Evaluation of
Skin and Ingestion
Exposure Pathways
Rosanne Aaberg
Pacific Northwest Laboratory
Richland, Washington
Joe E. Logsdon
Project Officer
Office of Radiation Programs
U.S. Environmental Protection Agency
Washington, DC 20460
1989
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FOREWORD
After a nuclear accident when there has been a release of radionuclides
into the atmosphere with consequential deposition on the ground, decisions are
necessary on whether protective action guides should be implemented. In order
to do this, several pathways for radiation exposure must be evaluated to
determine the projected dose to individuals.
The objective of this study, conducted by Pacific Northwest Laboratories
for the U.S. Environmental Protection Agency, is to provide background
information on exposure pathways for use in the development of Protective
Action Guides. The relative importance of three exposure pathways that are
usually considered to be unimportant compared to other pathways expected to
control relocation decisions following a nuclear power plant accident is
evaluated. The three pathways are the skin dose from contact with
radionuclides transferred from the ground, the skin dose from radionuclides on
the ground surface, and ingestion of radionuclides transferred directly to the
mouth from the hands or other contaminated surfaces. Ingestion of
contaminated food is not included in this evaluation, except for situations
where the food is contaminated as a result of actions by the person who
consumes the food (e.g., transfer of contamination from hands to food).
Estimates of skin and ingestion doses are based on a source term with a
radionuclide mix predicted for an SST2-type nuclear accident in an area where
the first year reference whole-body dose equivalent from whole body external
exposure to gamma radiation plus the committed effective dose equivalent from
inhalation of resuspended radionuclides is 1 rem.
Appendixes have been included to allow the reader to examine dose factor
calculations, source-term data, and quantification of contact and ingestion
parameters in more detail.
Washington, DC
Richard J. Guiftiond, Director
Office of Raanation Programs
Lf
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CONTENTS
Page
1. Introduction 1
2. Summary 2
2.1 Contact Pathway Dose .'.'.'.* 4
2.2 Surface Pathway 4
2.3 Ingest ion Pathway 5
3. Methods and Assumptions 5
3.1 SST2 Source Term 5
3.1.1 Source Term Averaging 6
3.1.2 Weathering 7
3.2 Dose Factors 8
3.2.1 Skin Contact 8
3.2.2 External g
3.2.3 Ingestion 9
3.3 Exposure Assumptions 9
3.3.1 Skin Contact 10
3.3.2 External 11
3.3.3 Ingestion 12
4. Results 13
5. Discussion . 14
6. References 17
Appendixes
A. Dose Factor Calculations and Data 19
B. Source-Term Data 25
C. Quantification of Contact and Ingestion Parameters ... 29
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EVALUATION OF SKIN AND INGESTION EXPOSURE PATHWAYS ,
1. INTRODUCTION . .
Proposed Protective Action Guides (PAGs) for relocation are based on
the whole-body dose equivalent from 1-yr external exposure to gamma
radiation plus the committed effective dose equivalent from inhalation of
resuspended airborne radioactive materials. The guides also,include dose
equivalent limits to the skin from 1-yr beta exposure. This study was ;
conducted by Pacific Northwest Laboratory (PNL) for the U.S. Environmental
Protection Agency (EPA) to determine the relative importance of three
additional exposure pathways that are usually considered to be unimportant
compared to other pathways expected to control relocation decisions
following a nuclear power plant accident. The dose equivalent from each
of the following exposure pathways was evaluated and compared, with the
total dose from the external gamma and inhalation exposure pathways:
- '-. ' ' ' '
Contact - from beta-emitting radionuclides transferred from the
ground to the skin;
Surface (external) - from the beta emitters deposited,on the ground;
and .....,-
Ingestion - from the ingestion of radionuclides transferred directly
to the mouth from the hands or other contaminated surfaces.
Ingestion of contaminated food is not included in this study.
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The dose calculations are based on the assumption of no protective actions
beyond normal bathing, clothing, and partial occupancy in low or
noncontaminated areas.
2. SUMMARY
The estimated dose equivalent to the skin from beta radiation and the
committed effective dose equivalent from ingestion for the maximally exposed
individual are given in Table 1. The estimated dose to the skin includes
contributions from both contact and external sources; the committed effective
dose equivalent from ingestion results from transfer of hand contamination to
the mouth.
The doses in Table 1 are from the first-year exposure following an SST2-
type reactor accident (Aldrich et al. 1983) of an individual who resides where
the projected effective dose from external gamma radiation plus the committed
effective dose from inhalation of resuspended materials is 1 rem . That is,
the doses are based on concentrations of radionuclides deposited per unit area
on the ground that would yield a reference whole-body dose rate of 1 rem from
first-year exposure, as calculated with the CRAC2 computer code (Ritchie et
al. 1984). A first-year average source term, calculated with the radioactive
decay and the weathering functions from WASH-1400, is used in CRAC2. For this
study, the contamination layer is assumed to be incorporated in a 1-mm-thick
surface layer of soil, or a 0.1-mm-thick layer of dust associated with a paved
surface.
Contact and surface (external) beta doses are computed as rem to
radiosensitive tissues at a depth of 70 pro (ICRP 1977). This depth was
selected to correspond to assumptions used in calculation of doses supporting
the selection of PAGs for skin.
The integrated dose considering radioactive decay and weathering
according to the WASH-1400 model (using the CRAC2 computer code)
normalized to 1 rem for the first-year exposure is herein referred to
as the "reference whole-body dose." Gamma exposure dominates this dose.
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TABLE 1. Dose for the First-Year Exposure to Maximally Exposed(a)
Individuals Residing in the 1-REM/Year "Reference
Whole-Body Dose" Zone Following an SST2-Type Reactor
Accident
Individual Exposure Pathway
Adult
Contact (skin) (d)
Surface (.skin) (e)
TOTAL SKIN
Ingestion (e)
Child
Contact (skin)
Surface (skin)
TOTAL SKIN
Ingestion
First-Year
Soil
0.7
1.7
TA
0.01
0.7
7.4
la
0.05
Dose (rem)(b)
Pavement(c)
6.7
1.7
874
0.1
6.7
7.4
TT~
0.5
(a) See Table 2 for average individuals.
(b) Skin dose is expressed as dose equivalent from the first-year
exposure and ingestion dose is expressed as the 50-yr committed
effective dose equivalent from the first-year exposure.
(c) Surface mixing: 1 mm for soil (160 mg/cm2), 0.1 mm for pavement
(16 mg/cm2) .
(d) Contact assumed: contamination residing on the skin 4380 h/yr
equivalent to 1.8 mg/cm2, or 1.1 percent of ground concentration
for soil, 11 percent of ground concentration for pavement.
(e) External (surface) dose based on 4380 h/yr at elevations of 1m
for adults and 30 cm for children.
(f) Based on ingestion of 100 mg/d for the adults; 500 mg/d for
children.
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2.1 CONTACT PATHWAY DOSE
Skin contact doses in Table 1 were calculated for maximally exposed
individuals who spend much time outdoors, work in soil, and bathe
infrequently. The critical group includes children playing in a yard or
playground. Contact doses depend on the deposition of contaminated
materials on the skin and duration of exposure to the skin, which in turn
depends on bathing practices. The maximally exposed individual is
o
assumed to have a dirt layer of 1.8 mg/cm on the skin (1 to 11 percent
of the ground surface concentration). This is based on a 50-ym layer of
dust or dirt on the skin (Hawley 1985). Dose factors for contact doses
are based on a skin depth of 70 urn.
For a given concentration of contaminants on the skin, the dose that
is calculated is the same regardless of whether a large or small area is
exposed. Although the skin is considered an organ, the dose to the skin
is not averaged over the entire skin. The dose of interest is to the
particular area of skin that receives irradiation from direct contact
with contamination. The dose is calculated to unclothed skin only
because this reflects the maximum dose rate. Contamination is assumed to
be in contact with the skin for 4380 h (from cycles of contamination,
bathing, and recontamination) for the first year after deposition of
contaminants.
