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2
PROTECTING HUMAN HEALTH
The primary objective of the proposed repository at Yucca
Mountain is to dispose of high-level raciioactive defense waste and spent
nuclear fuel in a safe manner. To determine whether the repository can be
designed to protect the public health from the risks associates! with
exposure to radiation from rarIionuclides that may be releasecI from the
repository, it is necessary to establish stanciards against which to judge
whether the design of the repository is acceptable. This target will be
embocliec] in a racliation protection standard to be issued by EPA.
In Section 801 of the Energy Policy Act of 1992, Congress directs
that EPA set these standards by specifying the maximum annual effective
close equivalent to indiviclual members of the public. In the same section,
Congress also asks three questions, the first of which is:
whether a health-based stanciard based on closes to
individual members of the public from raclionuclide
releases to the accessible environment . . . will provide a
reasonable standard for the protection of the health and
safety of the general public.
This chapter addresses this question. As background, we first
present a synopsis of the health effects of ionizing radiation and outline the
development of radiation protection standards on a national and
international basis. This discussion will illustrate the current status of
scientific investigation ant] consensus of expert judgment on which most
efforts to establish a stan~iard for high-level waste repositories are based.
We then turn to the question of whether a standard for Yucca
Mountain designed to protect individuals will, if met, also protect the
general public. We conclucle that the answer to this question is "yes,"
given the particular characteristics of the site ant! assuming that policy
makers ant! the public are prepared to accept that very low radiation doses
pose a negligible risk.
Because the current EPA standard for nuclear waste disposal in 40
CFR 191 takes an approach different from that required by Congress,
33
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34
YUCCA MOUNTAIN STANDARDS
:~ If:::
however, addressing only the question posed in Section 801 is too narrow
a response. Accordingly, we have expanded the discussion by
recommending the use of a standard} designed to limit individual risk rather
than individual dose and by ciescribing how a standard might be structured
on this basis. We then address the specific question of protection of public
health in the context of an individual-risk standard and compare this
standard with the one currently user! by EPA for sites other than Yucca
Mountain. Baser! on this analysis, we conclucle not only that an individual-
risk standard would protect the health of the general public, but also that
this form of standard is particularly appropriate for the Yucca Mountain
site in light of the site's characteristics.
Finally, standards are only useful if it is possible to make
meaningful assessments of future repository performance with which the
standarcis can be compared. In Chapter 3, we discuss our conclusion that
it is feasible to conduct such compliance assessments against an inclividual-
risk standard. Doing so, however, requires using the rulemaking process to
arrive at a regulatory decision about certain assumptions as part of the
standard, for example., about future human behavior. In the following
discussion of the standard, we have indicated the assumptions for which
this is required.
THE HEALTH EFFECTS OF IONIZING RADIATION
Cell and gene damage can be caused in humans exposed to
ionizing radiation (NRC, 1990a), (also referred to as the BEIR V report).
Extremely high closes of radiation can lead to quick death, as seen, for
example, in Nagasaki, Hiroshima, and Chernobyl. However, even much
lower levels of radiation can affect health. International scientific bodies
currently accept what is called the linear, or no-threshold hypothesis for the
tiose-response relationship. Most of what is known about effects of
radiation on human health comes from studying people exposed to large
doses of radiation. The empirical relationship between cancer induction
and radiation dose appears linear at the high doses received by the atomic
bomb survivors. The linear hypothesis postulates that this dose-response
relationship continues when extrapolated to very low doses. The no-
threshoic} hypothesis hoists that there is no dose, no matter how small, that
does not have the potential for causing health effects. To explain this
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PROTECTING HUMAN HEALTH
35
relationship of radiation to cancer, ant! other health effects, the following
outlines the interaction between radiation and the human body.
