranium, thorium, and radium is  estimated
     conservatively to be about one-third as large.  For a typical
     uncovered tailings pile, the external gamma dose rate is estimated
     to be 10 rem/yr at 3 feet from the surface of the pile, decreasing
     to less than 1 mrem/yr at 1.0 kilometer.

          It is estimated that the average individual exposed contin-
     uously over a lifetime to a dose equivalent of 8 rem/yr to the
     bronchial epithelium region of the lung will incur about a 70
     percent increase in the risk of bronchial cancer.   Average indi-
     viduals similarly exposed to a dose equivalent of 0.3 rem/yr and

     0.1 rem/yr would increase their risk of bronchial cancer by
     about 3 percent and 1 percent respectively.  The relative
     risk from particulate material blown from the pile is not as
     critical, because the radiation dose from inhaled radioactive
     particles is lower and, in addition, is delivered to the pul-
     monary region of the lung.  Irradiation of the pulmonary lung
     is believed to be less likely to produce a health effect
     compared to an identical dose equivalent delivered to the bron-
     chial epithelium.  The relative increase in risk caused by gamma
     radiation from the average tailings pile is also estimated to be
     less significant than the increase in risks caused by radon-222
     from the pile.

2.0  Introduction

2.1  Background

          In March 1974, the U.S. Environmental Protection Agency and
     the U.S. Atomic Energy Commission testified at hearings before
     the Joint Committee on Atomic Energy (JCAE) to present their
     respective views on pending bills S-2566 and HR-11378.  These
     were identical bills which provided for a cooperative program
     between the AEG and the State of Utah for the stabilization and
     containment of a uranium tailings pile at of the now
     inactive Vitro Mill near Salt Lake City, Utah.  Both Federal
     agencies acknowledged that there are other uranium tailings
     sites besides the Vitro Mill tailings pile that may present
     public health problems, and that legislation that deals with
     all inactive mill sites and not just a single site would be more
     effective in controlling these potential problems.  Consequently,
     both the EPA and the AEG recommended and subsequently proceeded
     to implement a study to determine the current condition of all
     inactive uranium mill sites in the various states.  This study is
     a cooperative two-phase undertaking involving the states concerned
     and appropriate Federal agencies.

          Phase I involved a visit to each site to determine the physi-
     cal conditions of the site, need for remedial action, ownership,
     proximity to populated areas, prospects that the number of people
     living around the site would increase, approximate areas of tailings,
     etc.  A preliminary report, which will serve as a basis for deter-
     mining if additional assessment is necessary, was prepared for each
     site.  These reports were provided to the JCAE (October 1974) before
     proceeding with Phase II.

          Phase II of the study, which started in July 1975, includes a
     detailed radiological and engineering assessment of the piles, an
     examination of alternative remedial measures and the preparation of
     cost estimates, detailed plans and specifications for various
     remedial actions.

      This part of the study also includes extensive physical measure-
      ments to determine the degree of potential radiation exposure of
      the public.  Additional research and development may be needed on
      stabilization techniques to prevent wind and water erosion of the
      tailings piles and to prevent radon emission from them.  An assess-
      ment will be made of the total environmental impact of proposed
      remedial actions.

           The EPA has recently attempted to quantify the potential
      public health problems associated with uranium mill tailings at
      both active and discontinued milling operations and place them
      in perspective relative to other segments of the uranium fuel cycle.
      The results of these analyses are contained in reference (1).

           These analyses were done on the basis of a model tailings
      pile which was assumed to be representative of all tailings sites.
      Numerous assumptions were made where information was lacking or
      where large uncertainties existed with the available information.
      This generic assessment indicated that uranium mill tailings piles
      present possible radiological health problems and that a more rig-
      orous analysis of them is required.

           Uranium mill tailings piles contain most of the decay products
      of the radioactive decay of uranium and a little residual uranium.
      The principal radionuclides of concern, in order of their appear-
      ance in the decay chain, are thorium-230, radium->-22-6,  radon-222,
      polonium-218, lead-214, bismuth-214, polonium-214,  and polonium-

           Uranium mill tailings piles release airborne radioactivity
      as radon gas and also as airborne particulates when wind blowing
      over the pile lifts material into the air.   Radioactivity may  be
      leached from the pile into surface and ground water by precipita-
      tion and by surface runoff waters.   Also,sufficient radioactivity
      is in the tailings to create a field of gamma radiation in the
     ..vicinity,of the tailings.   Because of the presence of  thorium-230
      in the tailings, which by its decay maintains the radium inventory,
      the .radioactivity in the pile can remain almost constant for
      thousands of years.  The scope of these problems and the remedial
      actions required for their control vary with each site and;  there-
      fore, must be determined for each inactive tailings pile.

2.2   Present State of Knowledge

           The Environmental Protection Agency and its predecessor
      radiological health program in the U.S. Public Health  Service  have
      been active for years in programs to determine the  nature  and  mag-
      nitude of the public health problems associated with uranium tailings.
      More widespread appreciation of the magnitude of the problems  involved
      has brought more support for these programs.

     Under a joint agreement between the AEG and PHS, a project
was begun in June 1967, in cooperation with the Colorado Depart-
ment of Public Health and the Utah State Division of Health to
evaluate the public health aspects of atmospheric concentrations
of radon-222 in the vicinity of uranium tailings piles.  The ob-
jectives of this project were to  (1) develop techniques for
taking integrated air samples for radon near uranium tailings
piles, (2) evaluate atmospheric concentrations of radon in areas
near tailings piles as an index of radiation exposure to the popu-
lation, (3) determine the effect of stabilization and covering on
the emission of radon gas from tailings piles, and  (4) develop
joint recommendations (or guidelines), if necessary, to control
exposures based upon measured radon concentrations.  The results
of the study are reported in a 1969 DHEW publication titled
"Evaluation of Radon-222 Near Uranium Tailings Piles" (2).

     Study areas were chosen to reflect conditions  at both opera-
ting and inactive uranium mills.  Locations were chosen at Grand
Junction, Colorado  (then an operating mill with uncovered tailings),
Durango, Colorado (inactive mill, uncovered tailings), and
Monticello, Utah (inactive mill,  covered tailings).  The Vitro Mill
tailings pile near Salt Lake City, Utah, was added  to the study in
October 1967.  The Vitro Mill tailings were uncovered, and the mill
ceased operation in July 1968, while the study was  being conducted.
At each of these study areas, a number of sampling  sites were
selected that were representative of onpile, near-pile, and general
community conditions.  Locations were chosen taking into considera-
tion the prevailing wind patterns, population densities, and geo-
graphical factors.

     Table 2.2-1 summarizes the measured radon concentrations at
onpile and offpile sampling stations at the four study areas.  For
perspective, these values may be  compared with the  following regu-
lations and guides.

     Federal regulations 10 CFR Part 20 (3) presently permit AEC
licensed operations to release average concentrations of radon-222
to the air in unrestricted areas of 3 x 10"^ yCi/ml (3 pCi/1) above
natural background.   [Such releases may be subject  to reduction if
it appears that a suitable sample of an_exposed population.may have
an intake of the decay product radionuclides greater than they would
have if exposed to average concentrations of one-third of the
3 x 10-9 yCi/ml, i.e., 1 pCi/1.]

     Recommendations of the International Commission on Radio-
logical Protection  (ICRP) state that the annual radiation dose
limits for individual members of  the general public should be
one-tenth of the corresponding annual occupational  dose for con-

          Table 2.2-1.  Summary of Onpile and Offpile Stations
                                at the Four Study Areas

    (From "Evaluation of Radon-222 Near Uranium Tailings Piles" (2))

Grand Junction, Colo.
Durango , Colo .
Station 54 )


j.,1 -
.50 -
.13 -

.48 -
.44 -
.09 -

.12 -
.03 -

1.6 -
2.3 -
.06 -




Va'average of about 17 measurements  at  each  location  over  a period of  12
; ^assumed to be the local background concentration
    bout 0.4 mi southeast  of  site
  'about 0.4 mi west of  site

      tinuous  exposure as  listed in the Committee II  report  (4).   Appli-
      cation of  these recommendations to the 168-hour concentration
      (10  pCi/1)  for occupational exposure to radon-222 (with daughter
      products present at  equilibrium concentrations) leads  to a  value
      of 1 pCi/1 for continuous exposure to individuals in the general
      population.   This concentration is in addition  to the  natural
      background radon concentration which is on the  order of 0.3 pCi/1.

           More  recently,  EPA has conducted environmental surveys (5)
      around the uranium tailings pile at the site of the now inactive
      Vitro Mill near Salt Lake City, Utah.  The surveys included measure-
      ment of  external gamma radiation and airborne radioactivity.  The
      results  of the surveys indicated that the external radiation levels
      on  the tailings area exceeded recommended dose  equivalent limits
      for  individuals in the general population (500  mrem/yr); that
      ambient  levels of radon over the pile and in some of the structures
      built Immediately adjacent to the tailings pile were above the
      currently  recommended concentrations for individuals in the general
      population (1 pCi/1); and that the working level (WL)  exposures in
      the  adjacent buildings exceed 0.01 WL.  The relation between radon
      concentrations in pCi/1 and working level (WL)  is discussed in
      section  3.3.