2.2 SURFACE PATHWAY
Doses resulting from the exposure of ground-deposited beta emitters
are dependent on the amount of time the individual spends outdoors in a
contaminated area. The maximally exposed individual is assumed to spend
8 h/d outdoors in the contaminated area. The first-year dose to the skin
from external exposure to beta emitters 1 m above the contaminated ground
is conservatively calculated to be 0.7 rem/yr, based on CRAC2 weathering,
which is very conservative for beta exposure. The dose factors for
external exposure are based on a skin depth of 70 pm, as are contact dose
factors.
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Dose factors at different distances are highly dependent on the beta
spectra. A reference height of 30 cm above contaminated ground is used
to estimate the potential dose to children. For the mix of radionuclides
in the SST2 source term, the dose at 30 cm would be approximately four
times greater than that at 1 m. The first-year beta dose at 30 cm
corresponds to about 2 to 3 rem to the skin with no credit for shielding
provided by clothing.
2.3 INGESTION PATHWAY
Ingestion doses evaluated here are only those resulting from
exposure to contaminated surfaces and from poor hand-washing practices.
For the ingestion pathway, the maximally exposed individual is a person
with contamination on the hands who does not wash his/ her hands before
eating. The critical group includes children playing outdoors and then
not washing their hands before eating, or small children who put their
hands or other objects in their mouths while playing in a contaminated
area. The ingestion pathway may potentially be a significant route. The
effective dose equivalent from the first-year intake by the maximally
exposed child may equal the 1 rem/yr of the reference whole-body dose.
Maximally exposed adults; include individuals who engage in frequent
hand-to-mouth activity (smokers) and also engage in outdoor activities,
such as construction workers or gardeners.
3. METHODS AND ASSUMPTIONS
3.1 SST2 SOURCE TERM
The source term used for this task is based on the initial mix of
radionuclides predicted for an SST2-type reactor accident in sufficient
quantity to produce a 1-rem first-year reference whole-body dose.
In-growth of daughters as we'll as physical decay and weathering
corrections are applied to the source term to yield first-year average
radionuclide concentrations on surfaces. Doses from individual
radionuclides are calculated and summed.
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3.1.1 Source-Term Averaging
The first-year average ground concentration (C.) for each radionuclide
is determined as follows:
C, = C01 (1.-
. t
2
where CQ = initial ground concentration (Ci/m ) of each radionuclide
i = individual radionuclide
x = effective decay constant (yr~ )
(In 2/physical half-life) + (In 2/weathering half-time)
t = time period (1 yr).
The contribution from daughter nuclides using simple decay chains is
calculated as follows:
r- - r-
U-, - U-,
(1 -
) (1 -
Xjt
where Ci
average ground concentration of daughter radionuclide
i, including contribution from parent radionuclide j
C. = initial concentration for the parent radionuclide
0
xi = decay constant of radionuclide i.
Source-term and chain-decay data used in the calculations are given in
Appendix B, Source-Term Data.
The value of C is equal to the surface concentration of nuclides
comprising the SST2 source term whose sum produces a 1-rem/yr reference
whole-body dose from the gamma exposure pathway, plus the inhalation
exposure pathway, as calculated with the CRAC2 computer code (Ritchie et al,
1984). The reference dose is mostly from external exposure to
gamma-emitting radionuclides that are on the ground; inhalation contributes
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only a small fraction of the total. The relative quantity of each
isotopeused to calculate skin and ingestion doses is directly proportional
to the SST2 particulate material source term.
3.1.2 Weathering
The skin and ingestion doses are evaluated using the weathering model
from WASH-1400 (USNRC 1975) that is used in the CRAC2 computer code (Ritchie
et al. 1984).^ The basis for this weathering model is gamma dose rates from
soils with Cs contamination distributed on the surface. Weathering of
paved areas such as sidewalks, driveways, and streets may be described by
the WASH-1400 model; the model has been confirmed to some 'degree'by field
studies (Warming 1982, 1984). -., -
This weathering model introduces extra conservatism :into the'
calculation of beta dose rates. Dispersion of gamma emitters into a thin
layer of soil would have a negligible effect on external exposure from gamma
radiation, but it would serve as a barrier to beta emitters. For example,
the range of a beta particle with a maximum energy of 1 MeV is about 3 mm in
soil, indicating that most of the beta energy will be attenuated by a soil
cover of a few millimeters. The dose rate from gamma radiation, however, is
attenuated to 10 percent of its original value only with a thickness of
about 30 cm of soil, or 100 times the range of a 1 MeV beta particle.
Assumptions concerning mixing of the surface layer, which are related
to weathering, lead to important consequences in the calculation of contact
doses. There is an.effective 1600-fold concentration difference between
contamination existing in surface dust layer of 10 g/m2 and contamination
dispersed in the top 1 cm of soil. Contamination that is not at the surface
would still be available for direct skin contact during gardening or'field
work, but at a concentration that has been reduced by dilution with soil.
For this study, the contamination layer is assumed to be incorporated in a
1-mm-thick (1600 g/m2) surface layer of soil, or a 0.1-mm-thick
(160 g/m ) layer of dust associated with a paved surface. The conversion
from mass to area is discussed in Appendix.C.
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Since leaching with rainwater could make a large difference in
surface concentrations of contaminants, the WASH-1400 weathering model
applied to beta emitters may best describe the surface conditions in a
relatively dry, unirrigated area. In the studies by Warming,
precipitation was sufficient to produce runoff only 3 percent of the time
in which precipitation occurred. The dose reduction due to weathering
could be greater in areas with heavier precipitation.
Experimental evidence shows that hosing of paved surfaces such as
sidewalks, driveways, and streets decreases the (gamma) dose rate by only
15 to 25 percent (Warming 1982, 1984). In these experiments, old asphalt
surfaces showed no significant weathering. It may be extremely
conservative to apply dose rates based on gamma emitters to contact beta
dose or ingestion because experiments show a marked decrease with time for
the ability to decontaminate (transfer contamination from) these surfaces.
3.2 DOSE FACTORS
Dose factors for skin contact found in the literature included
photons and electrons, and were not in the form required for this report.
Therefore, contact dose factors were developed for this task. Dose
factors for external exposure from beta radiation at 1 m were taken from
the literature (Kocher 1981b). VARSKIN, a PNL-developed computer code
(Traub et al. 1987), was used to estimate the ratio of dose factors at
different distances above the ground and to provide factors for
calculating doses at 30 cm above contaminated ground (see Appendix A for
dose-factor data). Skin dose is expressed as dose equivalent, and
ingestion dose is expressed as committed effective dose equivalent. The
exposure period for both is one year. Dose factors for ingestion are
based on ICRP 26 and ICRP 30 models (ICRP 1977, 1979).
3.2.1 Skin Contact
Radioactive material deposited directly on body surfaces is
considered to be on an infinite, thin plane. This is valid for very thin,
curved surface with a radius of curvature greater than the maximum
8
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beta-particle range. Skin contact dose factors for a skin thickness of
70 wm were computed using the Loevinger, Japha, and Brownell (1956)
solution to the thin-plane problem. The dose equation was solved for each
beta and electron energy level or group in the spectrum for each
radionuclide of interest. Healy (1971) used the same formula to calculate
allowable contamination levels, but used only the principal beta energies
and included photon dose in the results. Considering these differences
our calculated dose factors compared reasonably well with those by Healy.
