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OCR for page 218
11
Issues in Developing Guidances for TENORM
The primary purpose of this study has been to examine and report on
the scientific and technical bases of guidances developed by the Environmental
Protection Agency (EPA) for protection of the public from exposure to
technologically enhanced naturally occurring radioactive materials (TENORM).
The particular issue of concern to this study is whether the differences between
EPA guidances for TENORM and those developed by other organizations are
based on scientific and technical information or on policy decisions related to
risk management. If there are differences in the scientific and technical bases of
the various guidances, the relative merit of the different scientific and technical
assumptions should be evaluated.
This chapter presents several summary discussions related to the
purpose of the study, including discussions on:
· The question of whether the differences between EPA guidances
for TENORM and those developed by other organizations have a
fundamental scientific and technical basis.
· Specific areas in which the technical approaches to risk
assessment of radionuclides developed by EPA differ from the
approaches normally used by other organizations and the question of
whether the differences have been important in developing guidances
for TENORM.
· Specific areas in which the differences between EPA guidances
for TENORM and those developed by other organizations are based
on differences in policies related to risk management, rather than
scientific and technical issues.
218
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GUIDELINES FOR EXPOSURE TO TENORM
219
After those discussions, the chapter considers various alternatives for expressing
guidances for TENORM and their implications for risk assessment, particularly
with regard to the distinction between the risk-assessment issues that would
need to be addressed in developing guidances and the issues that would be
addressed in demonstrating compliance.
SCIENTIFIC AND TECHNICAL BASES FOR GUIDANCES
AS summarized in chapter 10, there clearly are differences in the
guidances for TENORM developed by EPA and similar guidances developed by
other organizations, both for indoor radon and for TENORM other than indoor
radon. Where there are differences, EPA guidances tend to be more restrictive.
However, this committee finds that the differences between EPA
guidances for TENORM and those developed by other organizations do not
have a scientific and technical basis. That conclusion is based on the
observations that all organizations that have developed guidance on indoor
radon have assumed essentially the same risk related to exposure to radon and
its short-lived decay products on the basis of data obtained from studies of
underground miners, and that all organizations that have developed guidances
for TENORM other than indoor radon have assumed essentially the same risk
related to uniform Radiation of the whole body on the basis of data obtained
primarily from studies of the Japanese atomic-bomb survivors. Thus, for
purposes of health protection of the public, including establishing guidances for
acceptable levels of indoor radon and acceptable levels of exposure to
TENORM other than indoor radon, all organizations have assumed essentially
the same risks related to radiation exposure.
The lack of a scientific and technical basis for the differences between
EPA and other guidances for TENORM does not imply that there are no
differences in the technical approaches used in assessing risks related to
radiation exposure. Indeed, this committee has learned of several such
differences, as discussed in the following section. But differences in the
technical approaches to risk assessment of radionuclides have not been the cause
of the differences in the various guidances for TENORM.
This committee also notes that the various guidances for TENORM
were developed at different times and that the basic assumptions about radiation
risks have changed over time. For example, when the existing federal guidance
on radiation protection of the public specifying an annual dose limit for
individuals of essentially 5 mSv was issued (FRC 1961; 1960), quantitative
information on the risks at low levels of exposure had not yet been developed by
such groups as the National Council on Radiation Protection and Measurements
(NCRP), the International Commission on Radiological Protection (ICRP), and
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ISSUES IN DEVELOPING GUIDANCES
the National Research Council. The genetic risk posed by radiation exposure
was believed to be a greater concern than the cancer risk, and the limits on
maximum and average annual doses in the federal guidance were based on a
largely unquantifiable expectation that exposures below the dose limits would
not result in an observable increase in cancers or genetic effects in exposed
populations. By the time the proposed revision of the federal guidance was
issued (EPA 1994d), the genetic risk was reduced in importance, cancer risks
had been estimated from the atomic-bomb survivor data, and the estimated risks
were used in conjunction with an assumption about the maximum tolerable risk
posed by radiation exposure as a justification for lowering the annual dose limit
for individuals to 1 mSv (see chapter 7~.
Thus, the difference between the federal guidance (FRC 1961; 1960)
and its proposed revision (EPA 1994d) clearly has a scientific basis. However,
the issue of concern to this study is the difference between current EPA
guidances for TENORM and those developed by other organizations, and this
committee has assumed that the proposed revision of the federal guidance
represents EPA's current views on requirements for radiation protection of the
public. Therefore, because all current EPA guidances for TENORM and the
guidances of other organizations have been developed or updated within the last
decade, the assumptions about radiation risks have been essentially the same in
all cases.
The committee was also asked to consider whether there is relevant
scientific information that has not been used in the development of
contemporary risk analysis of NORM. A particular concern is that some of the
important naturally occurring radionuclides are parents of long decay chains
involving complex mixtures of radioisotopes of different chemical elements,
and that exposure to such mixtures of radionuclides might necessitate novel
approaches to methods of risk estimation.
The decay chains of some naturally occurring radionuclides-
especially radium, uranium, and thorium are considerably more complex than
the decay chains of other radionuclides with regard to the number of decay
products and chemical elements involved. However, contemporary methods of
risk assessment that estimate doses and risks associated with ingestion or
inhalation of radionuclides by allowing any decay products produced in the
body to be redistributed and retained in the body according to the metabolic
behavior characteristic of the particular chemical element take this added
complexity into account by using the same methods that are applied to other
radionuclides with many fewer decay products. Thus, there is no evident need
for a different approach in dealing with the complex decay chains of some
naturally occurring radionuclides.
More generally, the committee is not aware of any evidence that there
should be differences in risks, and thus differences in approaches to risk
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GUIDELINES FOR EXPOSURE TO TENORM
221
assessment, associated with exposure to naturally occurring and human-made
radionuclides. Indeed, if one accepts the view currently held by all regulatory
and advisory organizations involved in radiation protection that estimates of
absorbed dose in tissue are the fundamental physical quantities that determine
radiation risks for any exposure situation (NCRP 1993a; ICRP 1991), then there
is no plausible rationale for any differences in risks between naturally occurring
and any other radionuclides, because absorbed dose in tissue depends only on
the radiation type and its energy but not on the source of the radiation.