           Because the mill tailings piles have not been the subject of
      as much  interest as some other sources of radiation, there has been
      little development of methods specifically designed for estimating
      exposures  to radiation originating with the pile.  Schiager (6)
      presents methods that require minimal data inputs for  estimating
      potential  radiaton exposures from abandoned uranium tailings piles
      based upon the average radium concentration in  the tailings.  The
      reductions in exposure rates that can be achieved by covering the
      tailings with earth or concrete are also discussed. Based upon a
      case study of data on one tailings pile (Vitro  pile),  Schiager
      concludes  that contributions to the average annual outdoor radon
      daughter concentrations exceeding 0.003 WL are  very unlikely on or
      near any tailings pile.  However, estimates of  indoor  radon daughter
      concentrations under the same conditions, which would  lead to a more
      accurate assessment of potential radiation exposures for most sites,
      are not  presented.

3.0   Radiological Assessment of Emissions from Uranium Mill
      Tailings Piles

           Mill tailings piles present a potential for exposure to radi-
      ation by several pathways.  The most important  pathway is believed
      to be that of the radon-222.  Radium-226 in the pile decays by alpha
      particle emission and becomes radon-222, a radioactive noble gas.
      The radon-222 gas that is released into the spaces between the grains

        of tailings material diffuses toward the overlying tailings sur-
        face; some reaches the surface and some undergoes radioactive
        decay enroute.  The radon-222 that reaches the surface escapes
        into the air above, where it is mixed into the passing airstream
        by normal local air turbulences.  The wind carries the radon-222
        with it and continually decreases the concentration by further
        mixing and dilution.  Persons downwind of the tailings pile will
        be exposed to some concentration of radon-222 and its particulate
        radioactive decay products in the air they breathe.  The air that
        enters a building downwind remains inside it for some period of
        time, depending on the ventilation rate, i.e., on the rate at
        which air enters and leaves the building.  The radon-222 in the
        air undergoes its normal rate of radioactive decay, forming a
        series of decay products which may be inhaled by persons in the
        building.  Some of the radioactive daughters of the radon-222
        are retained in the tracheobronchial region of the lungs, irra-
        diating the fluids and tissues, and thus increasing the risk of
        cancer formation there.  In contrast, persons outdoors who are
        exposed to the same radon-222 concentration from a nearby tailings
        pile may receive an exposure to the lungs from the radon decay
        products which is appreciably smaller (e.g., a factor of 10) than
        that received indoors, due to the lack of a delay time which
        permits daughter product ingrowth.

             Another pathway for exposure to radiation is that in which
        the wind lifts particles containing radionuclides from the sur-
        face of the tailings pile and carries them downwind,  with simul-
        taneous mixing, dilution, and deposition, until they reach persons
        inside or outside of buildings.  Inhalation of the particles leads
        to exposure in several ways,  but it is believed that the principal
        exposure is to the pulmonary region (the exchange space, non-cili-
        ated) of the lungs in this case.  It is likely that indoor exposures
        by this pathway are smaller than those outdoors because filtration
        and sedimentation processes reduce the particle concentrations.

             The third principal pathway for exposure to radiation from
        the tailings piles is that in which the radionuclides in the pile
        emit gamma radiation which may penetrate the overlying material
        and air to interact with the body tissues of persons on or near
        the tailings piles.

             The decay chain containing radon-222 and daughters is shown
        in figure 3.0-1.

  3.1   Radioactivity Emitted from Tailings Piles

3.1.1   Radon

             Most of the uranium ores being milled in this country are
        sandstones consisting of silica grains poorly cemented together

       Production of Radon-222
       23&     4-2 MeV a      }   234Th    0.19 MeV B

               4.51 x 109 yr              24.1 days

                                          4.7 MeV a
               1.17 min                   2.47 x 105 yr
       9qn     4.6 MeV a         09fi      4.8 MeV a           00<>
       230Th  - 7 - »•   226Ra   - "     222Rn
               8.0 x 104 yr               1,600 yr
       Short Half-Life Radioactive Decay Products of Radon-222
               m5.5 MeV a            91B    .   ..    6.0 MeV a _ .
          ^  - j.      218po  (RaA)  - "

               3.82 days                          3.05 min

       9iA_           0.7 MeV 3           O-IA            3.3 MeV B
       214Pb (RaB)   —7 - - "    214B±  (RaC)
                      26.8 m                            19.7 min
                        7.7 MeV a
                        164 u sec
       Lone Half-Life Radioactive Decay Products  of Radon-222
210pb (RaD)     0.02 MeV f    ,     210B1 (RaE)
                                                           1.1 MeV  f
                           21 yr                            5,01  days
       21°P°  ^      138 days"	"     *****
       Figure 3.0-1.  Uranitim-238  Natural Radioactive Decay Series


 with materials  such as  calcium carbonate (7} .   In the milling
 process,  the sandstone  is broken down  and  after the  uranium (and
 sometimes vanadium)  is  extracted by  chemical leaching, essen-
 tially  the whole original mass remains as  tailings.   The tailings
 consist of the  original sand grains  plus slimes made up of
 materials from  between  the sandstone grains and waste chemical
 products  of the mill process (7_).

      Sampling of the material  flow streams at numerous points in
 several mills has shown that the tailings  contain essentially all
 of the  radioactivity that was  originally in the ore  except  for
 the  uranium; usually less than 10 percent  of the original uranium
 content remains in  the  tailings (7_,8) •   However,  analyses of
 samples from inactive tailings piles show  lesser radioactivity
 concentrations  than  expected from uranium  concentrations  in the
 ore  (9,_10,_11) .  The  principal  decay  chain  in the tailings radio-
 activity  is that beginning with uranium-238 and ending with lead-
 206;  among its  many  members are the  long-lived  radionuclides
 thorium-230, radium-226, and lead-210.   The thorium-230 is  the
 effective beginning  of  this decay chain  in the  tailings.  The
 other radionuclide decay chain present,  beginning with uranium-
 235  and ending  with  lead-207,  contributes about 5 percent of  the
 total radioactivity.  There are no reports of radionuclides found
 in the  tailings in appreciable concentrations which  are not members
 of either of these two  decay chains.  For the majority of the
 uranium ores mined in the United States, the ratios  of the  con-
 centrations of  the radionuclides in the decay chains  are  approx-
 imately the ratios expected under the condition of secular  equi-
 librium (7); if the  uranium-238 decay chain was in undisturbed
 secular equilibrium, for each  0.1 percent by weight  of uranium
 oxide (UgOs) that had been in  the ore, the tailings  should  contain
 about 280 pCi/g of thorium-230, of radium-226,  and of  each  of  the
 succeeding members of the decay chain.  Measurements  of the radio-
 nuclides  in the tailings show  that commonly the activity  of the
 thorium-230 in  the tailings is  about the same as the  activity of
 the  radium-226  (11,12).

      The  radioactivity  that was pumped to the tailings  pond was
 disproportionately divided between the sands, the slimes, and
 the  dissolved material  (T) •  The dissolved material  generally
 contained less  than  one percent of the mass and less  than one
 percent of the  radioactivity of the tailings (7^8).  Although
 there is considerable variation from mill to mill, the slimes
 solids  are usually about one-third of the total weight  of
 tailings solids and  have most  of the radioactivity,  about three-
 quarters of the total.  It is  inferred from a statement of
 Tsivoglou et al. (7) that the  slimes particles may be  as  fine
 as minus-400-mesh (less than 25 micrometers in  diameter) like
particles of silt or clay, and  that the sands are generally ulus-
200-mesh  (greater than 74 micrometers in diameter), i.e., in  the
normal size range for sand particles.


           Culot, Olson, and Schiager (13) have measured radon emana-
      tion from the sand fraction of tailings from the Grand Junction
      pile and determined that about 23% of the total radium inventory
      in the sand releases radon to the atmosphere.  This emanating
      fraction will be assumed to apply to the slimes also, although
      Pearson (14) indicates that the smaller particles may release a
      greater fraction.  The overall release from tailings particles
      is assumed to be about 20% of the total radon produced by the
      decay of radium-226.

           The 20 percent emanation fraction does not completely deter-
      mine the radon release rate from tailings piles.  Radon released
      at a depth in the pile must migrate to the surface, and may decay
      during the process.  This migration of the radon, a diffusion
      process, produces a rate of radon emission from the upper surface
      of the pile which, for a given uniform radon production rate
      within the pile, is .a function of the depth of the pile.  This
      rate approaches an upper limit as the pile depth is increased.
      Schiager indicates the depth at which the limit is approached by
      stating that, for dry tailings, "6 to 10 feet in depth is required
      to produce the maximum radon flux" at the surface of the pile (6).
      This may be considered an "infinite thickness;" i.e., a thickness
      such that the rate at which radon released from all greater depths
      (even if they were infinite) reaches the surface is insignificant
      in comparison, increasing the total release rate negligibly.
      Schiager also provides a relation between the radon emission rate
      and the radium content.  He estimates on theoretical grounds that
      at the maximum rate of radon emission from the tailings, 1.6 pCi/m^-
      radon-222 are released for each pCi/g radium-226 in the tailings  (6).