Beta and electron spectra used in computations are those given by
Kocher (1981a). A low-energy cutoff of 100 keV (maximum energy) for beta
particles and 60 keV for electrons was assumed. Mono-energetic electrons
were treated as betas for this analysis. Additional information,
including equations and beta spectrum data used in SKINDOSE (developed for
this study) to calculate the dose factors, is given in Appendix A.
3.2.2 External
External dose equivalent to skin from nearby beta radiation-sources
was calculated for radiosensitive tissue at a single depth of 70 ym (ICRP
1977). Dose factors are from Kocher (19815), and estimates of dose
factors based on distance,of 1 ft above a surface are given in Appendix A,
Table A.2.
3.2.3 Ingestion .
Committed effective dose equivalents for ingestion are based on the
concepts of ICRP 26 and ICRP 30 (USEPA 1988). The dose factors for
ingestion are presented in Appendix A, Table A.I. Where, two values of
dose factors (committed effective dose equivalent) based on solubility
were presented, the larger value was used in this analysis.
3.3 EXPOSURE ASSUMPTIONS
Dispersion of contaminants in soil and transfer of contaminants to
skin and to the mouth are key parts of dose estimation.
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3.3.1 Skin Contact
The dose resulting from contamination on the skin is a function of
the concentration and duration of contaminants on skin. The concentration
of contamination on the skin is estimated based on concentration of
contaminants present on the ground over the first year after contamination
and the amount of dust or dirt residing on the skin. For this study, the
contamination layer is assumed to be incorporated in a surface layer of
soil 1 mm thick (1600 g/m2) or a layer of dust 0.1 mm thick (160 g/m2)
associated with a paved surface. Justification for the mixing layers is
provided in Appendix C.
The maximally exposed individual is assumed to have a dirt layer of
2
1.8 mg/cm on the skin (1 to 11 percent of the thickness of the
contaminant ground layer). The average individual is assumed to have
2 . .
1.0 mg of contaminated dust per cm of skin (0.6 to 6 percent of the
thickness of the contaminated layer). These estimates are based on
interpretations of experiments involving contamination on skin surfaces
(Hawley 1985, Shaum 1984). Conversion from mass to area is discussed in
Appendix C.
The transfer of contaminants to the skin is assumed to be
proportional to the concentration of contaminants on the ground. Because
the contamination on the ground is assumed to be mixed in a 160- to
2 2
1600-g/m dust layer, a dirt loading on the skin of 16 to 160 mg/cm
would be required to be equivalent to 100 percent of the concentration
present on the ground.
Although weathering is considered in the radionuclide source term,
the weathering model is conservative for beta emitters. Although dilution
of contaminants on the ground surface (mixing contamination into a thin
layer of soil) would have negligible effects on external exposure from
gamma emitters, it could decrease the dose from beta emitters by a
significant amount.
Despite the fact that most contamination would be on the hands and
arms, other areas of the body may also be affected (e.g., children with
10
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legs uncovered sitting on a contaminated surface). Skin contact also
applies to surfaces of the feet. Although contact may be considerable
for someone with bare feet, the skin of the feet is 5 to 10 times thicker
than skin on other areas of the body (Whitton 1973). Thus, beta doses to
sensitive tissues of the feet will be considerably less than that
estimated using a 70-um depth.
The dose equivalent for a maximally exposed individual is calculated
using the assumption that the affected skin area is contaminated for
43.80 h (half the number of hours in a year) in the year following the
contamination event. The average individual is assumed to have
contamination on a portion of the skin area for 800 h/yr. These time
periods, which are assigned arbitrarily, include many recontamination
events in the course of a year.
3.3.2 External
External exposure is; based on the number of hours per year the
individual is exposed to the contaminated surface (outdoors). Dose
factors for external exposure of the skin (beta radiation only) are based
o
on a height of 1 m above the contaminated surface (Kocher 1981b).
Dose factors are greater closer to the ground. Factors for 30 cm above a
contaminated surface were estimated using VARSKIN (Traub et al. 1987). A
ratio of dose factors (which depends on the energy of the particle) was
calculated by representing 30 cm and 1 m of air, plus tissue thickness of
0.007 cm with the equivalent thickness of unit density. Dose factors
from Kocher (1981b) were multiplied by this ratio to yield an estimate of
dose factors at 30 cm. To be conservative, no credit was taken for the
protection afforded by clothing.
The maximally exposed individual is assumed to be exposed to
external beta radiation for 4380 h/yr. This is equivalent to an
occupational exposure for someone who works outdoors in the
One meter is the standard height given in dose factor tables. Dose
factors from beta radiation are functions of height above the ground
and beta energy. For a. height of 1 m in air, the minimum electron
energy giving a non-zero dose-rate factor is about 320 keV; at ground
level (0.01 m) the cutoff is about 75 keV (Kocher 1981b).
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contaminated area 12 h/d, year-round. An average individual is assumed to
spend 2080 h/yr outdoors, or 40 h/wk. The exposure times for the
maximally exposed and average child are assumed to be equivalent to those
for adults.
3.3.3 Ingestion
Ingestion of radioactive surface contamination can occur when
radiocontaminants are transferred to the mouth via hands or foodstuffs.
Recent developments in estimating the ingestion of soil and dust (LaGoy
1987) are used to estimate doses to individuals. A discussion of
assumptions involved in ingestion calculations is given in Appendix C.
Because the source term for this study is given in terms of curies
per unit area, and previous studies involving radiocontaminants have used
concentrations per unit mass, ingestion of contaminants and contact with
contaminants are expressed in terms of surface area as well as unit mass.
For assessment of contact and ingestion doses by mass rather than
2
effective surface area, a conversion factor of 160 to 1600 g/m is used
to describe the extent of the contaminated layer. Relationships between
mass and surface area contamination for this assumption are discussed in
detail in Appendix C.
Individuals are assumed to ingest radiocontaminants proportional to
that found on a given surface area. The amounts are based on ingestion of
contaminated soil and are compared with results from methods that were
developed for occupational exposure to radiocontaminants. Data from LaGoy
(1987) suggest ingestion rates of lOO.mg/d for maximally exposed adults
and 25 mg/d for the average adult who does not participate in much outdoor
activity and does not smoke. Ingestion rates for children are taken as
500 mg for the maximally exposed child and 100 mg/d for the average
child. Since the units of the SST2 source term are in Ci/unit area, the
ingestion quantity must be converted from grams per day. This conversion
makes the source term units compatible with the units of ingestion. These
ingestion rates are equivalent to the total contaminated dust from 1.5 to
? 2
6.25 cm /d (based on pavement dust) for adults and 6.25 to 31 cm /d
for children (see Appendix C).
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The rate at which contamination is picked up from surfaces and
ingested, given by Dunster (1962) and Gibson and Wrixon (1979), is
2
10 cm /d for occupational exposure (8 h/d). For exposure to the
public, this value is multiplied by 16/8 (to correct to 16 h/d of
exposure) to yield 20 cm'"/d. These ingestion rates correspond
reasonably well with mass ingestion rates from LaGoy (1987) based on a
thin dust layer on a paved surface.
Children old enough to play outdoors, but young enough not to have
acquired good personal hygiene practices, are the critical group because
ingestion of contaminated soil is estimated to be higher than that for
the maximally exposed (adult) individual. According to LaGoy (1987), the
average child of 1 to 6 yrs of age may ingest 100 mg of dust or soil per
day, and the maximally exposed child of that age (excluding those with
habitual pica) may ingest 500 mg of dust per day. Age-specific dose
factors are not used in this analysis and are beyond the scope of this
report.
4. RESULTS
Dose equivalents, based on residency in an area within the 1-rem
first-year reference whole-body dose zone for the maximally exposed and
average individuals, are summarized in Table 2. The radionuclide source
term for exposures is a first-year average concentration based on the
WASH-1400 weathering model. The two columns in Table 2 correspond to
exposures based on soil (contamination mixed with the top 1 mm of soil)
and pavement (contamination mixed with the top 0.1 mm of surface dust).