Thus, in general, there should be no difference between NORM and
any other radioactive materials with regard to suitable approaches to estimating
doses and risks related to external or internal exposure. However, because
naturally occurring radionuclides are ubiquitous in the exposure environment,
there might be an increased opportunity, compared with many human-made
radionuclides, to use observational data on natural levels in different
environmental compartments (such as soil, water, air, plants, and animals) and
the fluxes between compartments to calibrate exposure pathway models for
TENORM. In contrast, the ability to use such natural analogue data in exposure
analysis must be tempered by the recognition that the physical and chemical
forms of TENORM can be substantially different from those of the same
elements in the natural environment, in which case observations on the behavior
of radionuclides in natural systems might not be relevant to the exposure
situation of concern.
DIFFERENCES IN TECHNICAL APPROACHES TO RISK
ASSESSMENT
During this study, the committee examined a white paper on risk
harmonization that had been prepared jointly by EPA and the Nuclear
Regulatory Commission (Nuclear Regulatory Commission/EPA 1995~. The
white paper includes discussions on similarities and differences in the methods
of risk assessment of radionuclides used by EPA and the Nuclear Regulatory
Commission.
The pr~rnary purpose of this section is to discuss and comment on the
differences between the EPA and Nuclear Regulatory Commission approaches
to risk assessment of radionuclides, and to discuss the importance of these
differences with regard to the development of guidances for TENORM. This
section also discusses the issues of truncation of risk assessments in time and
transferability of standards from one exposure situation to another, which are
particularly important for TENORM other than indoor radon.
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ISSUESINDEVELOPING GUIDANCES
Differences Between Environmental Protection Agency and Nuclear
Regulatory Commission Approaches to Risk Assessment
The Nuclear Regulatory Commission's approach to estimating risk
posed by chronic radiation exposure of the public normally is based on ICRP
recommendations on estimating doses per unit exposure and the risk per unit
dose. The Nuclear Regulatory Commission estimates lifetime risks on the basis
of estimates of annual doses that are the sum of the annual dose equivalent to
the whole body from external exposure and the 50-y committed effective dose
equivalent (ICRP 1977) from ingestion and inhalation of radionuclides. Lifetime
risk is estimated by multiplying the annual effective dose equivalent from
external and internal exposure by the assumed exposure time (for example,
70 y) and the nominal risk of fatal cancers caused by uniform whole-body
irradiation of 5 x 10-2 per sievert (ICRP 1991~. It is important to note that ICRP's
nominal risk factor takes into account the age dependence of radiation risk in the
whole population, which is based on data on the atomic-bomb survivors (ICRP
1991~.
EPA has developed a methodologically more rigorous approach to
assessing risk posed by chronic lifetime exposure to radionuclides, which is
particularly important for internal exposure and differs in several respects from
the simple approach described above.
First, EPA calculates the total risk by first calculating the risk in each
organ irradiated, which is based on the calculated absorbed dose and an assumed
risk per unit dose for that organ, and then summing the calculated risks for all
organs (Puskin and Nelson 1995; EPA 1994c). Thus, in estimating risk, EPA
does not use the calculated effective dose equivalent, with its assumption of
nominal risks per unit dose equivalent for various organs (which are intended
only to be approximate indicators of risk), multiplied by ICRP's nominal risk
related to uniform whole-body irradiation. If there were no other differences, the
two approaches would yield estimates of risk that differed only to the extent that
the risks for different organs assumed by EPA were substantially different from
the values assumed by ICRP in defining the effective dose equivalent, because
EPA and ICRP assume nearly the same risk related to uniform whole-body
irradiation. EPA's current risk estimate for whole-body irradiation (Eckerman
and others 1998) is about 12% higher than ICRP's, but this difference is not
significant.
EPA's approach described above gives different estimates of risk from
the approach used by the Nuclear Regulatory Commission, which is based on
the effective dose equivalent (ICRP 1977), and the current approach of ICRP
(1991), which is based on the effective dose, because EPA estimates risks to
specific organs that are not considered explicitly in calculating the effective
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GUIDELINES FOR EXPOSURE TO TENORM
223
dose equivalent (for example, organs of the gastrointestinal tract and the
kidneys) and has assumed different risks for many important organs. EPA's risk
estimate for bone is less than ICRP's current estimate (ICRP 1991) by a factor of
about 5 primarily because EPA recognized an error in ICRP's estimate that is
based on a confusion between dose to the radiosensitive endosteal tissues and
average skeletal dose (Puskin and others 1992; Bair and Sinclair 1992~. And
EPA's estimated risk factors for some other organs (such as the stomach, which
is not considered explicitly in the effective dose equivalent, and the lungs) differ
substantially from ICRP's current estimates (ICRP 1991), primarily because the
two organizations estimated risks in different populations with different organ-
specific background risks as a function of age (EPA 1994c). EPA's risk
estimates were developed for a US population, but ICRP's risk estimates were
developed for an average of several national populations (ICRP 1991~. In
addition, in developing the tissue weighting factors for the effective dose
equivalent (ICRP 1977) and the revised tissue weighting factors for the effective
dose (ICRP 1991), ICRP used rounded and binned values of the risk for the
different organs of concer~an approach that has not been used by EPA.
Second, in risk assessments of internal exposure to radionuclides with
radioactive decay products, the Nuclear Regulatory Commission and EPA use
different assumptions in calculating the dose due to ingrowth of decay products
in the body after intake of the parent radionuclide. The Nuclear Regulatory
Commission uses a model recommended previously by ICRP (1979) in which
most decay products are assumed to be retained in the organs of deposition of
the parent according to the retention function for the parent, even though the
metabolic behavior of the decay products often is different from that of the
parent. EPA has developed more-sophisticated models incorporating the
physiologically based biokinetic models developed more recently by ICRP
(1996; 1995; 1993a; 1989a), which assume that decay products are redistributed
and retained in the body according to their own metabolic behavior.