           A variety of factors influence the rate of radon release from
      a tailings pile.  The radon release rate at any one location is
      known to vary over a factor of 10 due to effects of weather, i.e.,
      wind speed, barometric pressure, atmospheric stability, rainfall,
      and snow cover (14).  Moisture in the tailings is particularly
      effective in reducing the emission rate; calculations (15) indi-
      cate that completely saturating a dry tailings pile with water will
      reduce its radon emissions by a factor of about 25.  Other data
      (13,^.4) indicate that addition of moisture occupying 15 to 85
      percent of the interstitial space will reduce the emission rate by
      a factor of 2 to 3.  It has been estimated from theory that a 2-foot
      layer of soil over the tailings will also reduce the radon emission
      rate by about 25 percent  (13)-

           A list of inactive uranium mill tailings piles is given in
      table 3.1-1.  The piles average 35 acres in area, with a range
      from 2 to 107 acres.  Their average radium-226 concentration is

                                                        Table 3.1-1  Inactive Uranium Mill Tailings Piles (1974)





Location W
Tuba City
Monument Valley(d>
Grand Junction
Rifle (new)
Rifle (old)
Slick Rock (2 piles)

Grants (f)
Falls City

Ray Point
Mexican Hat
Green River
Salt Lake City
Monti cello
Converse County

El Paso Natural
Gas Co. „;
Vanadium Corp. :.
Amax Uranium Corp;"
Colorado Venture
Union Carbide
Union Carbide
Union Carbide
Union Carbide

Foote Mineral
Foote Mineral
United Nuclear
United Nuclear
Foote Mineral
Oregon Pacific Ind.
Susquehana Western

Atlantic Richfield
A Z Minerals
U.S. Air Force
Vitro Corp.
Western Nuclear

Susquehana Western'' •
Porter Brothers Corp.
Acres <•'



race Area
M2 (xlO5)
•i 1.34
'•' 2.06


1.62 -

Quantity*"' »"'
Tons (x106)
T 1-9



Ra-226 Activity""



Ra-226 Quantit



(in prog.)
(in pit)
(in prog.)
        (a) Statement of Dr. James L. Liverman, USAEC, before the JCAE Subcommittee on Raw Materials, Hearings on Uranium Mill Tailings located in the
State of Utah (March 12,1974)
        (b) Data updated, June 1974, personal communication from Frank McGinley, USAEC
        (c) AEC correspondence from Martin B. Biles, Director of Operational Safety to Vernon S. McKenzie, Dept. Director, Bureau of Disease Prevention
and Occupational Health, PHS, HEW, March 13,1967
        (d) Tailings pile from ore upgrading process
        (e) 18 miles north (Ambrosia Lake) of Grants
        (f) 10 miles northwest of Grants

      about 620 pCi/g with reported concentrations (estimated from
      uranium production records) ranging from 90 pCi/g to 980 pCi/g.
      Assuming a uniform density of 1.6 g/cm^ (13), the average depth
      of all piles, calculated from their reported areas in table 3.1-1,
      is about 4.8 m, ranging from 1.5 to 12.6 m for individual piles.
      The piles are frequently roughly rectangular, but in general
      irregular in shape and, if unstabilized, are made more irregular
      by wind erosion.  Table 3.1-1 indicates that about two-thirds of
      the piles, both by number and by area, have been stabilized to
      some degree.  Photographs of more than a dozen tailings piles
      are in references (11) and (16) .  The Phase I study referred to
      in the introduction has produced recent photographs of these piles.

           The overall radon emission rate from tailings piles may be
      estimated by applying the theoretical factor of 1.6 pCi/m2-s
      radon-222 released per pCi/g radium-226 in the tailings to the
      areas and concentrations in table 3.1-1, with adjustments for
      conditions which influence the emission rate.  Because the
      analyses of samples from tailings piles (9_,JLO,_11) indicate lower
      radium-226 concentrations than those of table 3.1-1, a corre-
      sponding value of about 4/5 the average will be used, i.e., 500
      pCi radium-226 per gram will be used in all calculations that
      project the potential exposures from an "average" inactive
      uranium mill tailings pile.
           Based on meteorological data for the region (17_, _18_ ,19^t2Q) ,
      it is assumed that during the year, as a result of precipitation
      and freezing weather, there are about 80 days during which the
      release rate is reduced to 1/3 of the value for dry tailings.
      In effect, this reduces the yearly value by 15 percent.  This
      may be conservative, because some piles are moist years after
      cessation of mill operations (9) .

           An adjustment may be considered for areas of tailings piles
      which have less than an "infinite thickness" of tailings under
      them.  Although each pile is less than the "infinite thickness"
      at its edges, and although about one-quarter of the piles
      average less than 3 meters in depth, the overall effect of areas
      having less than the "infinite thickness" is small, and is esti-
      mated from the information given by Schiager (6) to be about 5
      percent .

           The overall radon-222 emission rate per unit area for the
      average unstabilized tailings pile is estimated from these considera-
      tions to be 500 pCi/g 226Ra x 1.6 pCi/m2-s 222Rn per pCi/g 226Ra
      x 0.85 x .95 = 640 pCi/m2-s radon-222.  For the average stabilized
      pile, this rate is estimated to be reduced by one-quarter to 480
      pCi/m2-s radon-222 (6).

           For the average inactive unstabilized pile of 35 acres, the
      total radon-222 release rate for the pile is about 90 uCi/s (2,900
      Ci/yr); for a similar stabilized pile, 68 uCi/s (2,200 Ci/yr) .  For

        the more than 800 acres of piles in table 3.1-1, the total is
        about 2 mCi/s (6 x 104 Ci/yr).

3.1.2   Radioactive Particulate Material

             The radium-226,  thorium-230, and more than a dozen other
        radionuclides in tailings piles are associated with particles
        which are picked up by the wind and transported through the
        environment.   This discussion will concentrate on those radio-
        nuclides whose presence in the  air is clearly due to wind action
        and on the thorium-230 in particular, because it is among the
        more hazardous once inhaled.

             The ability of the wind  to pick up and transport particles
        depends on the size and density of the particles and on the local
        wind speed.   It is likely that  the wind picks up only small
        numbers of particles bearing  thorium until the wind speed is
        sufficient to allow the wind  to move the larger sand grains.
        Under such conditions, the wind will pick up sand particles and
        with them the finer particles that were once the tailings slimes.
        The smaller particles have an unknown size distribution but indi-
        cations are that an appreciable fraction are smaller than 25 micro-
        meters in diameter (7.) •   The  larger grains are deposited quickly,
        after a short travel  distance.   At one site,  it has been noted
        that wind-carried tailings on the ground are readily visible
        beyond the original pile for  some 500 feet in the predominant
        wind direction (21).

             Meteorological data (17_,1.9,Z2)  indicate that,  allowing for
        rain and snow cover,  roughly  15 percent of the time the wind may
        have the opportunity  and sufficient speed to blow the tailing
        material around, and  about 0.1  percent of the time heavy dust
        blowing occurs.   It is likely that moisture is of lesser conse-
        quence in this case,  as the surface layers dry quickly.   The data
        are insufficient to permit determination of a quantitative depen-
        dence upon the wind speed.  Stabilized piles are sometimes con-
        sidered as if they release no tailings particles to the air.   How-
        ever, it appears that, due to the spread of contamination during
        the period preceding  stabilization,  some particulate radionuclides
        are available for resuspension  from the surroundings in every case.
        This is supported by  measurements showing higher than background
        concentrations (0.004 and 0.008 pCi/m^ thorium-230) in the air
        upwind of piles (23).

             Reported data (£,JLO,21,.23_)  indicate that average exposure
        rates on tailings piles may be  about 10 rem/year to the pulmonary
        region of the lung from airborne radium,  thorium, and uranium,
        and in the general range of 0.01 to 4 rem/year within 1,000 m of

        the pile,  with most data in the range of 0.1 to 1 rem/year.
        Measurements made under dusty conditions show exposure rates
        sometimes  exceeding the equivalent of 60 rem/year at locations
        on wind-deposited tailings near the pile (21).

             Thorium-230 and other uranium-related radionuclides are
        normally present in the air due to wind erosion of natural soil
        which contains concentrations on the order of 1 to 5 pCi/g of
        these radionuclides (13_ ^24), with additions to the air from the
        burning of coal and other human activities.  Their normal back-
        ground concentrations in air in the general environment are
        estimated  from reported, uranium concentrations (25) to be in
        the range  of 0.00001 to 0.0002 pCi/m3.

             A peculiarity of reported particulate concentrations in air
        in the vicinity of inactive mill tailings piles is that many show
        uranium concentrations comparable to the radium and thorium con-
        centrations, although milling operations commonly remove more
        than 90 percent of the uranium.  Some of the airborne uranium may
        originate with ore fragments contaminating the mill site.  However,
        it is difficult to believe this contamination is the principal
        source of  uranium in particulates because of the implication that
        it is also the source of the thorium and radium, thus exonerating
        the tailings piles.