Based on the assumed conditions, the ingestion of surface
contamination is a potentially significant exposure pathway. For the
maximally exposed individual, ingestion may account for up to 10 percent
of the reference whole-body dose for an adult or 50 percent for a child.
In order of importance, the major contributors to effective dose
equivalent from ingestion during the first year include 137Cs
(33 percent), 134Cs (31 percent), 132Te - 132I (6 percent), 131I
(8 percent), and Ce (7 percent). For the SST2 source term, these
isotopes account for about 85 percent of the ingestion dose equivalent.
13
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Contact doses are for exposure to the skin and are not directly
132 132
comparable to the reference whole-body dose. The isotopes Te- I
(28 percent), 137Cs (15 percent), 144Ce (14 percent), and 134Cs
(7 percent) account for the major part of the dose to skin by direct
contact for the first year after SST2 contamination. Doses to the average
individual (based on 1.0 mg/cm2 on skin for 800 h) are about 10 percent
of those for the maximally exposed individual. The average individual
would also have a smaller portion of contaminated skin area.
Exposure to beta emitters at 1 m for adults or 30 cm for children
above a contaminated surface accounts for a small increment to the total
dose. The surface (external) beta dose is equal to about a tenth of the
dose from direct contact for the maximally exposed adult but about twice
the contact amount for the average child. Estimates of external exposure
to beta radiation depend only on the number of hours spent outdoors and
not on personal cleanliness or other factors. The estimates of external
beta dose are conservative; actually clothing would provide some
protection from beta radiation. To be conservative, no credit is taken
for shielding provided by clothing. The dominant contributor to external
10p 1 Op
beta dose at 1 m is I (daughter of Te), which accounts for about
91
55 percent of the dose. Other contributors to external dose are Y
(17 percent) and 129MTe (9 percent). The dose estimated at 30 cm is
dominated by 132I (39 percent), 134Cs (17 percent), 91Y(10 percent),
and 127Te (9 percent). A larger proportion of the dose is from
radionucTides with softer betas.
5. DISCUSSION
Table 2 shows that dose equivalent to skin from beta emitters and
committed effective dose equivalent from ingestion via contaminated hands
may be significant compared with inhalation dose plus external dose from
gamma radiation.
The contact dose is closely related to weathering of the surface
layer of contaminants. Assumptions related to weathering have important
consequences in the calculation of contact doses. An effective 1600-fold
14
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TABLE 2. Dose for the Maximum and Average Individual Residing in the
1 REM/Year "Reference Whole-Body Dose" Zone Following an
SST2-Type Reactor Accident
Individual
Maximum
Adult
Average
Adult
Exposure Pathway
Contact (skin) (c)
Surface (skin) (d)
TOTAL SKIN
Ingestion (e)
Contact (skin) (c)
Surface (skin) (d)
First-Year
Soil
0.7
1.7
2.4
0.01
0.06
0.7
Dose (rem)(a)
Pavement(b)
6.7
1.7
0.1
0.6
0.7
TOTAL SKIN
Ingestion (e)
0.002
0.02
Maximum
Child
Contact (skin)(c) 0.7
Surface (at 30 cm)(d) 7.4
TOTAL SKIN 8.1
Ingestion(e) -0.05
6.7
: 7.4
14 ; .
0,5
Average
Child
Contact (skin)(c) 0.06
Surface (at 30.cm)(d) 3.2
TOTAL SKIN 3.3
Ingestion (e)
0,01
0.6
3.2
3.8
0.1
(a) Skin dose is expressed as dose equivalent from the first-year
exposure and ingestion dose is expressed as the 50-yr committed
effective dose equivalent from the first-year exposure.
(b) Surface mixing: 1 mm for soil (160 mg/cm^), 0.1 mm for pavement
(16 mg/cm2).
(c) Contact assumed: contamination residing on the skin 4380 h/yr
equivalerit to 1.8 mg/cm^, for maximally exposed, 800 h/yr and
1.0 mg/cm2 for average individuals.
(d) External (surface) dose based on 4380 h/yr for maximally exposed,
2080 for average individuals. 1 m for adult, and 30 cm for child.
(e) Based on ingestion of 100 mg/d for the maximally exposed adult,
25 mg/d for average adult; 500 mg/d for the maximally exposed child,
100 mg/d for average child.
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concentration difference between contamination in the surface dust layer
2
of 10 g/m and contamination dispersed in the top 1 cm of soil exist).
For this study, the contamination layer is assumed to be incorporated in
2
a 100 g/m surface layer of dust and soil. (Conversion from mass to
area is discussed in Appendix C).
A depth of 70 \im is used for the calculation of skin dose (for both
contact and external components) to correspond to assumptions used in
dose calculations supporting the selection of PAGs for skin. A value of
40 ym has been suggested as the appropriate depth of the radiosensitive
layer of skin (Whitton 1973). The use of dose factors calculated at the
40-ym depth would increase the resulting dose by about 40 percent.
Although the skin of the hands is likely to come in contact with surface
contamination, the skin thickness is greater, reducing the potential
damage to the sensitive layer.
Absorption of contaminants through the skin is another potential
pathway for internal exposure. A study of radioiodine determined an
absorption rate for iodine through skin of 0.008 percent h/cm
(Harrison 1963). For the maximally exposed individual, assuming
2
3000 cm of skin is contaminated for 3000 h, this would correspond to
about 3.5 percent of the intake of iodine by ingestion of contaminants
transferred from contaminated surfaces. This includes the assumption
that the particulate iodine in the contaminants are absorbed as well as
the aqueous solution used in the experimental procedure. A matrix of
soil rather than solvent can affect the absorption by skin; experiments
using TCDD (dioxin) showed that the soil matrix reduced the amount
absorbed by 85 percent (Hawley 1985). The soil matrix may have a similar
effect on absorption of iodine.
Many variables may affect doses to residents of a contaminated
area. In some areas, the time of year an accident occurs could have a
large effect. For example, residents are much more likely to come in
contact with dirt in spring or summer. Also, the mix of radionuclides
will change with time as the short-lived radionuclides decay.
16
-------
Assumptions about the transfer from surfaces to skin and the
residence time on skin are somewhat arbitrary and might be modified if
additional data become available in the future.
REFERENCES
ALDRICH, D. C., J. L. Sprung, D. J. Alpert, K. Diegert, R. M. Ostmeyer,
L. T. Ritchie, and D. R. Strip. Technical Guidance for Siting Criteria
Development. NUREG/CR-2239, SAND81-1549, prepared by Sandia National
Laboratories for the U.S. Nuclear Regulatory Commission, Washington,
DC, 1983.
DUNSTER, H. 0. Maximum Permissible Levels of Skin Contamination.
AHSB(RP)R28, United Kingdom Atomic Energy Authority, Authority Health
and Safety Branch Report, Harwell, Didcot, Berkshire, England, 1962.
GIBSON, J. A. B., and A. D. Wrixon. "Methods for the Calculation
of Derived Working Limits for Surface Contamination by Low-Toxicity
Radionuclides." Health Physics 36(3):311-321. 1979.
HARRISON, J. "The Fate of Radioiodine Applied to Human Skin."
Health Physics 9:993-1000, 1963.
HAWLEY, J. K. "Assessment of Health Risk from Exposure to
Contaminated Soil." Risk Analysis 5(4):289-302, 1985.
HEALY, J. W. Surface Contamination: Decision Levels. LA-4558-MS,
Los Alamos Scientific Laboratory, Los Alamos, New Mexico, 1971.
INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION (ICRP). Recom-
mendations of th£'International Commission on Radiological Protection,
ICRP Publication 26, Pergamon Press, New York, 1977.
INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION (ICRP).
Limits for Intakes of Radionuclides by Workers, ICRP Publication 30,
Pergamon Press, New York, ,1979.
KOCHER, D. C. Radioactive Decay Data Tables. DOE/TIC-11026, U.S.
Department of Energy, Washington, DC, 1981a.
KOCHER, D. C. Dose-Rate Conversion Factors for External Exposure
to Photons and Electrons. NUREG/CR-1918, prepared by Oak Ridge
National Laboratory for the U.S. Nuclear Regulatory Commission,
Washington, DC, 1981b.
LAGOY, P. K. "Estimated Soil Ingestion Rates for Use in Risk
Assessment". Risk Analysis 7(3):355-359, 1987.
17
-------
LOEVINGER, R., E. M. Japha, and G. L. Brownell. "Discrete
Radioisotopes Sources," Chapter 16. In Radiation Dosimetry, eds. G. H.
Hine and G. L. Brownell, pp. 693-799, Academic Press, Inc., New York,
1956.
RITCHIE, L. T., D. J. Alpert, R. P. Burke, J. D. Johnson, R. M. Ostmeyer,
D. C. Aldrich, and R. M. Blond. CRAC2 Model Description.
NUREG/CR-2552, SAND82-0342, prepared by Sandia National Laboratories,
Albuquerque, New Mexico and Livermore, California for the U.S. Nuclear
Regulatory Commission, Washington, DC, 1984.
SCHAUM, T. Risk Analysis of TcDD Contaminated Soil. Prepared for
EPA Office of Health and Environmental Assessment, Washington, DC, 1984.
TRAUB, R. J., W. D. Reece, R. I. Scherpelz, and L. A. Sigalla.
Dose Calculation for Contamination of the Skin Using the Computer Code
VARSKIN. NUREG/CR-4418, PNL-5610. prepared by Pacific Northwest
Laboratory for U.S. Nuclear Regulatory Commission, Washington, DC, 1987.
U.S. ENVIRONMENTAL PROTECTION AGENCY. Limiting Values of Radionuclide
Intake and Air Concentration and Dose Conversion Factors for
Inhalation, Submersion, and Ingestion. EPA-520/1-88-020. U.S.
Environmental Protection Agency, Office of Radiation Programs, 1988.
U.S. NUCLEAR REGULATORY COMMISSION. Reactor Safety Study: An Assessment
of Accident Risks in U.S. Commercial Nuclear Power Plants. Appendix
G: Calculation of Reactor Accident Consequences.WASH-1400 (NUREG
75/014).U.S. Nuclear Regulatory Commission, Office of Nuclear
Research, Washington, D.C. 1975.
WARMING, L. Weathering and Decontamination of Radioactivity
Deposited on Asphalt Surfaces. RISO-M-2273, Riso National Laboratory,
DK-4000 Roskilde, Denmark, 1982.
WARMING, L. Weathering and Decontamination of Radioactivity
Deposited on Concrete Surfaces.RISO-M-2473, Riso National Laboratory,
DK-4000, Roskilde, Denmark, 1984.
WHITTON, J. T.
Importance."
"New Values for Epidermal Thickness and Their
Health Physics 24:1-8, 1973.
18
-------
APPENDIX A
DOSE FACTOR CALCULATIONS AND DATA
A.I COMPUTATION OF SKIN DOSE FACTORS
Skin contact dose factors were calculated with the computer code
SKINDOSE, developed for this task. An equation presented by Loevinger,
Japha, and Browne!! (1956) was used to estimate doses resulting from
radioactive materials deposited directly on body surfaces. The equation
is a function of the distance from a thin-plane source:
D(x) = (1.07)v(Eavg)oa{c[(l + ln(c/vx) - e(1 ~ vx/c)] + e(1 ~ vx)}
given [ ] = 0 for x > c/v,
where D(x)
V
avg
a
j
= dose rate (rad/h) at distance x (gm/cm )
2
= beta absorption coefficient (cm /g)
= average beta energy (MeV)
2
= surface activity (yCi/cm )
a and c = functions of the maximum beta energy (E ):
max
max
0.17 - 0.5
0.5 - 1.5
1.5 - 3.0
0.260
0.297
0.333
2.0
1.5
1.0
19
-------
Skin depth x is assumed to be 70 ym which equates to
2 ^
7.8 E-3 gm/cm for a skin density of 1.12 gm/cm . The equation is
solved for D(x) for unit activity for each radionuclide of interest. The
dose factor for a radionuclide is calculated from individual beta and
electron energy levels of its beta spectrum weighted by frequency of
occurrence. Dose factors for the components of the spectrum are summed
giving a dose factor for the radionuclide.
A depth of 70 urn is used to calculate skin dose (for both contact
and external components) to correspond to assumptions used in dose
calculations supporting the selection of PAGs for skin. The use of a
different depth would change the dose factors.
Beta energy levels and some electron energies are grouped to
simplify calculations. Low-energy cutoffs were made for electrons that
cannot penetrate through 70 urn of skin. In addition, contributions from
short-lived (one hr or less) daughter decay products are included in dose
factors for the parent radionuclides.
Dose conversion factors for each of the isotopes in the source term
are given in Table A.I. This table gives dose factors for skin contact,
external exposure (from 1 m and 30 cm; beta only), and ingestion. Dose
factors for skin contact are from SKINDOSE, for external exposure at 1 m
are from Kocher (1981b), and for external exposure at 30 cm are
calculated using VARSKIN. The ingestion dose based on ICRP models is the
50-yr committed effective dose equivalent per unit of activity ingested
(USEPA 1988).
A.2 BETA SPECTRA
The beta spectrum of a radionuclide may have many components. Each
beta has two parameter energies, average (E ) and maximum (E ^ ),
avg max
and a frequency (intensity) associated with this mode of decay. The two
characteristic energies, £, and Em . are used in calculating dose
avg max
factors from beta spectra. Conversion and Auger electrons, however, have
one characteristic energy rather than a range. To use the same
techniques for calculating dose factors, average and maximum energies are
20
-------
TABLE A.I. Summary of Dose Conversion Factors for Skin Contact, External
Exposure to Beta and Electron Radiation and for Ingestion
DOSE FACTORS1 aj
Nuclide
58^0
60^0
86Rb
89Sr
90Sr
90Y
91y
95Zr
Nb
Mo
99MTc
103Ru
106Ru
105Rh
127je
127MTe
129Te
129Mje
13lMye
132Te
131 1
132j
134Cs
136Cs
137Cs
140Ba
141 Ce
143Ce
143pr
147 Nd
239Np
239pu
240Pu
241 PU
241Am
242Cm
244Cm
Contact, rem/yCI
per cm /h
1.20E+00
4.30E+00
8.30E+00
9.40E+00
7.20E+00
8.50E+00
8.20E+00
5.10E+00
8.40E-01
8.30E+00
5.80E-01
2.60E+00
8.50E+00
5.60E+00
7.80E+00
7.00E+00
2.20E+00
9.20E+00
5.30E+00
7.60E+00
3.00E+00
9.40E+00
8.10E+00
5.00E+00
5.40E+00
6.90E+00
7.40E+00
8.60E+00
6.80E+00
8.40E+00
1.20E+01
7.80E+00
7.20E+00
8.70E+00
OE+00
OE+00
OE+00
OE+00
2.10E-01
OE+00
OE+00
i External, rem/pCi/m^/yr Inqestion, rem/yCi
at 1 m
3.5E-04
OE+00
8.99E-01
7.88E-01
1.61E-02
1.24E+00
8.21E-01
2.54E-03
1.75E-03
4.18E-01
5.62E-04
4.4E-03
OE+00
9.58E-03
2.34E-01
6.03E-02
2.14E-03
6.59E-01
2.83E-01
5.77E-02
OE+00
2.13E-02
5.59E-01
3.81E-02
6.25E-03
2.75E-02
1.98E-01
6.59E-01
4.55E-03
4.33E-01
OE+00
2.33E-01
1.05E-01
2.52E-03
OE+00
OE+00
OE+00
OE+00
OE+00
OE+00
OE+00
at 30 cm
8.76E-04
OE+00
2.25E+00
2.1E+00
4.03E-02
3.11E+00
2.09E+00
6.35E-03
4.37E-03
1.05E+00
1.41E-03
1.1E-02
OE+00
2.4E-02
1.37E+00
4.16E+00
5.36E-03
1.93E+00
7.2E-01
1.44E-01
OE+00
1.47E+00
1.69E+00
9.53E-02
1.56E-02
6.86E-02
4.96E-01
1.94E+00
1.14E-02
1.08E+00
OE+00
1.47E+00
2.64E-01
6.3E-03
OE+00
OE+00
OE+00
OE+00
OE+00
OE+00
OE+00 ,
ingestion
3.6E-03
2.7E-02
9.4E-03
9.3E-03
1.4E-01
1.1E-02
9.5E-03
3.8E-03
2.6E-03
5.0E-03
6.2E-05
3.0E-03
2.7E-02
1.5E-03
6.7E-03
6.9E-04
8.3E-03
2.0E-04
1.1E-02
9.1E-03
9.4E-03
5.3E-02
6.7E-04
7.3E-02
1.1E-02
5.0E-02
9.5E-03
8.4E-03
2.9E-03
4.6E-03
2.1E-02
4.7E-03
4.4E-03
3.3E-03
3.2E+00
3.5E+00
3.5E+00
6.8E-02
3.6E+00
1.1E-01
2. OE+00
(a) The contact dose factor is for beta emitters residing directly on the
skin; the external dose factor is based on irradiation from contaminated
ground. Both are based on dose at a 70-pm skin depth. The ingestion
dose factor is based on ICRP dosimetry.