Third, in estimating doses and risks related to exposure to alpha
particles, the Nuclear Regulatory Commission uses a radiation quality factor of
20 to convert absorbed dose to dose equivalent for all irradiated organs, on the
basis of the ICRP recommendation (ICRP 1991, 1977) of a single radiation
quality factor for alpha particles that would apply to any tissue and stochastic
biologic end point of concern. However, in estimating risks related to irradiation
by alpha particles, EPA uses a relative biological effectiveness (RBE) of 1 for
leukemia, 10 for breast cancer, and 20 for all other cancer sites, on the basis of
organ-specif~c information on the risk per unit absorbed dose from alpha
particles (EPA 1994c). Thus, EPA's risk estimates for irradiation of bone
marrow by alpha particles are much less than the estimates used by die Nuclear
Regulatory Commission, and the risk estimates for the breast are somewhat less.
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ISSUESINDEVELOPING GUIDANCES
Finally, as noted previously, the Nuclear Regulatory Commission
normally estimates risks posed by chronic lifetime exposure on the basis of a
calculated annual effective dose equivalent, which includes the 50-y committed
effective dose equivalent from internal exposure for reference adults (ICRP
1977), an assumed time of exposure, and a nominal risk posed by uniform
whole-body irradiation. For internal exposure, use of the effective dose
equivalent in this way overestimates risk because it does not properly account
for the dose received as a function of age at exposure and time after exposure,
which are important concerns for chronic exposures of the public to long-lived
radionuclides that are retained in the body for long periods. For chronic external
exposure, EPA calculates lifetime risk essentially in the same way as the
Nuclear Regulatory Commission because the dose is received at the time of
exposure and, as noted previously, EPA assumes nearly the same nominal risk
related to uniform whole-body irradiation.
However, for internal exposure, EPA estimates risks posed by chronic
lifetime exposure of the public on the basis of age-specific dose rates and age-
specif~c cancer risks rather than committed effective dose equivalents for adults
and a nominal risk factor, as in the Nuclear Regulatory Commission approach
(Eckerman and others 1998; Dunning and others 1984, Sullivan and others
1981; Dunning and others 1980~. Particularly for internal exposure to long-lived
radionuclides with long retention times in the body, EPA's approach more
properly accounts for the dose received as a function of age at intake and time
after intake. In essence, EPA estimates risk posed by chronic lifetime exposure
as a convolution, over age at intake from birth to death and time after intake, of
(1) the dose rate as a function of time after intake for any age at intake, as
estimated with age-specific biokinetic and dosimetric models for ingestion and
inhalation of radionuclides, (2) the risk at any future age per unit dose received
at a given age, and (3) the probability of death from all competing causes as a
function of age, as obtained from US life tables. The risk at any future age per
unit dose received at a given age is estimated with an absolute-risk model for
bone, skin, and thyroid but with a relative-risk model for all other organs (EPA
1994c). The relative-risk model incorporates age-specif~c background cancer
risks from all causes in the US population.
Aspects of EPA's approach to risk assessment for radionuclides
described above have been used in several regulatory activities, including risk
assessments to support current standards for airborne emissions of radionuclides
in 40 CFR Part 61 (EPA 1989d; 1989b), development of radionuclide-specific
slope factors for use in risk assessments at contaminated sites subject to
remediation under the Comprehensive Environmental Response, Compensation,
and Liability Act (CERCLA) (EPA 1989c), and risk assessments to support the
development of site-cleanup standards for radionuclides (Wolbarst and others
1996) (see chapter 7~.
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GUIDELINES FOR EXPOSURE TO TENORM
225
Comments on Differences Between Environmental Protection Agency and
Nuclear Regulatory Commission Approaches to Risk Assessment
This committee offers the following comments on EPA's approach to
risk assessment for chronic lifetime exposure of the public, especially internal
exposure, as it differs from the approach normally used by the Nuclear
Regulatory Commission for similar exposure situations.
First, EPA's approach should provide more realistic estimates of risk
than the approach used by the Nuclear Regulatory Commission. All the factors
described in the previous section-the use of organ-specific risks for many
organs instead of risks based on the effective dose equivalent and a nominal risk
from uniform whole-body irradiation, the use of updated biokinetic models in
estimating dose from ingrowth of decay products in the body, the use of organ-
specific RBEs for alpha particles, and the use of age-specific dose rates from
internal exposure in conjunction with age-specific cancer risks-should result in
more realistic estimates of risks associated with chronic lifetime exposure.
Second, the differences between EPA and Nuclear Regulatory
Commission approaches to estimating radiation risks do not always result in
substantial differences in estimated risks. When the dose is due primarily to
external exposure or the internal dose is due primarily to short-lived
radionuclides that are distributed nearly uniformly in the body and emit only
low-LET radiations (photons and electrons), the differences in the risk estimates
between using EPA and Nuclear Regulatory Commission approaches are
insignificant, essentially because the risk posed by uniform whole-body
irradiation recommended by ICRP (1991) takes into account the age dependence
of both the radiogenic and background cancer risks. As noted previously, EPA's
risk estimate for these cases (Eckerman and others 1998) is only slightly higher
than ICRP's recommendation. The largest differences in estimated risks occur
for internal exposure to long-lived, alpha-emitting radionuclides (such as
thorium), which preferentially deposit in bone and have long retention times in
the body. In those cases, the important tissues at risk are red marrow and bone,
and the EPA approach can result in risk estimates for ingestion and inhalation
exposure that differ from the risk estimates obtained with the Nuclear
Regulatory Commission approach by more than an order of magnitude
(Eckerman and others 1998), with EPA's risk estimates generally being lower.
Third, the Nuclear Regulatory Commission does not always estimate
risks on the basis of the effective dose equivalent and a nominal risk related to
uniform whole-body irradiation. It uses organ-specific and age-specific risk
factors similar to EPA's assumptions in certain cases, including risk assessments
of reactor accidents and other situations where the particular individuals at risk
can be identified. Thus, the differences between EPA and Nuclear Regulatory
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ISSUESINDEVELOPING GUIDANCES
Commission approaches to risk assessment generally are important only for
prospective and hypothetical chronic-exposure situations.