Radiation that is sufficiently energetic to clisIo~ige electrons from
an atom is referred to as ionizing racliation. Impinging ionizing radiation,
colliding with atoms en c! molecules in its path, gives rise to ions ant! free
radicals that break chemical bonds and cause other molecular alterations
in affected cells. Any molecule in the cell can be altered by racliation, but
deoxyribonucleic acid (DNA), the clouble helix of base pairs that make up
the genes to be passer! on to the next generation, is the most critical
molecular target because of the uniquely important genetic information it
contains. Damage to a single gene, which might consist of thousands of
base pairs, can profouncily alter or kill the cell. Although millions of
changes in DNA are proclucec! in the body of every person each year by
exposure to natural background radiation ant] other influences, most of the
changes are reparable. If unrepairec! or misrepaired, however, the damage
might be expressed in the form of permanent genetic changes or mutations,
the frequency of which approximates 10-5 to IO6 per gene per Sievert
(Sv)~. Because the mutation rate tencIs to change in direct proportion to the
dose, it is inferred that the interaction of the gene with a single ionizing
particle might suffice in principle to mutate the gene. Damage to the
genetic apparatus of a cell can also cause changes in the number or
structure of its chromosomes, the thread-like structures on which the genes
are arranged. Such changes increase in frequency in proportion to the (lose
in the range below ~ Sv.
Radiation ciamage to genes, chromosomes, or other vital organelles
can be lethal to affected cells, especially dividing cells, which are highly
radiosensitive as a class. The survival of dividing cells, measured in terms
of their capacity to grow and divide, tencis to decrease exponentially with
increasing close, 1-2 Sv generally sufficing to recluce the surviving cell
population by about 50% (NRC, ~ 990a). The killing of cells, if sufficiently
extensive, can impair the function of the affected organ or tissue. In
general, however, too few cells are killed by a close below 0.5 Sv to cause
clinically detectable impairment of function in most human organs other
than those of the embryo. Because such effects on organ function are not
produced unless the radiation dose exceeds an appreciable threshoici, they
A unit of equivalent radiation dose, a Sievert is the product of the absorbed dose
and the radiation weighting factor. 1 Sievert equals 100 rem.
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YUCCA MOUNTAIN STANDARDS
are commonly viewer! as nonstochastic (or deterministic) effects, in
contradistinction to mutagenic effects of racliation, which are viewed as
stochastic effects because they might have no thresholds (see Glossary).
Carcinogenic effects of radiation, which can result from mutational
changes in the affected cells, are likewise viewer] as stochastic effects, the
frequency of which is assumed to increase as a linear, no-threshoIc]
function of the dose, although the possible existence of a threshold for such
effects cannot be excluded.
Natural background! radiation is estimated by the National Council
on Radiation Protection ant! Measurements (NCRP) to contribute 82% of
the average annual radiation exposure to a Uniter] States citizen, and
medical applications, an additional 15% (NCRP, 1987a). All other sources
of radiation exposure together contribute approximately 3% (Table 2-1~.
All sources combined give an average dose of 3.6 mSv/yr (360 mrem/yr).
Background radiation levels are not uniform. For example, the average
difference in background radiation between Denver, CO and Washington,
DC, is 0.3 mSv/yr (30 mrem/yr). One cross-country plane ride contributes
approximately 0.025 mSv (2.5 mrem) (NCRP, 1987a,b).
At the low-dose rates characteristic of natural background radiation
or occupational irradiation, the only health effects of radiation to be
expected are stochastic effects; that is, mutagenic and carcinogenic effects.
Although the risks of certain cancers have been significantly elevated in
some cohorts of radiation workers, especially those employed in the era
preceding modern safety standards, no definite or consistent evidence of
carcinogenic effects has been observed in workers exposed within present
maximum permissible dose limits or in populations residing in areas of
high natural background radiation. Hence, assessment of any cancer risks
attributable to irradiation in such populations must be based on
extrapolation from observations of the effects of exposure at higher dose
levels. Because a statistically significant increase in heritable
abnormalities is yet to be demonstrated in human beings at any dose level,
assessment of the risks of such effects must be based on extrapolation from
observations on laboratory animals. Because of the assumptions inherent
in the extrapolations that are involved, assessments of the carcinogenic and
mutagenic effects of low-level irradiation are highly uncertain. The
uncertainties notwithstanding, it has been possible to reach a reasonable
consensus within the scientific community on the relationship between
doses ant! health effects, that is generally considered to provide an
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PROTECTING HUMAN HEALTH
37
acceptable basis for evaluating the risks attributable to a given dose or the
degree of protection affordeci by a given limitation of exposure.