3.1.3   External Gamma Ray Exposures

             Tailings piles containing radium-226 are large enough so
        that for purposes of calculating the gamma ray exposures at the
        surface of the pile near its center, they may be treated as
        radiation-emitting slabs of infinite area.  Every disintegration
        of an atom of radium-226 eventually results in the production,
        through decay of its daughter products, of an average of 2.18
        photons.  These photons have a mean energy of 0.824 MeV.  More
        than 95 percent of this gamma energy is from radon daughters
        (24).  The total gamma ray flux at the surface of the pile is
        made up of unscattered primary gamma rays and secondary gamma
        rays that have been scattered by material inside the pile but
        still reach the surface.  The primary gamma ray flux is a func-
        tion of the gamma ray emission rate and energies, the linear
        attenuation coefficient of the tailings, and the thickness of
        the tailings.  The secondary or scattered gamma ray flux may be
        described by a buildup factor which is a function of the linear
        attenuation coefficient, the energy of the primary gamma rays,
        and the thickness of the slab.

             Schiager  (6) has performed some theoretical calculations for
        uranium mill tailings piles and has reduced them to the following:

                         x  (yR/hr) = 2.5 CRfl  (pCi/g)
14                                        Ra

      This simple formula allows the estimation of the exposure rate
      (x) above a tailings pile if the concentration of the radium in
      the tailings is known.  Schiager assumed that the density of the
      tailings pile was 1.6 g/cm3 and that the linear attenuation co-
      efficient was 0.11 cm~l.  The buildup factor increases only
      slowly after 1 to 2 feet of tailings.  Thus, the formula, which
      includes buildup, does not have to be corrected unless the depth
      of the tailings is less than about 1 foot, in which case the
      exposure at the surface of the pile will be smaller than pre-
      dicted by this equation.

           Moisture in the pile increases the gamma radiation field
      because of greater retention of radon which decays to gamma-
      emitting daughters.  Conversely, moisture reduces the gamma
      radiation by increasing the mass of material with which the
      gamma rays interact before escaping from the pile.

           For the average pile containing 500 pCi/g of radium, the
      gamma exposure rate at the surface is estimated to be approxi-
      mately 1,200 uR/hr or approximately 10 R per year.   In theory,
      each foot of packed earth (1.6 g/cm3) covering the pile (6)
      will reduce this exposure rate by a factor of approximately 10;
      it would require only about 4 feet'of earth to reduce the expo-
      sure rate at the surface from the pile to 1 mR per  year.   Diffu-
      sion and decay of radon-222 in the soil will, however,  produce
      gamma emissions from the upper layers,  thus counteracting in
      part the decrease in the gamma field due to attenuation.   For
      a typical uncovered tailings pile,  it is estimated  that the
      exposure rate will decrease to less than 1 mR/yr at 1.0 kilo-

           Experimental measurements of the gamma radiation over the
      Vitro uranium mill tailings pile by Salt Lake City  have been
      made using TLD dosimeters (_5).   This pile has been  reported
      (table 3.1-1)  to contain about 900 picocuries radium-226 per
      gram.  Schiagerfs equation using this concentration would
      predict an exposure rate on the surface of this pile of 2.2
      raR/hr.   TLD readings,  3 feet above the surface of the pile,
      ranged from 0.2 to 1 mR/hr at the edge of this pile and from
      approximately 0.4 to 2.2 mR/hr at the center of the top surface
      of this pile.   These predicted and measured dose rates  are in
      reasonable agreement.

3.2   Airborne Dispersion of Radionuclides

           The radionuclides released from the^tailings pile  surface
      are mixed into the passing airstream by local turbulences and
      carried off by the wind, which continually decreases the concen-
      tration by mixing and dilution.  This process of airborne dis-
      persion eventually delivers a dilute concentration  of radio-
      nuclides to the downwind locations of people who may inhale  them.


            Tailings.piles are in general large in area and not very
       high compared to the distance across them.   For computational
       convenience,  an example tailings pile has been chosen as a flat
       circular area emitting radon-?22 uniformly over its area.  The
       circumstances considered were the single plume case in which the
       wind blows continuously from one direction, and the annual average
       case in which the set of different wind directions and velocities
       occurring over a year are treated.

            There are several methods for treating single plume disper-
       sion from area sources.  It may be modeled by a virtual point
       source or by a line source (26).  It may be treated by integra-
       ting over the source, analytically or numerically (27).  For
       exposures near or on the source, it may be treated by considering
       a volume of air moving over the area source (6).  The evaluations
       made in this study were accomplished using a numerical integration
       over the area source for the single plume cases, and using a line
       source representation for the calculation of annual average
       exposures.  The area source considered is a circular area with a
       400 m (about 1/4 mile) diameter; it has an area of about 31 acres,
       a little smaller than the 35 acre average area of inactive piles.
       The single plume results were obtained from an adaptation of the
       RSAC code (28), and the annual average results from an adaptation
       of the AIREM code  (29).

            Meteorological data from the Ft. St. Vrain reactor site
       were adapted for the calculation of annual average exposures
       because the detailed meteorological data required for these cal-
       culations were not readily available for any of the specific
       tailings pile sites (17).  Although the Ft. St. Vrain reactor
       is not in the neighborhood of a tailings pile, it has meteor-
       ological conditions with many points of similarity to those at
       many tailings pile locations, including a highly anisotropic
       wind rose (30), in which the wind frequency ranges from 2.9 to
       17 percent per 22.5 degree compass sector.

 3.3   Radon-222 Dosimetry

            When a person breathes the air downwind of a tailings pile,
       he inhales the dilute concentration of radionuclides in the air.
       The effect of the radionuclides depends on the nature of the
       radiation they emit, on the rate at which they emit radiation,
       on the parts of the body they are retained in and the period of
       time retained there, and on the manner in which the radiation
       interacts with the body tissues on a microscopic scale.  The
       significance of radon-222 is believed to be primarily due to
       its radioactive decay products; it is the inhalation of these
       decay products which is considered to be a cause of cancer in
       the tracheobronchial region of the lung.

When the radon-bearing airstream is flowing around a building,
the radon enters the building through the ventilation system
and through the cracks and openings in it.  Some of the radon
daughters entrained in the air enter also, and additional
amounts of radon daughters are formed in the air inside the
building during the period the air remains there before the
ventilation replaces it with more air from outside.  Thus,
the air inside a building downwind from a tailings pile will
probably have greater concentrations of radon daughters than
the air outside.  The time period available for formation of
greater indoor concentrations of radon daughters depends upon
the ventilation rate; for normal ventilation (31) an amount of
air equal to that contained within the structure leaves it and
is replaced at least every hour.  Ventilation rates of 2 to 6
changes per hour are common.  The levels of radon daughter
products, greater thaja 0.01 WL, measured in buildings adjacent
to the Salt Lake City pile may be partly attributed to the
retention of radon in the structures (_5) .

     The assumption is made that an average individual chron-
ically exposed to a concentration in air of 1 pCi/m^ of radon-
222 receives a radiation dose of 4 ,mrem per year to the lung
from radon daughters (1).   This dose conversion factor is in
agreement with recommendations in a recent UNSCEAR report (32)
and assumes the person is inside a building with adequate ven-
tilation, a quality factor (Q) for alpha particles of 10 and a
penetration depth to cells at risk (bronchial epithelium of the
lung)  of 60 ym.  It does not represent the worst conceivable
case;  but, because of uncertainties,  it is considered an accept-
able conversion factor for estimating the average dose to large
segments of the general population.

     There are many uncertainties when the dose to the lung is
calculated from exposure to a known concentration of radon-222
(31).   The estimated dose is a function of:

     a.  The degree of equilibrium between radon and its radio-
         active daughters.

     b.  The fraction of  daughter ions Which remain free,  or
         unattached to aerosols at the moment of inhalation.

     c.  Assumptions with regard to the critical mode of expo-
         sure to certain tissues within the lung.   Cancers in
         uranium miners considered to have been caused by
         excessive exposure to radon daughters,  predominantly
         appear in the area of the large bronchi and are
         believed to occur as a result of deposition of alpha
         particle energy to the basal layer cells of the upper
         bronchial epithelium.

           Table 3.3-1 (33) gives calculated alpha dose rates to the
      basal cell nuclei of segmental bronchi resulting from the inha-
      lation of short-lived radon-222 daughter products in air con-
      taining 1 pCi/m3 radon-222.  The dose rate resulting from a
      given concentration of radon-222 is seen to have a wide range
      of values depending on the conditions of the exposure and the
      assumptions regarding the critical mode of exposure to tissues
      of the respiratory system.

           Working level months (WLM) is a term commonly used to
      express a miner's calculated exposure to radon daughter products
      found in mine air.  This unit is derived from the working level
      (WL), a unit which is defined as any combination of short-lived
      radon daughters in one liter of air that will result in the
      ultimate emission of 1.3 x 105 MeV of alpha energy (34).  A
      working level month  (WLM) is the exposure to 1 WL concentration
      of radon daughters for 170 hours or an equivalent product of
      radon-daughter concentrations and time.

           If the concentrations of radon daughters are known (or
      assumed) relative to the concentration of radon, the WL concen-
      tration can be expressed in units of pCi/m^ radon-222.  For a
      daughter ratio of 1.0/0.9/0.5/0.35, which is the ratio assumed
      for dose calculations in this report, 1 pCi/m3 of radon-222 is
      equivalent (table 3.3-2) to 5 x 10~6 WL.  This same ratio was
      used to calculate the radiation dose of 4 mrem/yr resulting
      from chronic exposure to 1 pCi/m3 radon-222.  These relation-
      ships are summarized in table 3.3-2.