Fourth, EPA does not always use the more rigorous approach to risk
assessment described in the previous section but, in some cases, uses the same
approach as the Nuclear Regulatory Commission. EPA uses the more rigorous
approach only in assessing risks for purposes of reaching decisions on rule-
making, such as decisions on the feasibility of establishing standards and the
effects of alternative standards. However, when radiation standards are
expressed in terms of dose equivalent, as is often the case, EPA uses the same
approach to dose assessment as the Nuclear Regulatory Commission for
purposes of demonstrating compliance. The dosimetric quantities currently used
by EPA for compliance purposes are effective dose equivalents for reference
adults (Eckerman and others 1988), which do not incorporate any of EPA's
current assumptions for purposes of risk assessment involving organ-specific
and age-specific doses and risks or biokinetic models for radionuclides and their
decay products in the body. EPA has taken the customary approach
incorporating ICRP recommendations in demonstrating compliance with
standards expressed in terms of dose to maintain a stable and uniform regulatory
framework for the nuclear community. Furthermore, in using an assumed limit
on lifetime risk to derive a limit on annual effective dose equivalent from
exposure to all radionuclides of concern for use in standards (see chapter 7),
EPA uses essentially the same nominal risk per unit effective dose for any
radionuclide as does ICRP (1991~; but EPA does not take into account the
results given by the more sophisticated models that continuous intakes of
different radionuclides corresponding to a given annual committed effective
dose equivalent for reference adults can correspond to substantially different
lifetime risks.
Finally, given the differences between EPA and Nuclear Regulatory
Commission approaches to risk assessment and the fact that EPA and the
Nuclear Regulatory Commission use the same approaches in demonstrating
compliance with radiation standards expressed in terms of dose, it is important
to appreciate that the simplified approaches to risk assessment developed by
ICRP (1991; 1977) and used by the Nuclear Regulatory Commission were
believed to be reasonable for the needs of these organizations. ICRP and the
Nuclear Regulatory Commission are concerned only with radiation protection,
in which case dose provides a measure of risk; and the effective dose equivalent
and, later, the effective dose were developed by ICRP to provide a reasonable
surrogate for risk in any exposure situation. Furthermore, radiation protection is
concerned with control of exposures without undue concern for the risks posed
by actual exposure situations, provided that applicable dose limits and the
ALARA (as low as reasonably achievable) objective are met. Therefore, for
purposes of radiation protection, the use of effective dose equivalents and a
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GUIDELINES FOR EXPOSURE TO TENORM
227
nominal risk factor for uniform whole-body irradiation in estimating risks posed
by chronic exposure to any radionuclides was believed to be satisfactory.
ICRP also recognized that there are radionuclide-specific differences in
lifetime risks related to internal exposure for the same annual effective dose
equivalent, but the simplified approach to estimating risk was judged to be
satisfactory as long as these differences were within about a factor of 3 of the
risk posed by external exposure. However, the recent EPA analyses indicating
that more rigorous estimates of risk associated with chronic lifetime intakes can
differ from estimates based on the effective dose equivalent and a nominal risk
factor by substantially more than an order of magnitude for some radionuclides
(Eckerman and others 1998) call into question the general suitability of using
the effective dose equivalent (ICRP 1977) in estimating risk even for purposes
of radiation protection.
Many of the differences between EPA and Nuclear Regulatory
Commission approaches to risk assessment described in this section result from
the use by the Nuclear Regulatory Commission, and other federal and state
agencies, of the now outdated effective dose equivalent. ICRP has replaced this
quantity with the effective dose (ICRP 1991), which incorporates a greater
number of organs and updated information on organ-specif~c risks, and ICRP
also has developed age-specif~c effective dose coefficients for inhalation and
ingestion which incorporate the newer physiologically-based biokinetic models
for radionuclides and their decay products (ICRP 1996; 1995, 1993a).
Thus, EPA's current approach to risk assessment differs from the
approach to estimating risk based on current ICRP methods mainly in three
respects. First, EPA estimates risk on the basis of age-specific absorbed dose
rates and radiogenic risks, instead of committed effective doses and a nominal
risk factor. Second, EPA estimates risk for a US population with a longer
average lifespan and different background cancer risks as a function of age than
ICRP, the risk factor for bone is the corrected value developed by EPA, and the
cancer risk for breast is based on data for the United States, rather than the
atomic-bomb survivors. Third, EPA uses different RBEs for alpha particles for
leukemia and breast cancer than the standard radiation weighting factor of 20
used by ICRP.
The effect of the differences described above is that EPA's risk
estimates are slightly higher than ICRP's for external exposure and for internal
exposure to radionuclides with short retention times in the body, but EPA's risk
estimates are substantially less than those obtained by using ICRP methods for
internal exposure to some of the long-lived, alpha-emitting radionuclides
Occulting in TENORM. For 232Th, for example, EPA's risk estimates for
inhalation and ingestion are less than the estimates based on current ICRP
methods by a factor of 4-5 (Eckerman and others 1998~.
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ISSUESINDEVELOPING GUIDANCES
importance of Approaches to Risk Assessment for Guidances for
TENORM
The potential importance of the differences between EPA and Nuclear
Regulatory Commission or ICRP approaches to risk assessment described above
for the development of guidances for TENORM is difficult to evaluate. The
concern here is only with guidances for TENORM other than indoor radon
because all organizations use essentially the same assumptions in assessing risk
related to indoor radon.
As summarized in tables 10.3 and 10.4, current EPA guidances for
TENORM other than indoor radon (Luitig and Weinstock 1997; EPA 1994d)
are expressed in terms of the annual effective dose equivalent. In these cases,
EPA's more rigorous approach to risk assessment was not used in developing
the particular dose criteria based on an assumed acceptable risk, but ICRP's
nominal risk factor for all radionuclides (ICRP 1991) was used instead.
Furthermore, the approach of calculating effective dose equivalents for
reference adults (Eckerman and others 1988) would be used in demonstrating
compliance with the guidance. Therefore, on the basis of the discussions in the
previous two sections, the more rigorous approach to risk assessment would be
used by EPA only for investigating the feasibility of any particular guidance for
TENORM.
However, TENORM other than indoor radon has some unique
characteristics among the various controlled sources of public exposure that
could encourage a reexamination of the conventional approach to developing an
annual dose criterion based on an assumed acceptable risk and ICRP's nominal
risk factor. In contrast with human-made radionuclides from the nuclear fuel
cycle, only a few radionuclides are of concern (isotopes of uranium, thorium,
and radium and their shorter-lived decay products), and most of the
radionuclides of concern are long-lived alpha-emitters that deposit in bone.