21
-------
hypothesized. Beta and electron spectra used in the computations were
from Kocher (1981a). A low-energy cutoff of 100 keV (E ) for beta
ffl3X
particles and 60 keV for electrons was used. Mono-energetic electrons
are treated as betas for this assessment. Some energy levels are grouped
1 y-j
for simplicity. An example of how data from Kocher for Sb is
interpreted for determining skin dose factors is given in Table A.2.
Mono-energetic electrons were assumed to behave as if they were beta
particles, with a maximum energy (E ) corresponding to that expected
iflcLX
for a beta particle with the same average energy. This assumption led to
reasonable results, based on comparisons of tabulated mean energy emitted
per unit of accumulated activity (column 4 in Table A.2) in units of
rad/h per yCi/g. Resulting dose factors from beta particles and
electrons were compared with mean energy emitted. They correlated well.
Table A.2. Example Application Using Data for 127SB From Kocher (1981a)
as Input to Skindose
Radiation
Type
Auger-L
Auger-K
ce-K- 1
ce-L- 1
ce-K- 6
ce-K- 19
cd-K- 30
Beta-1
Beta-2
Beta-3
Beta-4
Beta-5
Beta-6
Energy
(keV)
3.19
22.7
22.29
56.16
220.6
441.19
653.4
max 258
avg 72.7
max 291
avg 82.9
max 425
avg 127.5
max 441
avg 132.8
max 504
avg 155.1
max 657
avg 211.1
Intensity Delta
(percent) (rad/hr per yCi/
3.9
0.53
3.47
0.45
0.43
0.220
0.212
0.110
0.610
0.8
1.25
5.22
1.25
0.0003
0.0003
0.0022
0.0005
0.0020
0.0021
0.0017
0.0002
0.0011
0.0022
0.0035
0.0172
0.0056
SKINDOSE Input
g) Average Maximum
Below
Below
221
441
653
81
130
504
657
Threshold
Threshold
660
1240
1740
285
435
155.1
211.1
22
-------
Kocher (1981b) assessed doses from mono-energetic electrons by a
more rigorous method. The equation for mono-energetic sources included a
term for the specific absorption fraction. The value of this quantity is
evaluated by interpolation of tabulated values obtained using Monte Carlo
techniques. For this analysis, the more rigorous approach was not
required.
A.3 DOSE FACTORS FOR EXTERNAL EXPOSURE
Dose factors used for external exposure of skin to a depth of
70 ym from electrons (betas and electrons, as distinguished from photons)
are from Kocher 19815 (given in Sv/Bq/cm2). These factors are for 1 yr
of chronic exposure of skin to beta emitters at 1 m above a contaminated
surface and are given in Table A.I.
Dose factors for a distance of 30 cm above contaminated ground were
estimated using VARSKIN (Traub et al. 1987). A ratio of dose factors was
calculated by representing 30 cm and 1 m of air plus the tissue thickness
of 0.007 cm by the equivalent thickness of unit density. Dose factors
from Kocher (1981b) were multiplied by this ratio to yield an estimate of
the dose factor 30 cm above contaminated ground.
The dose factors for the nucTides of interest averaged about four
times greater at 30 cm than for 1 m above contaminated ground. These
factors may be appropriate to assess external exposure from surface
contamination to children. However, the potential dose from exposure to
beta radiation from contaminated surfaces is less than the potential dose
from the contact pathway.
A.4 DOSE FACTORS FOR INGESTION
Dose factors for ingestion are 50-yr effective committed dose
following 1 yr of chronic uptake based on ICRP 26 and ICRP 30 models
(USEPA 1988). Ingestion dose factors for radionuclldes of interest, in
units of rem/uCi ingested, are given in Table A.I. In cases where there
were dose factors for different chemical forms, the larger factor was
selected.
23
-------
REFERENCES
KOCHER, D. C. Radioactive Decay Data Tables. DOE/TIC-11026, Technical
Information Center, U.S. Department of Energy, Washington, DC, 1981a.
KOCHER, D. C.
Photons and Electrons.
Dose-Rate Conversion Factors for External Exposure to
NUREG/CR-1918, prepared by the Oak Ridge National
Laboratory for the U.S. Nuclear Regulatory Commission, Washington, DC, 1981b.
LOEVINGER, R., E. M. Japha, and 6. L. Browne!1. "Discrete Radio-
isotope Sources," Chapter 16. In Radiation Dosimetry, eds. G. H. Hine and
G. L. Browne!!, pp. 693-799, Academic Press, Inc., New York, 1956.
TRAUB, R. J., W. D. Reece, R. I. Scherpelz, and L. A. Sigalla. Dose
Calculation for Contamination of the Skin Using the Computer Code VARSKIN.
NUREG/CR-4418, PNL-5610, prepared by Pacific Northwest Laboratory for U.S.
Nuclear Regulatory Commission, Washington, DC, 1987.
U.S. ENVIRONMENTAL PROTECTION AGENCY. Limiting Values of Radionuclide Intake
and Air Concentration and Dose Conversion Factors for Inhalation,
Submersion, and Ingestion. EPA-520/1-88-020.U.S. Environmental Protection
Agency,Office of Radiation Programs, 1988.
24
-------
APPENDIX B
SOURCE-TERM DATA
B.I SST2 SOURCE TERM
The source term used for this report is based on the initial mix
of radionuclides predicted for an SST2-type reactor accident in
sufficient quantity to produce a 1-rem first-year reference whole-body
dose.
In-growth of daughters as well as physical decay and weathering
corrections are applied to the source term to yield first-year average
radionuclide concentrations on the ground surfaces. Decay chains and
the effect of weathering options are described in this appendix.
B.2 DECAY CHAINS
Table B.I lists radionuclides in the source term and shows the
parent radionuclides used in simple decay chain calculations.