3.4   Airborne Thorium-230 and Radium-226 Dosimetry

           Table 3.4-1 gives the dose conversion Jractors for thorium-
      230 and radium-226 used to estimate the average dose to large
      segments of the general population (1).

           Particulate material from a tailings pile that is lifted
      into the air by the wind will contain  thorium-230, radium-226,
      and radon-222 and its daughter products.  If these particles
      are inhaled, the resultant radiation doses will depend pri-
      marily on:

           a.  The activity levels of the radionuclides.

           b.  The particle size distribution of the aerosol particles.

           c.  The solubility of the particles in fluids of the lung.

           d.  The energies of the alpha particles emitted in the lung.


Table 3.3-1.  Calculated Alpha Dose Rates (millirem/yr) from Inhalation
       of Short-Lived 222Rn Daughter Products to the Basal Cell
                   Nuclei of Segmental Bronchi (33)
Penetration Depth to Cells at Risk

Living Accommodation:
adequate ventilation'3'
Living Accommodation:
inadequate ventilation^)
Industrial Premises (c)
Air Conditioned Sites (d>
(a)222Rnj 218Po> 214pb,
pCi/mr respectively.
and 214BJ
I concentrations:
0.90, 0.51,
 (b,)222Rn,^218po, 214Pb, and 214Bi concentrations:  1.0, 0.95, 0.70, 0.57,
  c-)222Rll) 218p0> 214Pb> and 214Bi concentrations:  1.0, 0.97, 0.84, 0.78,
m  respectively.
 W)222Rnj 218PO) 214pb, and 214Bi
                               Bi concentrations:   1.0, 0.88, 0.49, 0.35,

           Table  3.3-2.   Conversion Factors and Equivalents
                       for Radon-222 and Daughters
  Concentration                Working                   Dose
  of Radon-222                  Level                  Equivalent

  Continuous'3'             Continuous              4 mrem/year
  exposure to               exposure to             to  the  lung
  1 pCi/m3                  5 x 10~6 WL

  Continuous                Continuous              8 x 10^ mrem/year
  exposure to               exposure to             to  the  lung
  2 x  105 pCi/m3            1 WI/C)

  Continuous                Continuous              1 mrem/year
  exposure to               exposure to             to  the  lung
  0.25 pCi/m3               1.3 x 10~6 WL
        (a) Assumed radon-222 and daughter ratios 1.0/0.9/0.5/0.35  (for
   living accommodation with normal ventilation) .
                delivered to basal cell nuclei of segmental bronchi;
   Q, 10; penetration depth to cells at risk, 60 urn.

        ^C'A combination of short-lived radon daughters in 1 liter of  air
   that will result in the ultimate emission of 1.3 x 105 MeV of alpha
   energy .

              Table 3.4-1.  Dose Conversion Factors for Airborne
                        Thorium-230 and Radium-226 (1)


Soluble (D)
Insoluble (Y)
Soluble (D)
Insoluble (Y)
Dose Conversion Factor
mrem/yr per

        Pulmonary region of the lung.
       'Assumes particle has an AMAD of between 0.5 and 1.0 y,  the pulmonary
region of the lung weighs 450 gms,  and the mean retention time  of an insoluble
class Y particle is 1,000 days in the pulmonary region of the lung.
its daughters.
        Assumes only radium-226 contributes to the dose and not radon-222 nor

          The  probability of  inhaled particles being retained in the
      lung is determined  by the particle size distribution of  the
      aerosol  (33).   Some of the particles are exhaled,  and a  fraction
      of  the others that  is initially retained in the lung is  subse-
      quently removed to  the gastrointestinal tract.   Of the remaining
      retained  particles, those that are insoluble will  remain a long
      time and  deliver their radiation primarily to their region of the
      lung;  those that are soluble are dissolved and their radionuclides
      will be  translocated by  the blood to other organs  where  they may
      be  retained selectively.  Soluble thorium and radium are bone
      seekers;  thus, the  critical organ doses will be delivered to the

          Dose conversion factors given in table 3.4-1  are suitable
      for aerosol particles with a particle size distribution  charac-
      terized  by an activity mean aerodynamic diameter (AMAD)  of 0.5
      to  1.0 micrometer.   As an illustration of the effect of  particle
      size,  table 3.4-2 gives  the deposition of particles in the pul-
      monary region of the lung as a fraction of the AMAD and  indicates
      how the  dose conversion factor could change as a function of par-
      ticle  size distribution.  For insoluble particles, the dose con-
      version factors are also directly related to the mass of the pul-
      monary region of the lung (450 g) and the mean residence time of
      particles in the lung (tb = 1,000 days).  For insoluble  particles
      with a t|j of 250 days and an AMAD of 5 micrometers, the  dose con-
      version factors could be as much as a factor of 10 smaller.  The
      dose conversion factors in table 3.4-1 represent upper limit or
      conservative values, with the exception of the assumption that
      only the radium contributes to the dose, and not its decay product
      radon-222 nor radon daughters.

3.5   Gamma Ray Dosimetry

           The gamma ray exposure from the surface of a uranium mill
      tailings pile is assumed to be whole body radiation potentially
      delivering an absorbed dose with a quality factor of 1.0.  The
      exposure rate in mR/hr is therefore assumed equal to a Dose
      Equivalent Rate in units of mrem/hr to the whole body.

4.0   Results

4.1   Air Concentrations

           Figure 4.1-1  shows the radon-222 concentrations in  air as
      a function of distance for a single plume from an ar^a source
      compared with those from a point source.  The downwind distance
      is measured from the center of the area source to the receptor.

Table 3.4-2.  Percent Deposition of Particles in the Pulmonary
                Region of the Lung as a Function of Activity
                       Mean Aerodynamic Diameter (33)
                                       Deposition of  Particles
     Particle                          in the Pulmonary Region
       AMAD                                  of  the Lung
    Micrometers                                  %


                      • EDGE OF AREA SOURCE
                               FROM POINT SOURCE
•FROM 400 M


        0.1                          1.0







0.5   E

           Figure 4.1-2 shows a similar comparison of the radon-222
      concentrations in air as a function of distance in the cross-
      wind direction at a downwind distance of 300 m.

           Figure 4.1-3 shows the calculated annual average radon-222
      concentrations as a function of distance and averaged over all
      directions, compared to the measured data from which table 2.2-1
      was compiled (2).  The majority of the measured concentrations
      did not differ appreciably from the concentration assumed to be
      the local background.  Only measured results which did differ
      appreciably from background are shown; their statistical error
      ranges well over 100 percent.

4.2   Radiation Doses
           If the dose conversion factor of 4 mrem/yr per pCi/mJ
      radon-222 is applied to the radon-222 concentrations in figure
      4.1-3, the dose to the bronchial epithelium of the lungs of
      persons in buildings at the respective distance from the center
      of the tailings pile is obtained.   The estimated average radia-
      tion dose due to radon emission is about 8 rem to the lung per
      year at 50 m from the edge of  the pile, about 0.3 rem per year
      at 1 km and about 0.1 rem per  year at 2.2 km.

           For individuals outdoors  near tailings piles,  the exposure
      to the airborne decay products of  radon-222 released from the
      pile is probably insignificant,  because of dilution by the wind
      before much radioactive decay  occurs.   However,  most people spend
      the majority of their time indoors.

           Another mode of exposure  is that from particulate radio-
      active material airborne due to wind action.   The average expo-
      sure rate from airborne uranium, thorium, and radium to the lungs
      of individuals within 1.0 km of a  tailings pile is  estimated to
      be generally one-third or less of  that from decay products of
      radon released from the pile.   The principal exposure from the
      particulate uranium, thorium,  and  radium is believed to be to the
      non-ciliated, pulmonary region of  the lung, in contrast to the
      principal exposure by daughters of airborne radon being to the
      bronchial epithelium.  Another contrast between these modes of
      exposure to the lung is that the exposure from particles is
      likely to be less indoors than out,  because of removal of par-
      ticles by filtration and sedimentation from the airstream
      entering a building.

           The average exposure rates from gamma radiation to the whole
      body are similar to the exposure rates to the lung  from airborne
      uranium, thorium,  and radium at locations close to  a tailings
      pile, but decrease more rapidly with distance,  to less than 1
      mR/year at 1.0'km.



    - 30,000
    - 20,000
                                           PLUME AXIS
                              POINT SOURCE
                                        AREA SOURCE
    - 10,000
100         0         100
 FIGURE 4.1-2
PLUME).                     '




                           DISTANCE FROM PILE CENTER, Km

           The presence in the neighborhood of contamination such as
      scattered ore fragments and windblown tailings may cause expo-
      sures to remain higher with distance from the pile than these
      estimates indicate.

5.0   Discussion

           Radiological dose assessment calculations were performed
      for the average, uncovered, dry tailings pile 1.3 x 10^ m2
      (31 acres) containing 500 pCi/g each of thorium-230 and radium-
      226.  For piles of different radionuclide concentrations, the
      results given in figure 4.1-3 can be scaled linearly to estimate
      approximately the corresponding radiation dose rates.  For example,
      the Vitro pile is approximately 3 times larger and contains
      almost twice the radioactivity as the average pile.  Therefore,
      allowing for its size, radiation doses from radon and thorium
      are expected to be 2 times greater  in adjacent areas and, at a
      distance, 5 or 6 times greater than those predicted for the
      average pile.  In addition, the dose rate in the prevailing wind
      direction may be twice that of the  average dose rate.  The "worst
      case" is downwind from the Vitro pile where dose rates in buildings
      adjacent to the pile are estimated  to be about 3 times that pre-
      dicted for the average dose rate in the vicinity of the average
      inactive tailings pile.