Within recent years, the risks attributable to low-level irradiation
have been assesses! in detail by the Uniter! Nations Scientific Committee
on the Effects of Atomic Radiation (UNSCEAR, 1988), the National
Research Council Committee on the Biological Effects of Ionizing
Radiation (NRC, 1 990a), ant] the International Commission on
Radiological Protection (ICRP, 19911. The last of these assessments,
which cirew on ant} extender} the previous two, arrived at risk assessments
for carcinogenic effects and for heritable effects, which are shown in Table
2-2. Carcinogenic effects, which are expresser! only in exposed inclivi~iuals
themselves, are estimated to account for the bulk (80%) of the overall risk
of harm. The lifetime risk of developing a fatal cancer from irradiation is
estimated to be 5 x 10~2/Sv for a member of the general population.
Nonfatal cancers, although projected to be pro~iucect more frequently than
fatal cancers, were judged to contribute less to the overall health impact of
irradiation because of their lesser severity in affecter! individuals ant! were,
therefore, weighted accordingly (Table 2-21. Of the total risk of heritable
effects, about one-fourth is projected to be expressed in the first two
generations alone, the remainder during subsequent scores of generations.
This table indicates that if ~ 00 people were each to receive ~ Sv of
radiation over their lifetimes, which is about 300 times greater than the
overall average annual natural background level of radiation in the United
States, five wouIct be expecter! to die from cancer induced by that radiation.
Since it accounts for the great bulk of the potential harm that might be
attributed to low-level radiation, the above risk estimate for fatal cancer is
often used to calculate the expecter! number of fatalities attributable to low-
dose irradiation in a population. For example, if one million persons were
each exposed to a dose equivalent to that received from a transcontinental
plane ride (0.025 mSv), the resulting collective dose (25 person-Sv) would
be estimated to cause one extra fatal cancer in the population in addition
to the 200,000 fatal cancers that would! be expected to occur in the same
population from all other causes combined. Because the added risk, if any,
is calculated to be such a small fraction of the total cancer risk, it is not
surprising that epidemiological data have revealeci no significant
differences in the rates of cancer or other diseases among populations
exposed to far larger variations in natural background} radiation levels
(NRC 1990a).
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YUCCA MOUNTAIN STANDARDS
Table 2-l Average Amounts of ionizing Radiation Received Yearly
by a Member of the U.S. Populationa
Source
Doseb
(mSv/yr) (°/0)
Natural
Radons 2.0 55
Cosmic 0.27
Terrestrial 0.28
Internal 0.39 ~ ~
Total Natural 3.0 82
Anthropogenic
Medical
X-ray diagnosis 0.39
Nuclear medicine 0.14
Consumer products 0.10
Occupational ~ 0.01
Nuclear fuel cycle ~ 0.01
Nuclear fallout < 0.01
Miscellaneous ~ 0.01
Total anthropogenic
Total Natural ant}
Anthropogenic
11
<0.3
0.63
< 0.03
< 0.03
< 0.03
18
3.6 100
a From NRC (199Oa) and NCRP (1987a)
b Average effective dose equivalent
c Dose to bronchial epithelium alone
d DOE facilities, smelters, transportation, etc.
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PROTECTING HUMAN HEALTH
39
Table2-2. Estimated Frequencies of Radiation-Induced Fatal
Cancers, Nonfatal Cancers, and Severe Hereditary
Disorders, Weighted for the Severity of their Impacts on
Affected Individualsa
No. of cases per
100
per Svb
Fatal cancers
Nonfatal cancers
Severe heredity disorders
Total
5.0
1.0
1.3
7.3
a From ICRP (1 99 1 ~
b Numbers of cases, weighted for severity of their impacts on affected individuals
over their lifetimes, attributable to low-level Radiation of a population of all ages.
DEVELOPMENT OF RADIATION PROTECTION STANDARDS
There is a worIc~wide interest in the development of radiation
protection standards, including those for the disposal of high-level
radioactive waste, and a consiclerable belly of analysis anti informer}
judgment exists from which to draw in formulating a standard for the
proposed Yucca Mountain repository. EPA's process for setting the Yucca
Mountain standard is presumably not bound by this experience, but a sound
technical approach should include a review of other relevant work to date.