Those are precisely the kinds of radionuclides for which the differences between
EPA and Nuclear Regulatory Commission approaches to risk assessment are the
most important and there is the greatest incentive to use a more rigorous
approach to risk assessment to establish a dose criterion based on an assumed
acceptable risk.
Furthermore, because only a few radionuclides are of concern in
regulating TENORM, regulatory criteria conceivably could be expressed in
terms of allowable concentrations of radionuclides in environmental media
rather than dose. If the acceptable environmental levels are based on an assumed
acceptable risk, they could be derived with EPA's more rigorous approach to
risk assessment.
A factor that argues against this approach is that EPA's preliminary risk
assessments for various scenarios of exposure to TENORM other than indoor
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ISSUES IN DEVELOPING GUIDANCES
external radiation in the field. The standards for mill-tailings sites would not be
appropriate for other exposure situations where the specified concentrations
were not reasonably achievable.
Second, the cleanup standards for 226Ra in contaminated soil at uranium
mill tailings sites correspond to an annual dose that is an appreciable fraction of
the annual dose limit of 1 mSv (100 mrem) for all controlled sources combined
in EPA's proposed federal guidance on radiation protection of the public (EPA
1994d) (see chapter 7~. Therefore, if a standard for 226Ra that would apply to
other exposure situations is intended to correspond to a limit on annual dose that
is only a small fraction of the dose limit for all controlled sources combined, the
standards for mill-tailings sites might not be appropriate, especially for large-
volume sources.
Third, the external dose from localized sources of 226Ra can be
substantially less than the external dose from large-volume sources, such as a
large extent of contaminated soil (for example, more than 100 m2) with the same
activity concentration. Therefore, using a single concentration standard for 226Ra
without regard for the size of the source could result in unduly restrictive
regulation of localized sources if the standard is intended to correspond to a
particular annual dose for any exposure situation.
Finally, as noted in chapter 7, the cleanup standards for 226Ra in
contaminated soil at uranium mill tailings sites are expected to correspond to
concentrations of indoor-radon decay products of about 4 x 10-7 J/m3 (0.02 WL).
The assumed correspondence between radium concentrations in soil and levels
of indoor-radon decay products applies only to materials in which the emanation
rate of radon is similar to that in mill tailings. Therefore, if exposures to indoor
radon are a potential concern, the radium standard for mill-tailings sites might
not be appropriate for other situations where the emanation rate of radon from
the materials in contaminated soil is substantially different from the emanation
rate from mill tailings.
The issue of transferability of standards, especially standards in the
form of concentration limits of radionuclides, is not easily resolved, primarily
because radiation protection involves compliance with the ALARA objective, as
well as a limit on dose or risk. Therefore, for example, the cleanup standards for
226Ra in contaminated soil at uranium mill tailings sites could be applied to other
exposure situations involving 226Ra if the standards were reasonably achievable,
even when there would be substantial differences in doses and risks. In
transferring standards from one situation to another, it is important to investigate
whether the standards are reasonably achievable for a variety of exposure
situations of concern, especially if the doses and risks are substantially different.
Differences in the physical and chemical forms of radionuclides in the different
situations also need to be considered because the dose from internal exposure
pathways can depend significantly on the form of the materials. Such
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GUIDELINES FOR EXPOSURE TO TENORM
233
considerations are important to ensure that standards that are reasonable for one
exposure situation are not applied inappropriately to other situations.
POLICY-BASED DIFFERENCES IN GI1IDANCES FOR TENORM
As indicated earlier in this chapter, this committee finds that the
differences between EPA and other guidances for TENORM do not have a
scientific and technical basis but, rather, result essentially from differences in
policies for risk management. This section discusses a number of ways in which
that is the case, including
· Selection of a limit on acceptable dose.
· Application of EPA's groundwater protection strategy to
regulation of TENORM.
· Differences between the Nuclear Regulatory Commission's
standards for decontamination and decommissioning of contaminated sties
and EPA's preferred approach to radiation-site cleanup standards.
· EPA guidance on indoor radon vs. NCRP and ICRP
recommendations.
.
EPA guidance on dose limit for all sources of exposure combined
vs. NCRP's recommendation on a remedial-action level for exposure to
natural sources.
guidances.
The general treatment of natural background in establishing
All those considerations are potentially important in developing guidances for
TENORM.
Limit on Acceptable Dose
The white paper on risk harmonization (Nuclear Regulatory
Commission/EPA 1995) indicates that EPA and the Nuclear Regulatory
Commission have fundamentally different views about a limit on acceptable risk
related to radiation exposure and, therefore, about a limit on acceptable dose
that might be included in guidances for TENORM other than indoor radon and
for any other controlled sources of exposure. In particular, the white paper
indicates that the annual dose limit of 1 mSv (100 mrem) specified in 10 CFR
Part20 (Nuclear Regulatory Commission 1991) is acceptable for individual
Nuclear Regulatory Commission licensees, whereas the white paper and other
guidance (Luftig and Weinstock 1997; EPA 1994d) indicate that in EPA's view,
the dose from individual sources should normally be limited to substantially less
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than the annual dose limit from all sources combined of 1 mSv (100 mrem).
EPA evidently favors an annual dose constraint for individual sources of
0.15 mSv (15 mrem), on the basis of the objective of achieving a lifetime risk of
about 10-4 (Luitig and Weinstock 1997~.
This committee offers the following comments on the issue of a limit
on acceptable risk and, therefore, acceptable dose. First, the determination of an
acceptable risk for any exposure situation clearly is entirely a matter of
judgment (risk-management policy) which presumably reflects societal values.
Inasmuch as EPA and the Nuclear Regulatory Commission have used
essentially the same assumptions about the risks posed by radiation exposure in
establishing radiation standards, it is clear that the determination of a limit on
acceptable dose for any exposure situation also is entirely a matter of judgment.
Therefore, any differences between the views of EPA and the Nuclear
Regulatory Commission on an acceptable dose have no scientific or technical
basis.
Second, a simple comparison of the Nuclear Regulatory Cornrnission's
annual dose limit for individual licensees of 1 mSv (100 mrem) (Nuclear
Regulatory Commission 1991) with EPA's preferred annual dose constraint for
individual sources of 0.15 mSv (15 mrem) (Luftig and Weinstock 1997) gives
the impression that EPA's dose constraint would be considerably more
protective of human health. However, this committee believes that such a
comparison is quite misleading and, therefore, that the resulting impression is
basically incorrect.