B.3 WEATHERING
Average ground concentrations of radionuclides listed in Table
B.2 are first-year average concentrations calculated with CRAC2
weathering (Ritchie et al. 1984), a two-step exponential weathering
model from WASH-1400 (USNRC 1975).
25
-------
TABLE B.I. RadionucTides and Decay Chains
in SST2 Source Term
Nuclide
58f)Q
60Qo
86 Rb
89Sr
90Sr
90Y
91Y
95Zr
99MO
99mTc
103Ru
106Ru
105Rh
127$b
127Te
127mje
129Te
129mje
ISlmje
132Te
131 1
132j
134CS
136CS
137cs
140Ba
140La
141 Ce
143Ce
144Ce
143Pr
147Nd
239Np
242cm
238Pu
239Pu
244Cm
240Pu
241 pu
Half-Life, days
7.130E+01
1.921E+03
1.865E+01
5.200E+01
1.026E+04
2.670E+00
5.880E+01
6.550E+01
3.510E+01
2.751E+00
2.508E-01
3.959E+01
3.690E+02
1.479E+00
3.800E+00
3.896E-01
1.090E+02
4.861E-02
3.340E+01
1.250E+00
3.250E+00
8.040E+00
9.521E-02
7.524E+02
1.300E+01
1.099E+04
1.279E+01
1.676E+00
3.253E+01
1.375E+00
2.844E+02
1.358E+01
1.099E+01
2.350E+00
1.630E+02
3.251E+04
8.912E+06
6.611E+03
2.469E+06
5.333E+03
1.581E+05
Parent
90Sr
91Sr
95Zr
99MO
105Ru
127Sb
129Sb
131mje
132Te
140Ba
143Ce
242Cm
239Np
244cm
241Pu
26
-------
TABLE B.2. Initial Deposition and First-Year Average
Concentration Calculated with CRAC2
(WASH-1400) Weathering Model.
Average Concentration
Nuclide
58Co
60 Co
86 Rb
89Sr
90Sr
90Y
91Y
95^r
95 Mb
99MO
99MTc
103Ru
106Ru
105Rh
12755
127Te
127MTe
129Te
129MTe
13lMTe
132Te
131 1
132 j
134QS
136Cs
137cs
140Ba
140La
141ce
143Ce
144 Ce
143Pr
147 Nd
239Np
242Qn
238Pu
23gpu
244Cm
240Pu
241 pu
241Am
Initial,
Ci /m2
1.4E-08
l.OE-08
4.3E-08
6.5E-07
3.2E-08
1.4E-07
3.3E-06
4.3E-06
4.3E-06
2.1E-06
2.0E-06
1.6E-06
3.7E-07
9.4E-07
1.7E-05
1.7E-05
2.2E-06
4.1E-05
7.8E-6
2.3E-05
2.4E-04
1.7E-05
3.5E-05
2.6E-06
1.4E-06
3.6E-06
1.1E-06
4.3E-06
4.0E-06
3.6E-06
2.7E-06
3.7E-06
1.6E-06
4.2E-05
1.7E-08
l.OE-09
8.9E-10
1.2E-10
8.6E-10
1.6E-07
1.1E-10
Decay Only,
Ci/m2
3.8E-09
9.3E-09
3.2E-09
1.3E-07
3.1E-08
3.3E-08
7.6E-07
1.1E-06
1.7E-06
2.3E-08
2.4E-08
2.6E-07
2.7E-07
6.0E-09
2.5E-07
2.8E-07
8.5E-07
3.1E-08
l.OE-06
1.1E-07
3.1E-06
6.6E-07
3.1E-06
2.2E-06
7.4E-08
3.6E-06
5.3E-08
8.2E-08
5.2E-07
1.9E-08
1.8E-06
2.2E-07
6.8E-08
3.9E-07
8.4E-09
1.1E-09
9.0E-10
1.2E-10
8.6E-10
1.6E-07
2.4E-10
CRAC2,
Ci/m2
3.2E-09
7.0E-09
3.0E-09
1.2E-07
2.3E-08
2.5E-08
6.6E-07
9.5E-07
1.4E-06
2.2E-08
2.4E-08
2.3E-07
2.0E-07
5.9E-09
2.5E-07
2.8E-07
7.0E-07
3.1E-08
9.5E-07
1.1E-07
3.1E-06
6.4E-07
3.1E-06
1.7E-06
7.1E-08
2.7E-06
5.1E-08
7.9E-08
4.8E-07
1.9E-08
1.4E-06
2.1E-07
6.6E-08
3.9E-07
6.8E-09
7.9E-10
6.7E-10
8.7E-11
6.4E-10
1.2E-07
1.7E-10
27
-------
The average concentration for CRAC2 weathering is calculated by the
following algorithm:
C1 Co1
0.63J-
-R1-,
fr°-v
K1-, /
-e-'V
where Rli = (1.13 + x^t
R2i = (0.0075+ x.j)t,
t = years
The correction for in-growth of a daughter product from parent j is
as follows:
0.63
.37^(1 -e-R2j) (1 -e-
R2-
where x-j and Xj are the decay constants of nuclides i and j.
REFERENCES
RITCHIE. L. T., D. J. Alpert, R. P. Burke, J. D. Johnson, R. M. Ostmeyer,
D. C. Aldrich, and R. M. Blond. 1984. CRAC2 Model Description.
NUREG/CR-2552, SAND82-0342, prepared by Sandia National Laboratories.
U.S. NUCLEAR REGULATORY COMMISSION. Reactor Safety Study; An Assessment
of Accident Risks in U.S. Commercial Nuclear Power Plants. Appendix
6: Calculation of Reactor Accident Consequences^WASH-1400 (NUREG
75/014).U.S. Nuclear Regulatory Commission, Office of Nuclear
Research, Washington, D.C., 1975
28
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APPENDIX C
QUANTIFICATION OF CONTACT AND INGESTION PARAMETERS
C.I SURFACE CONTAMINATION/MIXING ASSUMPTIONS
Exposure to two types of surfaces with differing contaminant
concentrations is hypothesized. The first is a hard surface (paved) on
which a very thin layer of dust is present; the second surface is soil,
which has a thicker mixing layer.
For the hard surface, the amount of dust in which contamination is
mixed is assumed to be 160 g/m . This corresponds to a dust layer
0.1 mm thick, based on a density of 1.6 g/cm3, or approximately 1 year
of dustfall in a moderately dusty area (Hawley 1985). Hawley gives
2
dustfall for the community of Niagara Falls as 1.2 mg/cm per 30 days,
or 400 mg/m2/d, which is described as moderate dustfall. A dust layer
of 160 g/m2 is 1 year of dustfall accumulating at a rate 10 percent
higher than the rate for Niagara Falls.
For comparison, the mass of a dust layer was calculated using mass
and concentration data for lead. Using lead in dust measurements
reported by Gallacher et al (1984) and lead concentration data from
Duggan and Williams (1977) or Lepow et al (1975) results in an estimate
of the mass of the dust layer on paved surfaces that is. much lower than
the 160 g/m2 given above. The quantity of lead in dust per unit area
from a study in Wales (Gallacher et al 1984) was given as 8.5 mg/m for
paved areas adjacent to houses on roads with heavy traffic and
2.7 mg/m for similar areas on cul-de-sacs. Typical lead
concentrations in street dust measured in 5 residential areas in greater
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London range from 920 to 1840 ppm (Duggan and Williams 1977); A U.S.
study (Lepow et al 1975) reported mean lead levels in street dirt of
1200 ppm. Assuming concentration of lead in dust in the range of 500 to
2000 vig/g, the mass of dust would be only 1.3 to 17 g/m2. This
corresponds to a thickness of 0.01 mm or less. This estimate of the dust
layer seemed unreasonably thin and was not comparable with measured
dustfall.