Accorciingly, we summarize below the status of relevant work on racliation
protection standards both in the Uniter! States and abroad.
General Consensus in Radiation Protection Principles and
Standards
A number of international ant! nongovernmental national bodies
(such as the International Atomic Energy Agency (IAEA), ICRP and
NCRP) have recommended radiation protection principles ant! standards.
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YUCCA MOUNTAIN STANDERS
These recommendations, in turn, usually are considered by the national
agencies that set radiation protection standards, which then are codified
into pertinent rules and regulations. Of the international bodies, the
International Commission on Radiological Protection (ICRP) is perhaps the
most influential. Its counterpart in the U.S. is the National Council on
Radiation Protection and Measurements (NCRP).
In the United States, several agencies establish radiation protection
standards in their areas of responsibility. Among them are the following:
the U.S. Environmental Protection Agency (EPA), the U.S. Nuclear
Regulatory Commission (USNRC), ant! the U.S. Department of Energy
(DOE). These three agencies play key roles in programs involving public
health and safety, environmental protection, health ant! safety in the
nuclear industry, and radioactive waste management and disposal.
Recommendations for radiation standards to protect the public
health and safety are frequently based on the analyses of radiation risks
clevelopeci by the United Nations Scientific Committee on the Effects of
Atomic Radiation (UNSCEAR) and the TCRP on the international level ant]
by the Committees on Biological Effects of Ionizing Radiation (BEIR) in
the United States. The most recent analyses are presented in the
UNSCEAR (19X8) and NRC (1990a) reports, respectively.
Concurrent with the development of radiation protection concepts
internationally and in this country, a consensus has emerged among the
organizations involves! in performing analyses and making
recommendations (ICRP, NCRP, NRC's BEIR V, and UNSCEAR) anti
those that promulgate regulations (EPA, USNRC, and DOE). This
coalescence of views and resulting consensus can be seen in the general
uniformity in the system of radiation close limitation, fundamental units
and terminology, health effects factors, occupational and public dose
limits, dose apportionment, anti use of the critical-group concept. The
latter two concepts are defined and discussed later in this chapter.
Consistent with the current understanding of the related
consequences, ICRP, NCRP, IAEA, UNSCEAR, and others have
recommended that radiation doses above background levels to members of
the public not exceed 1 mSv/yr (100 mrem/yr) effective close for
continuous or frequent exposure from radiation sources other than medical
exposures. Countries that have considered national radiation protection
standards in this area have endorsed the ICRP recommendation of 1 mSv
per year radiation dose limit above natural background radiation for
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PROTECTING HUMANHEALTH
41
members of the public. In the Uniter! States, DOE, in Order 5400.5, and
USNRC, in 10 CFR 20, have set the close stanciarc! for public exposure to
ionizing radiation at 1 mSv per year above natural background] level. EPA
is in the process of cleveloping similar guidance for all U.S. federal
agencies (EPA, 19931.
This framework, with an effective dose limit of ~ mSv per year, is
user] as a basis for protecting the public health from routine or expected
anthropogenic sources of ionizing radiation (i.e., resulting from human
activity) other than medical exposures. It inclucles any exposures to the
public derives! from the management and storage of high-level radioactive
defense waste ant! spent nuclear fuel. We note that guidance to date has
been for expecter] exposures from actual routine practices. There is little
guidance on potential exposures in the far distant future.
{CRP (1985a) proposeci apportionment of the total allowable
radiation close from all anthropogenic sources of racliation, excluding
medical exposures. Thus, for radioactive waste management, including
high-level radioactive defense waste and spent nuclear fuel, the national
authorities coup! apportion, or allocate, a Faction of the 1 mSv per year to
establish an exposure limit for high-level waste facilities. EPA in 40 CFR
191 notes} that its requirement for the WIPP transuranic waste facility, at
a level of 0.15 mSv/yr (l 5 mrem/yr), is consistent with ICRP's concept of
apportionment.
Most other countries also have endorsed the principle of
apportionment of the total allowed radiation close. Apportionment values
that have been established by various countries for high-level radioactive
waste range from 5% to 30%, corresponding to radiation doses ranging
from 0.05 mSv (5 mrem) per year to 0.3 mSv (30 mrem) per year.
.