As emphasized in chapter 7, requirements for radiation protection of
the public include implementation of the ALARA objective, as well as
compliance with a dose limit for all controlled sources combined and dose
constraints for individual practices or sources; and the ALARA objective is
included in existing and proposed federal guidance on radiation protection of
the public (see chapter 7) and the Nuclear Regulatory Com~nission's radiation-
protection standards in 10 CFR Part 20. Thus, although the Nuclear Regulatory
Commission allows annual doses as high as 1 mSv (100 mrem) for individual
licensees, it also requires that all licensees implement an ALARA program. The
effect of vigorous application of the ALARA objective has been that doses to
the public achieved by nearly all licensees are only a few percent or less of the
dose limit. Therefore, the practical effect of Nuclear Regulatory Cornrnission
requirements is that doses from nuclear facilities currently operating under
Nuclear Regulatory Commission or Agreement State licenses are limited to
levels that EPA would judge acceptable according to its preferred annual dose
constraint of 0.15 mSv (15 mrem). The principal difference between EPA and
Nuclear Regulatory Cornrnission approaches to radiation protection is that EPA
imposes dose constraints on particular classes of sources (such as operating
nuclear fuel-cycle facilities) as a means of implementing the ALARA objective,
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235
whereas the Nuclear Regulatory Commission usually applies the ALARA
objective only on a site-specific basis. That difference evidently has little
practical importance in determining doses actually experienced.
Another important consideration in comparing EPA and Nuclear
Regulatory Commission views on an acceptable dose is that EPA's preferred
annual dose constraint for individual sources of O.l5mSv (lSmrem) is a
regulatory goal in the case of cleanups of radioactively contaminated sites, and
the goal for cleanups can be waived if achieving the goal is not feasible (Luftig
and Weinstock 1997~. Therefore, as in the case of the Nuclear Regulatory
Commission's annual dose limit of 1 mSv (100 mrem), EPA's dose constraint
can be modified by ALARA considerations when applied to cleanup of
contaminated sites. In this case, however, the important difference is that EPA's
criterion can be relaxed, whereas the doses allowed by applying the ALARA
objective to Nuclear Regulatory Commission licensees are always lower than
the Nuclear Regulatory Commission's dose limit.
This committee also notes that it is somewhat misleading to label
annual doses approaching 1 mSv (100 mrem) as "acceptable," even though they
are allowed for individual Nuclear Regulatory Commission licensees under
unusual circumstances. The ICRP (1991) has emphasized that annual doses
approaching 1 mSv (100 mrem) are only "barely tolerable" and expects that
doses usually can be reduced to well below barely tolerable levels by the use of
source constraints at less than the dose limit and further site-specific
applications of the ALARA objective. As noted previously, this is the case for
most licensed sources. Doses are properly termed "acceptable" only when they
are below the dose limit and are ALARA.
Application of the Environmental Protection Agency's Groundwater-
Protection Strategy to TENORM:
An important element of EPA's approach to protection of public health
and the environment is its groundwater-protection strategy (EPA 1991b). The
strategy defines protection of groundwater in terms of compliance with
standards (maximum contaminant levels, MCLs) for radionuclides and other
contaminants in public drinking-water supplies (see chapter 7), and it specifies
that human activities today should not cause levels of contamination in
groundwater that would entail later costs for removal if the groundwater is used
as a source of drinking water. The application of MCLs in drinking water as
standards for limiting contamination of groundwater from current operations,
cleanup of contaminated sites, and waste disposal clearly has important
implications for establishing guidances for any radioactive materials. That is
especially so for TENORM because the radionuclides of concern occur naturally
in all groundwaters.
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As in the case of a limit on acceptable dose discussed in the previous
section, application of EPA's groundwater-protection strategy in establishing
guidances for TENORM and other radioactive materials clearly is a matter of
risk-management policy. As discussed in chapter 7, MCLs for naturally
Occurring radionuclides in drinking water are based on considerations of
existing levels in public drinking-water supplies and judgments about the cost
effectiveness of reducing these levels with available technology for water
treatment, but they are not based on an a priori judgment about an acceptable
dose or risk related to exposure to radionuclides in drinking water. Furthermore,
the judgments about levels of radioactivity that are reasonably achievable in
public drinking-water supplies can change (EPA 1991a).
Given the basis for the MCLs for radionuclides in drinking water
described above, it is clear that EPA's groundwater-protection strategy should be
interpreted as defining a goal, rather than a requirement that must be met
without regard for other circumstances. Therefore, application of the
groundwater-protection strategy to guidances for TENORM is justified only to
the extent that compliance with MCLs in groundwater that is a potential source
of drinking water is reasonably achievable for the exposure situations of
concern. In considering levels of contamination in groundwater that are
reasonably achievable for any particular situation, it is important to consider not
only the costs of achieving any particular levels in relation to projected health
risks averted, but also such factors as the costs of primary treatment at the
source in relation to potential future costs of secondary treatment by a water-
supply system, the volume of groundwater that could be affected in excess of
drinking-water standards, the period over which the projected effects could
occur, and the ability of institutional controls to prevent future uses of
contaminated groundwater and the associated costs of such controls.
Differences Between Nuclear Regulatory Commission and Environmental
Protection Agency Approaches to Site-Cleanup Standards
The Nuclear Regulatory Commission recently issued standards for
decontamination and decommissioning of licensed nuclear facilities (Nuclear
Regulatory Commission 1997a) that define radiologic conditions for license
termination and release of sites for unrestricted or restricted use by the public
(see chapter 9~. Sites generally are acceptable for unrestricted use if the annual
effective dose equivalent from all exposure pathways, including use of
groundwater as a source of drinking water, does not exceed 0.25 mSv
(25 mrem) for a period of 1,000 y. Conditions for restricted release also are
specified, and the standards allow for alternative criteria for license termination,
provided that the annual effective dose equivalent from all sources combined
does not exceed 1 mSv (100 mrem).