The choice of dust thickness is rather arbitrary due to the large
number of variables affecting dust deposition and accumulation. Some of
the variables include roughness of surface, wind frequency and velocity,
and amount and intensity of rainfall. The value of 160 g/m2 was chosen
as the mixing layer for paved surfaces. A smaller value comparable to
the estimates based on lead deposition may be more appropriate for some
circumstances, but this assumption will be offset, to some degree, by
assumptions for the amount of dust transferred to skin from paved
surfaces (see Section 2.0).
Q
For a nonpaved surface, a mixing layer of 1 mm, or 1600 g/m was
used. The 1 mm thickness is used in an attempt to describe a surface
that is mixed by forces other than cultivation. A relatively thin layer
was used to provide conservatism (i.e., to bias the dose calculation on
the high side, if at all). Concentrations may be calculated with a
greater mixing depth, but this would provide additional dilution, thus be
less conservative.
C.2 DUST/DIRT RESIDING ON SKIN SURFACES
The amount of dust or dirt that may reside on the skin of a
maximally exposed individual is assumed to be 1.8 mg/cm2. The dirt
loading on the skin of an average individual is assumed to be
2
1 mg/cm . These values may be high (conservative) for paved surfaces
where the relative roughness of the surface tends to reduce the amount of
dust transferred to the skin. The quantities are based on estimates by
Schaum (1984) and Hawley (1985), and studies by Lepow (1975) and Reels
(1980). The range of 0.5 to 1.5 mg/cm2 was assumed by Schaum (1984) to
represent an average value for the entire exposed area of the body.
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Hawley (1985) estimates the dirt loading on the skin of an adult
2
equivalent to 1.8 mg per cm , which is based on a 50-ym layer of dust
o
with density of 0.7 g/cm (to account for the voids between dust
particles, however, an effective density of 0.35 g/cm3 was used).
The Lepow (1975) study deals with ingestion of lead from dirt on the
hands of children. Preweighed self-adhesive labels were used to sample
dirt on the hands. The mean weight of hand dirt samples was 11 mg for a
2 2
21.5-cm label, or 0.5 mg/cm . This weight was assumed to be a lower
bound for soil residing on the skin of children because the labels are not
100 percent efficient in transferring contamination. The estimate of dirt
on skin derived from the Roels et al. (1980) study is 1.5 mg/cm2, which
is based on the quantity of lead on the hands of 11-year-old children
playing on a playground.
The concentration of contamination present on the surface of the skin
may be calculated from the quantity of contaminants per unit area on the
ground, the mixing depth and mass, and the mass per unit area on the skin.
This concentration may be calculated as a fraction of the contamination on
the ground, as follows:
2 ?
Ci/m (skin) = Ci/m (ground) x
-------
be a function of age, which means the individual referred to in Table C.I
is the same for an adult or child.
Table C.I Fraction of Ground Concentration Assumed to be on the Skin
Surface.
Surface
Pavement
Ground
Fraction of Surface
Mixing Maximum Individual^
(loading = 1.8 mg/cm )
160 g/m2 0.11
1600 g/m2 0.011
Contamination on Skin
Average Individual^
(loading = 1 mg/cm )
0.063
0.0063
The activity residing on skin of the maximally exposed individual is,
2 2
therefore, estimated to be 1.8 mg/cm divided by 16 mg/cm for a dust
layer on pavement, or 11 percent of the total ground concentration. For a
soil surface, this is equivalent to 1.1 percent of the contamination mixed
2
in the 160 mg/cm surface soil layer. For the average individual, the
2
contamination is assumed to be 1.0 mg/cm , or about 6 percent of the
dust layer or about 0.6 percent of the surface soil layer.
C.3 INGESTION OF DIRT/DUST
Ingestion of contaminants can occur when surface contamination is
transferred from a surface to hands, foodstuffs, cigarettes, or other
items. Ingestion of dirt from contaminated hands has been investigated in
the research of lead ingestion by children and more recently in the
context of exposure to other hazardous contaminants. Soil ingestion
studies have been used to estimate the amount of soil on skin surfaces and
age-dependent ingestion quantities.
Ingestion rates assumed for this analysis based on LaGoy (1987) are
100 mg/d for the maximum adult individual, 500 mg/d for the maximum child,
25 mg/d for the average adult, and 100 mg/d for the average child. The
quantity of contaminants ingested is dependent on the concentration of the
contaminants in the surface dust or dirt layer, or in other words, on the
32
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assumed mixing layer. Assumptions concerning the mixing layers, noted
2 2
above, are 160 g/m for a dust layer on pavement and 1600 g/m (1 mm
thick) for other surfaces.
Since the source term is in units of Ci per unit area, the ingestion
rate must be converted to an area basis. Ingestion rates based on area
then depend on the mass of dust layer calculated as follows:
mg/d ingested
g/m dirt
1000 mg
10,000 cm
2
= cm /d ingested
m
Ingestion rates based on contamination from a given surface area have been
used to assess ingestion of removable radioactive contamination residing
on surfaces. Table C.2 presents the area-equivalent contamination
ingested for use with the contaminant source term. The ingestion rates
used for this analysis are equal to the contamination from about 0.2
to 31 cm /d.
These ingestion rates are reasonably comparable to rates calculated
2
by Healy (1971) to estimate ingestion doses. Healy assumed that 1 cm
of surface contamination could be taken into the mouth per hour; thus,
2
ingestion rates for workers of 8.0 cm /d and for the public of
2
24 cm /d were assumed. The higher ingestion rate for the public was
presumed to allow for higher intake by children.
Table C.2 Ingestion Rate of Contaminated Soil for Area Based Source Term
Surface
Mixing
Adult Child
Maximum Average Maximum Average
100 mg/d 25 mg/d 500 mg/d 100 mg/d
Pavement
Ground
160 g/m2
1600 g/m2
6.3 cm2
0.63 cm2
1.6 cm2
0.16 cm2
31 cm2
3.1 cm2
6.3 cm2
0.63 cm2
The intake rates given in Table C.2 are used to estimate dose from
ingestion of contaminants in the SST2 source term in the text of this
paper.
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REFERENCES
DUG6AN, M. 0. and S. Williams. 1977. "Lead-in-Dust in city Streets."
The Science of the Total Environment, pp. 91-97. Elsevier Scientific
Publishing Company, Amsterdam.
GALLAGHER, 0. E. 0., P. C. Elwood, K. M. Phillips, B. E. Davies, and
D. T. Jones. 1984. "Relation Between Pica and Blood Lead in Areas of
Differing Lead Exposure." Archives of Disease in Childhood 59:40-44.
HAWLEY, J. K. 1985. "Assessment of Health Risk from Exposure to
Contaminated Soil." Risk Analysis 5(4):289-302.
HEALY, J. W. 1971. Surface Contamination: Decision Levels.
LA-4558-MS, Los Alamos Scientific Laboratory, Los Alamos, New Mexico.
LAGOY, P. K. 1987. "Estimated Soil Ingestion Rates for Use in Risk
Assessment." Risk Analysis 7(3):355-359.
LEPOW, M. L., L. Bruckman, M. Gillette, S. Markowitx, R. Robino, and J.
Kapish.1975. "Investigations into Sources of Lead in the Environment
of Urban Children." Environmental Research 10:415-426.
ROELS, H. et al. 1980. "Exposure to Lead by the Oral and Pulmonary
Routes of Children Living in the Vicinity of a Primary Lead Smelter."
Environmental Research 22:81-84.
SCHAUM, J. 1984. Risk Analysis of TCDD Contaminated Soil. Prepared for
EPA Office of Solid Waste and Emergency Response by EPA Office of
Health and Environmental Assessment, Washington, D.C.
*U.S. GOVERNMENT PRINTING OFFICE: 1991517-003/47006
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