Table 2-3 presents the limits established by various countries on
individual exposure from high-level waste disposal facilities. The
information in this table suggests a general consensus among national
authorities and agencies to accept and use the principle of radiation dose
apportionment.
THE FORM; OF THE STANDARD
A standard is a societally acceptable limit on some aspect of
repository performance that should not be exceeded if the repository is to
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YUCCA MOUNTAIN STANDARDS
be judges! safe. There is, however, a variety of ways in which this limit can
be formulated. It can, for example, be imposer! at several points in the
chain of events that might ultimately lead to adverse effects on public
health. Thus, the limit could apply to the amount of radionucliries released
from the repository, to the racliation doses to persons resulting from those
releases, to the number of health effects associated with the doses, or to the
level of risk. Risk, close, or health effect limits can be states! for
inclivicluals or for populations.
We recommend the use of a stanciard that sets a limit on the risk to
inclividuals of adverse health effects from releases from the repository. In
this context, risk is the probability of an individual receiving an adverse
health effect. it is essential to define specifically how to calculate this risk,
however, for otherwise it will not be clear what number to use to compare
with the risk limit established in the standards.
From the scientific perspective, the calculation of health risks
should take into account all of the uncertainties involved in analyzing
repository performance over very long time periods. Because many of the
elements of the calculation are not well known, they must be dealt with by
using distributions that represent the analysts's state-of-knowledge. The
first step in calculating risk is therefore to clevelop a distribution of closes
received by indivicluals, taking into account all of the events that go into
determining whether a dose is receiveci.2 A probabilistic distribution of the
health effects associated with these closes can then be cleveloped as the
product of each value of close received and the health effects per unit dose.
In this report, we choose to define risk as the expected value of the
probabilistic distribution of health effects.3
2 This does not mean that every event needs to be treated probabilistically; some
might be represented by a single bounding estimate, for example. The
definition does require, however, that all of the parameters that determine the
dose be considered in developing the probabilistic distribution of dose.
3 It is both easier and common practice to calculate doses received over an
individual lifetime. One reason is that the effects of radiation might not appear
until years after the dose is received. The lifetime calculation can be
annualized by dividing by the duration of an average lifetime. Since this
annualized risk is often more convenient for comparison to other risks, we
recommend it be used.
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PROTECTING HUAtINHEALTH
Table 2-3 Quantitative High-Leve' Waste Disposal
Objectives/Criteria at International Level and in
OECD Countriesa
Organization/Cou Main Other Main Comments
ntry Objective/Crite Maturers)
ria
NEA (1984) Max. indiv. risk Individual No consensus on
objective risk/dose= best ALARAJoptimiz
10-5/yr criterion to judge ation
(all sources) long-term
acceptability
ICRP 1 mSv/yr Both prob. and ALARA useful,
Publication 46 (normal doses should be notably to
(1985) evolution taken into compare
scenarios) account in alternatives, but
1 0~5/yr ALARA might not be the
(probabilistic most important
scenarios) for siting factor
individuals (all
sources)
IAEA ICRP Also includes .
Safety Series 99 Publication 46 qualitative .
(1989) technical criteria .
on disposal
system features
and role of
safety analysis
and quality
assurance
CANADA Max. indiv. risk Period of time Additional
AECB regul. obj. 1 0~6/yr for qualitative,
Document R. 104 demonstrating nonprescriptive
(1987) 104yr requirement and
No sudden and guidelines in
dramatic regulatory
increase for documents
times > 1 04yr No explicit
optimization
required
43
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YUCCA MOUNTAIN STANDARDS
occur. We recognize that there are significant uncertainties in the
supporting calculations and that the uncertainties increase as the time at
which peak risk occurs increases. However, we see no technical basis for
limiting the period of concern to a period that is short compared to the time
of peak risk or the anticipated travel time.
Nevertheless, we note that although the selection of a time perioc!
of applicability has scientific elements, it also has policy aspects that we
have not addresseci. For example, EPA might choose to establish
consistent policies for managing risks from disposal of both long-lived
hazardous nonradioactive materials and radioactive materials.
Another time-relatec! regulatory concern can affect the formulation
of the safety stanciarci. This is based on ethical principles, and is the issue
of intergenerational equity (Berkovitz, 1992; Hol(lren, 1992; Okrent, 19941.