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EPA has taken strong exception to the Nuclear Regulatory Commission
standards for unrestricted release of contaminated sites (Luttig and Weinstock
1997; Trovato 1997~. EPA believes that the standards are not adequately
protective of human health and the environment in two important respects. First,
the Nuclear Regulatory Commission's annual dose constraint of 0.25 mSv
(25 mrem) for unrestricted release of contaminated sites does not comply with
EPA's lifetime risk objective of 104, which is applied in establishing preliminary
remediation goals under CERCLA. EPA prefers a lower annual dose constraint
of 0.15 mSv (15 mrem) to achieve the risk goal. Second, the Nuclear Regulatory
Commission standards do not include a separate provision for groundwater
protection in accordance with existing standards (MCLs) for public drinking-
water supplies, and compliance with the Nuclear Regulatory Commission's
annual dose constraint of 0.25 mSv (25 mrem) from all exposure pathways
could result in radionuclide concentrations in groundwater in excess of
drinking-water standards. The inclusion of such a provision would be in
accordance with EPA's groundwater-protection strategy discussed above and
with CERCLA and its implementing regulations, which specify that federal
drinking-water standards are applicable or relevant and appropriate
requirements (ARARs) for cleanup of groundwater (see chapter 7~.
On the basis of the discussions in the previous two sections, the
disagreement between EPA and the Nuclear Regulatory Commission over the
adequacy of the Nuclear Regulatory Commission standards for unrestricted
release of contaminated sites clearly is a matter of policy with no scientific or
technical basis. The issue clearly is not whether the Nuclear Regulatory
Commission standards protect human health and the environment because * is
not the case that the resulting risks would be acceptable under EPA's approach
but intolerable under the Nuclear Regulatory Commission's. The difference
between an annual dose of 0.15mSv and 0.25 mSv cannot reasonably be
regarded as substantial, especially when the Nuclear Regulatory Commission
also requires that the ALARA objective be applied in reducing doses below the
specified dose constraint. Furthermore, the difference between an annual dose of
0.15 mSv and 0.25 mSv normally cannot be distinguished reliably in a dose
assessment, given the substantial uncertainties in exposure pathway and
dosimetry modeling. EPA's desire for a separate groundwater-protection
requirement that complies with drinking-water standards also is not based on an
a priori judgment about levels of contamination that are required for protection
of public health without regard for the feasibility of achieving the standards.
Thus, the disagreement between EPA and the Nuclear Regulatory Commission
over appropriate cleanup standards for contaminated sites is entirely a matter of
differences of opinion about reasonable approaches to risk management.
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Differences in Guidances for Indoor Radon
The differences between EPA guidances for indoor radon and the
recommendations of NCRP and ICRP are discussed in chapter 10. EPA's
mitigation level for indoor radon is somewhat more restrictive than those
recommended by NCRP and ICRP. This committee reiterates that these
differences do not result from differences in the scientific and technical basis for
the guidances. Rather, they result primarily from EPA's greater emphasis on
reducing risks in the whole population on the basis of cost-benefit analysis,
whereas the NCRP and ICRP guidances were based primarily on a concern for
reducing exposures of the relatively few people who experience the highest
risks.
Difference Between Environmental Protection Agency and National
Council on Radiation Protection and Measurements Guidances for
TENORM Other Than Indoor Radon
As discussed in chapter 10, EPA has issued proposed federal guidance
on radiation protection of the public that includes an annual dose limit of 1 mSv
(100 mrem) for all controlled sources combined, including human-made
radionuclides and TENORM other than indoor radon (EPA 1994d).
Furthermore, the proposed guidance specifies that the annual dose from
individual sources or practices, including individual sources of exposure to
TENORM other than indoor radon, should be limited to less than 1 mSv (100
mrem). In contrast, NCRP's recommended annual dose limit of 1 mSv (100
mrem) per year for members of the public (NCRP 1993a) does not apply to
TENONS. Rather, the NCRP developed a separate recommendation that
remedial actions be undertaken when the annual dose from exposure to natural
sources only, including undisturbed natural background and TENORM other
than indoor radon, exceeds 5 mSv (500 mrem). Therefore, although a direct
comparison of the two guidances is not straightforward, the proposed EPA
guidance, which applies to all sources of exposure to TENORM combined,
should in most cases be considerably more restrictive than NCRP's
recommended remedial-action level.
The difference between EPA and NCRP guidances for TENORM other
than indoor radon does not result from differences in the scientific and technical
basis of the guidances, in that both organizations assumed essentially the same
risk related to radiation exposure for purposes of establishing the guidance.
Rather, the difference results from differences in the approaches to risk
management for TENORM. EPA regards TENORM other than indoor radon as
a type of controlled source similar to sources of human-made radionuclides, so
exposures to TENORM other than indoor radon are included in radiation
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239
protection guidance that applies to human-made sources. However, NCRP
regards TENORM as an enhanced form of natural background which should be
treated separately from human-made sources for purposes of radiation
protection. In addition, the difference between EPA's annual dose limit of 1 mSv
(100 mrem) and NCRP's remedial action level of 5 mSv (500 mrem) reflects a
difference in judgment about acceptable risks related to exposure to TENORM.
Again, judgments about acceptable risk are strictly matters of policy.
Treatment of Natural Background in Establishing Guidances
Natural background radiation has played various roles in establishing
guidances for control of exposure to radionuclides in the environment,
depending primarily on whether or not the particular guidance applies to
naturally occurring radionuclides (see chapters 5 and 7~. EPA regulations that
apply only to specific sources or practices involving human-made radionuclides
generally do not take into account the magnitude and variability of natural
background, because standards that were judged to provide an acceptable risk or
to be reasonably achievable did not consider exposure to natural background.
However, the annual dose limit of 1 mSv (100 mrem) for all controlled sources
combined, including human-made radionuclides and TENORM other than
indoor radon, in EPA's proposed federal guidance on radiation protection of the
public (EPA 1994d), although it excludes exposures to natural background, was
developed in recognition of the magnitude and variability of natural background
(NCRP 1993a; ICRP 1 99 1~.