           The credibility, accuracy, and precision of estimates of
      dose equivalent from emissions from tailings piles can be greatly
      improved with site-specific information on:

           a.  The emission rate of radon from inactive piles, experi-
               mentally measured.

           b.  Meteorological data, assembled for the inactive piles
               for comparison with assumptions used in this report.

           c.  Population data around each pile.

           d.  Measurements of airborne radionuclide concentrations,
               correlated with meteorological conditions.

           e.  Activity vs. particle size distributions of airborne
               particles containing thorium-230 and radium-226.

 5.1   Results for the Average Inactive Tailings Pile

           For  persons  in dwellings  in  the neighborhood of a 31 acre
       inactive  tailings  pile  assumed  to  contain  500 pCi/gm radium-226,
       the estimated  average radiation dose rates  are given in

section 4.2.  These dose rates, high in comparison to  those
around nuclear facilities, indicate a need to evaluate measures
to reduce exposures near inactive tailings piles.

     The comparison of the area source with a point source indi-
cates that the size of the source causes average concentrations
of radon in the neighborhood of a pile to be more evenly dis-
tributed around the compass than average concentrations from a
point source.  Thus, a location upwind of the pile in  the most
frequent wind direction will not be as favored as might be
expected from the wind frequency data.

     The results indicate that the suspension of fine particles
by the wind is not as large a problem as that of the radon
emissions.  Stabilization of piles by addition of a foot or more
of soil cover is believed to effectively stop further spread of
wind suspended contamination as long as the cover is not dis-
placed; previously distributed contamination not cleaned up at
the time of stabilization can continue to contribute small dose

     Similarly, the potential gamma radiation dose rates,  ranging
up to about 10 rem/yr on a tailings pile,  may be largely avoided
by stabilization of the pile with one or more feet of soil cover
and cleanup of distributed contamination.   The permanence of the
benefits of such stabilization may be questioned.  Measurements
at the Mpnticello pile indicate that the cover soil has reduced
the gamma exposure rate by a factor of about 5 (35).

     Data in the report "Evaluation of Radon-222 Concentrations
Near Uranium Tailings Piles" (2)  may be compared to the estimated
average values of the example calculation, shown in figure 4.1-3.
The data representing measurements on the  piles (see table 2.2-1)
may be compared to the estimated value in figure 4.1-3 of  roughly
2,000 pCi/m^ at 50 m from a pile; all average concentrations on
piles are higher, some by a factor of 8.   Annual average concen-
trations measured in Durango, Grand Junction, and Salt Lake City
(2) are plotted in figure 4.1-3 for comparison,  after subtracting
background.   Predicted values are not in major disagreement with
the measured values, considering their wide scatter.

     It appears from the results of this assessment that the
principal long term radiological health problem associated with
tailings piles is caused by radon releases.   The critical  pathway
is radon emission from the pile surface followed by diffusion,
holdup in dwellings long enough for radon  daughters to grow in,
and finally radiation exposure to the lung due to the inhalation
of the radioactive daughters of radon.   Radon is not appreciably
contained by covering the pile with a few feet of earth cover.


       Therefore,  the evaluation of remedial actions for individual
       tailings piles to prevent excessive exposure to members of the
       general population must include a consideration of the radia-
       tion dose delivered through the radon exposure pathway.

            Particulate material of respirable size carried off a pile
       by wind delivers a radiation dose to the lung which is less
       than the radon dose.  Radiation exposure through this pathway
       appears to be relatively easily controlled by stabilizing the
       pile with a few feet of earth cover.  Direct gamma radiation
       from the pile is also reduced although not eliminated by a few
       feet of packed earth.

            Radioactive material may be carried from the pile by winds
       or by human actions, creating offsite exposures from radon and
       from gamma radiation.  Wind action brings tailings and conse-
       quent radiation exposures to locations whose distance once pro-
       vided appreciable protection from the radioactivity in the pile.
       Radium can be leached from the pile if the pile is exposed to
       water.  These are special cases of contamination, not considered
       in this report, that must be considered for each individual
       tailings pile site.

            When radon-222 is released from a tailings pile there is,
       in addition to the exposure of persons living near the pile, a
       potential for national and world wide exposure.  Radon being a
       noble gas does not deposit quickly on the ground like particu-
       late matter; thus, the 3.8 day half life allows sufficient time
       for wind to carry most of the radon very long distances before
       radioactive decay occurs.  At long distances from the pile, the
       radon-222 exposure pathway is identical to the local exposure
       pathway.  But in addition to the radiation dose to the bronchial
       epithelium caused by short half-life daughters, radon-222 even-
       tually decays to long half-life radionuclides of lead, bismuth,
       and polonium.  These deposit on the ground and have a potential
       for radiation exposure through food pathways.  These potential
       national and world wide radiological impacts are not evaluated
       in this report.  They are very small when compared to the
       natural radiation background as caused by these same radio-
       nuclides .

 5.2   Uncertainties

            The radiological assessment of uranium mill tailings piles
       performed in this report makes use of some parameters for which
       the values are not well known.

5.2.1   Source Term
             One of the primary factors affecting the emission of radon-
        222 is the quantity of its precursor, radium-226, that is in the
        tailings.  Under the assumption that the radium-226 content of
        the ore is in secular equilibrium with the uranium-238 content,
        the quantity of radium-226 that went to the tailings at each
        mill site has been estimated from the mill's production records;
        the values in table 3.1-1 were derived in this manner.  Analyses
        of samples from tailings piles (£,.10,1^) show smaller concentra-
        tions, down to half the expected value.  Whether these  discrepan-
        cies are due to errors in the predicted concentration or as a
        result of sampling which is not representative is not clear; indi-
        vidual samples from the Mexican Hat pile differed by a factor of
        3 or more (9) .  The ratio of sand to slimes particles in the
        sample may influence the analysis,  as the slimes solids contain
        higher radium-226 concentrations.  It is probable that in some
        instances the assumption of secular equilibrium is incorrect; for
        example, ground water may have leached some elements from the ore.
        A review of analyses of more than 50 soil samples (j.2) having
        radioactivity concentrations characteristic of uranium ore (i.e.,
        concentrations of radionuclides that,  in equilibrium, would be
        associated with uranium concentrations greater than 0.05%),
        showed that about one-third had concentrations of radium-226
        and thorium-230 within 10% of equilibrium and one-third had  con-
        centrations differing from equilibrium by more than 50%,  the
        remaining third falling in between.   At Mexican Hat, the sample
        analyses show thorium-230 concentrations five times the radium-
        226 concentrations (9_).   In general,  either case is possible;
        that the ore had uranium-238 radioactivity greater than its
        radium-226 radioactivity,  or smaller.

             The radon emitted from the surface of the pile may be
        assumed to have originated at a distance below the surface which
        is less than the "infinite thickness;" however,  the radium may
        not be uniformly distributed in the pile.   At various times,
        ores having different radium concentrations may have been proc-
        essed by a mill,  with their associated tailings  ending in different
        parts of the pile.   Another factor  is  the greater radioactivity
        of the slimes solids (T)  because any process that tended  to
        separate slimes solids from sands in the tailings would make the
        pile non-uniform in its distribution of radium-226,  and thus non-
        uniform in its radon-222 emission rate over the  surface.   At least
        one mill had separate tailings ponds for the sands and for the
        slimes (8).

             Another source of uncertainty  in the radon  emission  estimate
        is the 0.2 emanation fraction.   The determination of a 0.23  frac-

       tion by Schiager et al. (13), although carefully performed, is
       properly applicable only to tailings sands from the Grand Junction
       pile.  The emanation fraction may be different for the slimes
       solids, and may differ from pile to pile.  It is unlikely that
       it is much less than 0.2 and also that it is greater than 0.8.

            Although the tailings piles have been discussed in terms of
       an average pile, they are not uniform in character.  Though fre-
       quently roughly rectangular, the piles are generally irregular
       in size and shape.  Several of the piles listed in table 3.1-1
       are actually two or more piles in the same vicinity, with the
       possibility that each has individual characteristics as a radio-
       activity source.

            The piles may vary considerably in the soil characteristics
       of the material comprising the pile.  Some piles, from opera-
       tions called upgrading operations or concentrators, may be
       relatively pure sand with little of the fines from the slimes
       and little radioactivity.  This is because the slimes have been
       shipped to another mill for further processing (7) .  Piles of
       this type might be expected to be easily windblown, having no
       fines to help hold the material together.  Other piles may have
       a large fraction of the fine particles from slimes in their make-
       up, due to the nature of the ore they came from and possibly to
       slimes from upgrading operations having been added to them.
       Reference to piles being muddy (_5) and clay-like (9) is reason-
       able confirmation of characteristics unlike sand.  Such piles
       will retain moisture longer and have reduced radon emission rates
       as a result.  They may also be. somewhat less subject to being
       wind drifted.