Whether and how best to be fair to future generations is an important
societal question. Although current generations are assumer} to have
benefited from activities, such as electricity production or national defense
programs that have causer] radioactive wastes to accumulate, far future
generations will not benefit clirectly, but might be exposed to risks when
any radioactive materials eventually escape the proposed repository. In
crafting standards, EPA should as a matter of policy address whether future
generations should have less, greater, or equivalent protection.
The responsible institutions have considered the question of the
protection to be afforded future generations. For example, in her
presentation to us, Margaret Federline (USNRC, personal communication,
May 27, 1993) spoke about a "societal pledge to future generations" that
wouIc! "provide future societies with the same protection from radiation we
wouic! expect for ourselves." The {AEA document, Safety Principles ant]
Technical Criteria for HLW Disposal, Safety Series 99, has as one
objective the "responsibility to future generations." Under this
responsibility to future generations, IAEA recommends that "the degree of
isolation of high-level radioactive waste shall be such so there are no
predictable future risks to human health or effects on the environment that
would not be acceptable today." In this IAEA establishes that "[tithe level
of protection to be afforded to future individuals should not be less than
that provided today."
A health-based risk standard could be specified to apply uniformly
over time and generations. Such an approach wouici be consistent with the
principle of intergenerational equity that requires that the risks to future
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PROTECTING HUMAN HEALTH
57
generations be no greater than the risks that wouIc! be accepted today.
Whether to aclopt this or some other expression of the principle of
intergenerational equity is a matter for social judgment.
PROTECTING THE GENERAL PUBLIC
Earlier in this chapter, we recommend the form for a Yucca
Mountain standard! based on inclividual risk. Congress has asked whether
standards intended to protect individuals wouIc! also protect the general
public in the case of Yucca Mountain. We conclude that the form of the
standards we have recommendecl would do so, provider] that policy makers
and the public are prepared to accept that very low radiation doses pose a
negligibly small risk. This latter requirement exists for all forms of the
stan~iarcis, including that in 40 CFR 19 ~ . We recommenc! adciressing this
problem by adopting the principle of negligible incremental risk to
individuals.
The question posed by Congress is important because limiting
individual dose or risk lions not automatically guarantee that adequate
protection is provider] to the general public for all possible repository sites
or for the Yucca Mountain site in particular. As ciescribed in the previous
section, the individual-risk stanciarc! should be constructed! explicitly to
protect a critical group that is composed of a few persons most at risk from
releases from the repository. The standards are then set to limit the risk to
the average member of that group. Larger populations outside the critical
group might also be exposed to a lower, but still significant, risk. It is
possible that a higher level of protection for this population represented by
a lower level of risk than the one established by the standards might be
considered.
For purposes of this discussion, the "general public" can be
thought of as inciu~iing global (hemispheric or continental) populations that
might receive very small risks from repository releases, as well as local
populations that lie outside the critical group but that might still be expose(l
to risks not much lower than those imposed on the critical group. The
issues are different for these two types of populations, and we discuss them
separately.
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YUCCA MOUNTAII;STAND~=S
PROTECTING THE GLOBAL POPULATION
Radiation releases from a repository can in principle be distributed
to a global, or other large and dispersed population, in several ways. For
example, food contaminated by raclionuclicles could be shipped to regions
far from the repository area, or contaminates! ground] water could enter a
major river and the drinking water supplies that it serves. The global
distribution of releases from a repository is assumed as the exposure
scenario for the containment requirements in EPA's regulation 40 CFR 191.
In the case of Yucca Mountain, there would be no releases to major rivers,
ant! therefore the most likely pathways for global distribution are gaseous
releases of carbon dioxide containing the radioactive isotope of carbon,
i4C, that eventually will escape from the waste canisters, or by widespread
distribution of foodstuffs grown with contaminated water.
In general, the risks of radiation proclucec! by such wicle dispersion
are likely to be several orders of magnitude below those to a local critical
group. As noted earlier in this chapter, however, the "linear hypothesis"
implies that even very small increments to background closes might cause
effects from cancer induction in the same ratio (5xI0-2/Sv) as larger doses.