Natural background is important in developing guidances that apply to
naturally occurring radionuclides. Current guidances for alpha-emitting
radionuclides in drinking water, uranium and thorium mill tailings, and indoor
radon are concerned only with naturally occurring radionuclides, and the
development of guidances for these situations clearly required consideration of
background levels of the radionuclides of concern. In the case of alpha-emitters
in drinking water, the controllable exposures are due almost entirely to natural
levels of radionuclides in groundwater or surface water; in the case of mill
tailings and indoor radon, the levels of natural background provide a floor for
any standards because the levels of the radionuclides of concern cannot be
reduced below background.
Background also has been taken into account in different ways even for
the same exposure situation involving naturally occurring radionuclides. A case
in point involves guidances for indoor radon at uranium mill tailings sites. The
initial EPA guidelines for homes built on sites contaminated with uranium mill
tailings in Colorado specified remedial-action levels in excess of background
(Harley 1996~. When these guidelines were incorporated into EPA's uranium
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ISSUESINDEVELOPING GUIDANCES
mill tailings standards in 40 CFR Part 192 (see chapter 7), the remedial-action
level for indoor radon included background.
The issue of the most appropriate way of taking natural background
into account in establishing guidances for radiation exposure is particularly
important for TENORM other than indoor radon. As indicated by the
discussions in the previous section, two approaches could be taken. Exposures
to TENORM could be regulated without regard for the magnitude and
variability of natural background, even though all radionuclides of concern are
part of natural background. This approach is embodied, for example, in EPA's
proposed federal guidance on radiation protection of the public (EPA 1994d)
and the current guidance on cleanup of contaminated sites (Luftig and
Weinstock 1997~. Or, guidance could be developed for exposure to TENORM
other than indoor radon and natural background combined; this is the approach
recommended by NCRP (1993a).
Both approaches have advantages and disadvantages. The advantage of
regulating without regard for the magnitude and variability of natural
background is that controlled sources of exposure to TENORM would be
regulated in the same way as human-made radionuclides; this would provide a
desirable consistency in regulating all controlled sources. The disadvantage is
that naturally occurring radionuclides resulting from human activities must be
distinguishable from the undisturbed background of the same radionuclides. The
distinction can be made if the difference between the levels of TENORM and
natural background is sufficiently high, but the ability to measure TENORM
with confidence depends on the magnitude and variability of background.
Indeed, in some cases, it might be difficult to measure TENORM corresponding
to low doses and risks, such as annual doses of 0.15 mSv (15 mrem) or lifetime
risks of 10 4. That disadvantage would probably be particularly important in
establishing guidances for TENORM in soil, given the doses and risks
associated with undisturbed natural background (see, for example, tables 2.8,
2.9 and 2.10).
Conversely, developing guidances for TENORM that include natural
background has the disadvantage that controlled sources of TENORM would be
regulated differently from human-made radionuclides. An advantage is that
there would be no need to distinguish between TENORM and natural
background; this could reduce the difficulties in verifying compliance with
standards by means of environmental measurements.
Regardless of the approach used in taking natural background into
account in developing guidances for TENORM, there is no fundamental
scientific or technical basis for the choice. The choice would be based on risk-
management policy and on considerations of the practicality of implementing
the guidance, especially the ability to verify compliance by means of
environmental measurements.
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IMPLICATIONS OF GUIDANCES FOR RISK ASSESSMENT
241
The particular form that a guidance for TENORM might take has
important implications for risk assessment, particularly with regard to issues that
would need to be addressed in developing the guidance and issues that would be
addressed in demonstrating compliance. That concern arises only with
guidances for TENORM other than indoor radon, because of the availability of
epidemiologic data that directly link concentrations of radon decay products in
an exposure environment with increased risks of lung cancer.
This committee assumes that a guidance for TENORM other than
indoor radon could be expressed in one of three ways: a limit on acceptable risk,
a limit on acceptable dose, or limits on acceptable concentrations of
radionuclides in various environmental media. Each has different implications
for risk assessment.
If a guidance is expressed in terms of a limit on acceptable risk, all that
is required in establishing the guidance, in principle, is a judgment about an
acceptable risk for the exposure situations of concern. All issues for risk
assessment could be addressed in demonstrating compliance with the limit. In
practice, however, risk assessments normally would be used in developing
guidances expressed only in terms of acceptable risk. For example, such
assessments are required by the National Environmental Policy Act whenever a
guidance would have substantial economic or environmental effects. In addition,
some type of risk assessment normally would be needed to demonstrate that a
proposed risk standard is reasonably achievable.
If a guidance is expressed in terms of a limit on acceptable dose that is
based on the objective of achieving a particular risk, the one issue for risk
assessment that would need to be addressed in developing the standard is the
numerical value of the risk per unit dose. As indicated above, EPA normally
uses the standard assumption for risk of 5 x 10-2 per sievert in establishing a dose
standard based on a limit on acceptable risk. However, particularly in the case of
TENORM, where only a few radionuclides are important, EPA could develop
radionuclide-specif~c risk factors by using the methods discussed earlier,
although this option would be attractive only if internal exposure to long-lived
alpha-emitting radionuclides were more important than external exposure. With
a dose standard, all other issues of risk assessment, particularly assessments of
exposure pathways and the dose per unit exposure, would be considered in
demonstrations of compliance.
Finally, if a guidance is expressed in terms of limits on acceptable
concentrations of radionuclides in the environment, which are directly
measurable, all issues of risk assessment including exposure-pathway analysis,
estimates of dose per unit exposure, and the approach to estimating risk must
be addressed in developing the standard, but none would need to be considered
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in demonstrations of compliance. This approach would allow the greatest
opportunity for applying EPA's more rigorous methods of risk assessment
discussed above in developing guidances for TENORM. However, it could be a
considerable challenge to develop a standard expressed in terms of measurable
quantities that reasonably could be applied to the variety of exposure situations
of potential concern. Such complexity makes a standard expressed in terms of
concentrations of radionuclides in the environment less attractive than a dose
standard, which is the usual approach.
The particular form of guidances for TENORM that would be the most
appropriate means of providing protection of human health and the environment
is largely a matter of judgment, and there is no scientific or technical basis for
the choice. The important concerns in choosing the particular form of any
guidance include clarity of the regulatory approach, ease of implementation, and
consistency with the approach used in other regulations, including those for
human-made radionuclides.
Representative terms from entire chapter:
regulatory commission