            These differences in the soil characteristics of a tailings
       pile affect its relaxation length for radon (a measure of the
       depth from which radon can migrate to the surface) when wet or
       dry and the rate at which the pile will dry out when moistened
       by precipitation.  Some piles appear to dry slowly (£); their
       radon emission rates may be higher in the future when they have
       dried out.  The dry depth available for radon emission may also
       be restricted by the underlying soil characteristics and topog-
       raphy, which may prevent rapid drainage of moisture from the pile.
       Moisture, especially together with a high slimes content, will
       reduce the radon emission rate, but it is unlikely that it would
       be reduced by more than two-thirds.  Moisture may also serve to
       leach radium and other radionuclides into the underlying soil and
       possibly into the cover soil if it is applied for stabilization.
       Stabilizing a pile by the addition of soil and by growing vegeta-
       tion may further reduce the radon emission rate by the enhance-
       ment of moisture retention.

     Although we have used c.  callings density of 1.6 g/cm^,  it
is known that the dry density of tailings varies from one pile
to another  (13).  One effect of uncertainty in the density is
to introduce uncertainty in our calculation of the average depth
of tailings in  a pile from its area and tonnage.  In regard  to
other pile characteristics, it should be noted that the slimes
solids have been reported as being more dense than the sands  (8).
The uncertainty in density may also affect the relation (6)
between the radium concentration in pCi/g and the rate of radon
emission in pCi/m^-s from the surface of a pile of "infinite
thickness."  There seems to be some discrepancy among the pa-
rameters relating density, emanation fraction, and infinite
thickness; if the density (1.6 g/cm-*) and emanation fraction
(0.2) are reasonably accurate, the infinite thickness (6 to 10
feet) appears to be underestimated.  It is unlikely that the
density would be less^-than 1.4 or greater than 2.4 g/cm^.

     The thorium-230 content of the tailings piles is subject
to greater uncertainties than the radium content, because it has
not been so widely measured.  It has the same types of uncer-
tainties in quantities, concentrations, and distribution as the
radium.  Since its chemical behavior is different from that of
radium, its distribution may be different from the radium, both
by natural causes in the ore and by the physical and chemical
processes in the mill and tailings.

     The soil characteristics of the tailings may also have a
large influence on the amount of material that becomes airborne
in that a relatively clean sand of coarse grains may not release
many particles to the wind whereas tailings noticeable for their
dusty appearance may release large quantities (23).  Tailings
which are subject to wind action may be so blown around that the
source is enlarged, and the reported exposures may reflect wind-
blown tailings much nearer than the tailings pile.  This may
account in part for the slow decrease in exposure rate with down-
wind distance (23).

     Some of the radioactive radon-222 decay products in the air
may have been lifted into the air in particulate form (in the
same manner as the uranium,  thorium, and radium)  rather than
being formed directly in the air by radon-222 decay.   The  avail-
able data does little to distinguish airborne radon daughters by
their history, especially if they are associated with particles.
The degree and nature of their association with particles  plays
an important role in their potential for exposure of different
regions of the lung or of other organs.  Our estimates of  doses
from radon daughters include only those daughters formed in the

             Some of the factors that affect the radon emission estimates
        also affect the estimates of the gamma radiation field around the
        piles; differences in density, in composition, and in moisture
        content can affect the linear attenuation coefficient and the
        buildup factor.  The linear attenuation coefficient will vary
        almost directly with the bulk density of the tailings and thus
        varies as the moisture content affects the density.  Comparison
        of estimated radium concentrations with measured exposure rates
        indicates that the theoretical factor employed in the gamma radi-
        ation estimates may estimate exposure rates which are high in some
        cases, perhaps up to a factor of 3.  Since most of the external
        gamma dose is due to radon and its decay products in the pile,
        the rate at which radon escapes influences the gamma dose rate.
        Sealing the tailings surface with a thin coating preventing
        radon escape could be expected to result in an increase in the
        local gamma field, but not more than 60 percent (13).  The
        external gamma dose rate from the airborne material is small,
        less than one mrem per year.
5.2.2   Meteorology
             The meteorological data employed, from the Fort St. Vrain
        reactor site in Colorado (17), is different from that at specific
        tailings piles.  The uncertainties due to differences at each
        tailings location include the uncertainties due to the effects
        of terrain on the meteorology at low elevations.  These effects
        are quite local so that a good estimate of the annual average
        dispersion to locations within a half mile of a pile would
        require data obtained from that neighborhood.

             The dispersion of radon from the piles was estimated with
        adaptations of the common model for Gaussian diffusion from a
        continuous point source emitting at a uniform rate.  The only
        changes made were to consider an area source rather thamsa point
        source.  It has been observed that the radon emission rate is
        affected by meteorological conditions and, thus,  is not uniform.
        One effect is reported to be an emission rate proportional to wind
        speed (14).  Such an effect could deplete the radon in the upper
        layers of a pile so that following a high wind, the emission rate
        would be depressed, under circumstances in which the common -Gaussian
        diffusion model would, due to its inverse dependence era windspeed,
        predict an increased concentration exposure rate.  It is-.possible
        that there are other meteorological factors,  such as variations
        in barometric pressure and stability of the lower atmosphere,
        which may cause average exposures to differ from those predicted
        by the model for a uniform continuous source.  Pearson {14)
        suggests that atmospheric instability may produce a depletion
        analogous to that suggested for high wind speeds.  Either or

both of these depleting effects would lower  the annual  average
exposure to radon from the piles.

     It should be noted that not all the airborne radioactivity
in the vicinity of an inactive tailings pile originates  in  the
pile.  The poor correlation between the direction of the wind
across the tailings pile and the measured concentrations (2)
lends support to this.  Some airborne activity is due to natural
background which may be anomalously high because of higher  than
average radionuclide concentrations in the soils and rocks  in  the
vicinity, and some may be due to contamination of the surroundings
by ore fragments and windblown tailings from the mill site  (_5,j^,
J.O^,J.l) .  Urban areas may have higher concentrations of some par-
ticulate radionuclides in the air due to increased dust  levels
resulting from vehicular traffic and construction activities (19).
Thus, the radioactivity levels reported in the vicinity  of several
piles (_2,J>) may arise from several sources.  These different sources
complicate field measurements and the interpretation of  the data
obtained.  The assumption that the tailings pile is the  only source
may lead to overestimating its effects.

     Several conditions can lead to high dust concentrations
and related higher thorium and radium concentrations.  The lack
of protective vegetation on the tailings piles permits higher
wind velocities at the surface so that dust-blowing conditions
may result more frequently than in vegetated areas.   Measurements
made in dust-blowing conditions may show exposure rates  ten or a
hundred times the average (21).

     Dust devils provide another mechanism for providing high
dust concentrations and thus higher potential exposure rates
(19,23).  Dust devils occur commonly on bare dry ground which
has been heated by the sun, warming the adjacent air so  that it
is appreciably lighter than the overlying air (36).   Most dust
devils last only a few minutes but in some locations they may
occur frequently (36).  It has been reported that one which
lasted several hours removed about a cubic yard of sand per
hour (36).   Measurements made with dust devils on a tailings
pile show exposure rates as much as ten times the average (23).
They are unlikely to be present more than a few percent of the
time, however.

     Since construction activities and vehicular traffic add sig-
nificantly to dust concentrations (19),  allowance should be made
in planning actions to stabilize or dispose of tailings piles to
avoid unnecessary exposure of workers and persons living in the


5.2.3   Dose Conversion Factors

             The dose conversion factor for radon-222 is a function of
        the degree of equilibrium between radon and its daughters, the
        fraction of daughters which remain unattached to aerosols at
        the moment of inhalation, and assumptions with regard to the
        critical mode of exposures to certain tissues within the lung.
        Depending on the conditions of the exposure and assumptions
        regarding the way tissues of the lung are exposed, the dose
        conversion factor ranges widely in value as seen in table
        3.3-1.  The value of the factor used in this report of 4
        mrem/yr per pCi/m-* radon-222 does not represent the worst
        case, but because of uncertainties it is considered acceptable
        for estimating the average dose to large segments of the general

             The dose conversion factors for airborne thorium-230 and
        radium-226 depend greatly on knowledge of the solubility and
        size of the radioactive aerosol particles.  For thorium-230,
        the dose conversion factor varies by a factor of 3.5 between
        insoluble and soluble particles; for radium-226 by a factor of
        330.  The dose conversion factor for both may vary by a factor
        of 2, larger or smaller, depending on whether the particle size
        distribution may be described by an activity mean aerodynamic
        diameter  (AMAD) as small as 0.1 ym or as great as 10 ym.  The
        AMAD value of 0.5 to 1.0 ym assumed in this report is believed
        to be reasonably typical of airborne particles carried more than
        short distances by average wind velocities.  Measurements reported
        by Breslin and Glauberman  (23) support this assumption; they indi-
        cate a respirable fraction  (to the pulmonary region) of about

             The solubility of  the particles in lung fluids and in passing
        through the gastrointestinal  tract is undetermined.  However,
        experience with uranium miners indicates that most of the ore dust
        they are exposed to is  insoluble  (34) and investigations of mill
        flow streams show that  very small percentages of the radium and
        thorium are dissolved as  the material passes through the mill  (8)
        The assumption made is  that the airborne participates in the vicin-
        ity of tailings piles are  insoluble.

             The dose conversion^factors used in this assessment for air-
        borne, insoluble particles containing thorium-230 and radium-226
        assume that the mass of the lung is 450 g and that the particles
        have a half-value residence time  (tfe) in the lung of 1,000 days.
        These  assumptions, which represent  the best judgment of the Envi-
        ronmental Protection Agency,  have been selected with a tendency to
        make  the dose  conversion factors conservative.  For example, if t^
        were  250  days,  the  factors would be 4 times smaller; if the lung
        mass were assumed  to be 1,000 grams, they would be 2 times smaller.

          The dose conversion factors used in this assessment for air-
     borne, soluble particles containing thorium-230 and radium-226
     and for gamma radiation are derived directly from recommendations
     of the International Commission on Radiation Protection (ICRP)
     and as such represent generally acceptable values.

5.3  Health Risk Estimate

          There is believed to be a potential health risk to persons
     living near uranium mill tailings as a result of the pile's
     radon-222 emissions.  The potential health risk associated with
     this exposure to radon-222 and its daughters is a form of  lung
     (bronchial) cancer considered to be 100 percent fatal.   It has
     a latent period of 10 to 15 years after the onset of radiation
     exposure that is followed by a plateau period of elevated  risk
     which lasts from 15 years to possibly a lifetime.

          The potential health risk from windblown particulate  material
     from the pile is probably not as significant because the radiation
     dose is smaller than the dose from radon-222, plus the  particulate
     material dose is delivered to the pulmonary region of the  lung.
     The radon-222 dose is delivered to the bronchial epithelium region
     of the lung, and data from the BEIR (37)  report indicates  that the
     bronchial epithelium is much more sensitive to radiation compared
     to the pulmonary region of the lung.

          The potential health risk from the pile's gamma radiation is
     also of lesser significance because the whole body gamma radiation
     dose is considerably smaller than the dose to the  bronchial epithe-
     lium from radon-222 daughter products.   While more health  effects
     are produced per unit dose equivalent of whole body radiation than
     per unit dose equivalent delivered to the bronchial epithelium only,
     the much smaller relative dose from gamma rays causes the  overall
     potential health risk from gamma radiation to be considerably less
     than the potential health risk from radon-222 emissions.

          Because bronchial cancer is believed to have many  possible
     causes, the bronchial cancer risk from radon daughters  is  best
     expressed in terms of the percentage increase in the expected
     cancer risk, i.e., the relative risk, rather than the absolute
     number of cancers expected per person-rem of exposure.   The rela-
     tive risk is not considered to be a function of the level  of
     absolute risk and is therefore the same for everyone while the
     absolute risk of bronchial cancer to an individual depends
     strongly on his personal history.   For example, habitual cig-
     arette smoking may increase the risk of bronchial  cancer by a
     factor of 2 to 10.  Because the relative risk coefficient  for
     bronchial cancer from radon exposure is thought to be relatively
     independent of smoking history, exposure to radon  daughters

     sufficient to double the risk of bronchial cancer for
     nonsmokers will also double the risk for smokers.

          The health risk associated with exposure to radon-222
     and its daughter products cannot be accurately assessed at
     this time.  Although epidemiological studies have indicated
     increased incidence of bronchial cancer among uranium miners
     and certain metal,  hematite, and felspar miners who have been
     exposed to high airborne concentrations of radon daughters,
     the use of health risks derived from these results to provide
     estimates of risk to members of the general population involves
     considerable uncertainty.  There are many differences between
     the uranium miner populations and the general population living
     near tailings piles, and there are critical differences between
     radiation exposure conditions found in mines compared to houses

          It is current best judgment that exposure to 110 working
     level months (WLM)  doubles the risk of bronchial cancer in
     uranium mining populations (42) where "working level months"
     is a measure of integrated exposure to radon daughter products
     (38).  Because uranium miners usually have chronic bronchitis
     which reduces the dose owing to the protection provided to the
     epithelium by the increased thickness of the mucus layer, an
     equivalent radon daughter exposure level delivers a greater dose
     to members of the general population than to mining populations.
     The BEIR report (37) uses a factor of 2 to account for existing
     chronic bronchitis when estimating the dose to the basal cell
     layer of the epithelium of the larger bronchi on a probabilistic
     basis.  Therefore,  despite uncertainties, 55 working level months
     exposure will be assumed to double the risk of bronchial cancer
     for members of the general population.

          The general population doubling exposure of 55 WLM lies within
     the limits of 250 WLM (40) in United States uranium miners and 29
     WLM (41) in non-uranium miners which can be obtained in other
     studies although the uncertainties in risk estimation below 100
     WLM is very high (41).  This general population risk estimate is
     still likely to be low by a factor of 2 to 4 because of other
     significant differences in exposure conditions between members of
     the general population and the uranium mining population (42) which
     have not been adequately assessed.  The potential risk is assumed
     to vary in direct proportion to exposure to radon daughter products
     in units of WLM.  Additional discussion is included in Reference (42)

          The individual living in the immediate vicinity of the average
     inactive tailings pile  (50 meters) is estimated to receive a poten-
     tial dose equivalent of 8 rem/yr to the bronchial epithelium.  This

     dose equivalent is assumed to be equal to an exposure to 0.5
     WLM/yr (38) of radon daughters.  It would, therefore, require
     approximately 100 years of exposure to radon daughters at the
     rate of 0.5 WLM/yr to double the individual's risk of bronchial
     cancer.  This is the same as a 1 percent increase in potential
     risk for each year of exposure or a 70 percent increase in
     potential risk over a lifetime of 70 years.   Also, 0.5 WLM/yr
     is approximately 1/10 of the radiation guidance for the protec-
     tion of underground uranium miners issued by the Environmental
     Protection Agency (43).   Individuals similarly exposed to 0.3
     rem/yr at 1 km and 0.1 rem/yr at 2.2 km incur an increased
     bronchial cancer risk of about 3 percent and 1 percent respec-
     tively for a lifetime of exposure to an average inactive uranium
     mill tailings pile.

6.0  Conclusions

          Atmospheric concentrations of radon-222,  thorium-230,  and
     radium-226, and direct gamma radiation levels from inactive
     uranium tailings piles have been estimated,  and the potential
     radiation doses to individuals in the general population were
     presented in section 4.   These calculations  indicate that indi-
     viduals may be receiving significant doses to the lung out  to
     distances of approximately 1 kilometer from  the tailings sites
     from radon-222 and its daughter products,  and that this is  the
     most significant long-term exposure pathway.   Since some of
     these sites are located  in or near urban areas,  large popula-
     tion groups may be receiving these lung doses,  thus increasing
     the health risks.   Stabilization of piles with compacted earth
     may not be especially effective in reducing  the radon emission
     problem since, in theory,  2 feet of earth cover reduces the
     emission rate by only about 25 percent and about 20 feet are
     required to reduce it by more than 90 percent (6).

          Windborne particulates contining uranium,  thorium-230,
     and radium-226 which are fine enough to be transported distances
     of a kilometer or  more result in other lung  doses which are
     smaller than those from radon daughters and  which are delivered
     to different lung  tissues.   Several feet of  earth cover are
     expected to be adequate  to stabilize the pile so that wind  ero-
     sion of radioactive particulates is significantly reduced or

          The direct gamma radiation exposure rate at about 3 feet
     from the surface of an average tailings pile containing 500 pCi/g
     of radium-226 is estimated to be about 1,200 yR/hr (^ 10,000
     mrem/yr).   In theory, each foot of compacted earth cover will
     reduce this exposure rate  by a factor of 10  (6).  Therefore, 4

     feet of  earth cover should eliminate any problems associated
     with direct gamma radiation doses.   The actual reduction in
     exposure rate provided by earth cover appears to be less and
     should be confirmed by experiment.

          There is considerable need for field studies to refine the
     various  uncertainties discussed in section 5.2.  If the poten-^
     tial radiological impact of an inactive uranium mill tailings
     pile is  to be more accurately estimated, the following param-
     eters should be measured:

          a.   Radium-226 and thorium-230 content of the pile.

          b.   Radon emission rate from the surface of the pile.

          c.   Average radon-222 and decay product concentration,
     the concentration of ambient aerosol particles, particle size
     distribution, and the fraction of the total potential alpha
     energy that is uncombined with particles in the environment
     around the pile out to a distance of at least 1 kilometer.

          d.   Radon-222 and daughter product concentrations and
     the fraction of the total potential alpha energy that is
     uncombined with particles inside a selected number of
     dwellings around the tailing pile.

          e.   Direct gamma radiation levels as a function of
     distance from the tailings pile.

          f.   Airborne levels of thorium-230 and radium-226 around
     the pile as a function of distance, both indoors and outdoors,
     together with the determination of the particle size distribu-

          g.   Solubility class of the radionuclide bearing particles
     and their specific activity.

          Finally, there is a need to refine dose conversion factors
     for radon-222 and its decay products, and for thorium-230 and
     radium-226, under those conditions typical of uranium mill
     tailings pile sites.

          In order to completely assess the environmental impact of
     a tailings pile, the total health effects to the population at
     risk would have to be considered on a national and worldwide
     basis.  This will be the subject of future investigations by the
     Environmental Protection Agency.

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36.  IDSO, S.B.  Tornado or Dust Devil:  The Enigma of Desert Whirlwinds,
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