Using the linear hypothesis to calculate the effects of very low doses on
large populations requires multiplying this factor by the cumulative dose
imposed on populations numbered] in the trillions over the life of the
repository.
There are, however, important cautions to be noted with this
procedure. With respect to small increments to natural background]
radiation levels, the BEIR V report (NRC 1990a) states that:
Finally, it must be recognized that derivation of risk
estimates for low doses and dose rates through the use of
any type of risk mode] involves assumptions that remain
to be validated. At low doses, a mode! dependent
interpolation is involved between the spontaneous
incipience and the incipience at the lowest doses for which
data are available. Since the committee's preferrer! risk
models are a linear function of dose, little uncertainty
should be introducer! on this account, but departure from
linearity cannot be excluder! at low closes below the range
of observation. Such departures could be in the direction
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PROTECTING HUMAN HEALTH
of either an increased or decreased risk. Moreover,
epidemiologic data cannot rigorously exclude the
existence of a threshold in the millisievert dose range.
Thus the possibility that there may be no risks from
exposures comparable to external natural background
radiation cannot be ruled out. At such low doses and dose
rates, it must be acknowledged that the lower limit of the
range of uncertainty in the risk estimates extends to zero.8
59
The doses to global populations involved in gaseous release from
Yucca Mountain are likely to be well below the mSv range noted in BEIR
V. For example, let us assume that the repository inventory of 91,000 Ci
(3.37 x 10~5 Bq) (Wilson et al., 1994) of i4C is released into the air over
10,000 years. Using EPA's dose conversion factor 1.1 x 10-~° Sv/Bq
(EPA, 1992), the population dose over 10,000 years would be 3.7 x 105
person-Sv, or an average of 37 person-Sv/year over the 10,000-year period
(Nygaard et al., 19931. Assuming that the ~4C is well mixed with air over
the globe, and for an average global population of 12 billion people during
this period, the corresponding average individual dose rate is 3.1 x lO~9
Sv/yr (3.1 x 10-4 mrem/yr). For comparison, the dose set by EPA in 40
CFR 191 is I.5 x 10~4 Sv/yr (15 mrem/yr), and this is the limit to be
applied for the persons likely to receive the highest doses from the
repository. Therefore, there is great uncertainty about the number of health
effects that would be imposed on the global population because of the
difficulties in interpreting the risks associated with such small incremental
risks from ~4C releases at Yucca Mountain.
NEGLIGIBLE INCREMENTAL RISK
To acidress scenarios of widespread but extremely low-level doses,
the radiation protection community has introduced the concept of
negligible individual dose. The negligible individual dose is defined as a
level of effective dose that can, for radiation protection purposes, be
dismissed from consideration. NCRP has recommended a value of 0.01
In this paragraph "low doses" applies to very small increments to the dose from
the natural background.
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YUCCA MOUNTAIN STANDARDS
mSv/yr (I mrem/yr) per radiation source or practice (NCRP, 1993), which
currently would corresponci to a projected risk of about 5 x 1 0~7/yr for fatal
cancers, assuming the linear hypothesis. In its considerations, NCRP
clecideri on this level of close or risk taking into account risk in relation to:
l . Natural risk of the same health effects;
2. Risk to which people are accustomed;
3. Estimated risk for the mean and variance of natural
background radiation exposure levels;
4. Perception of, and behavioral response to, risk levels; ant!
5. Difficulty in detection ant} measurement of dose anti health
effects.
Others over the years have advocated the use of a negligible otiose
or risk level (Comer, 1979; Eisenbud, 1981; Schiager et al., 198619. The
general consensus of these authorities was that a negligible value would
be useful in many applications. Federal and state approaches for the
regulation of chemical carcinogens are in keeping with this view, which
generally take a 10-6 lifetime risk as an acceptable level (Travis et al.,
1987; EPA, 1991), as are the exposure limits for radioactive waste adopted
by most nations in the Organization for Economic Cooperation and
Development (OECD) (Dejonghe, 1993~. The Federal German Radiation
Protection Commission, for example, has recommender! ignoring
individual closes of less than 0.003 mSv per year (Smith and Hocigkinson,
19881.~°
We believe that the concept of a negligible incremental dose can
be extended to risk anti can be applied to Yucca Mountain. Defining the
level of incremental risk that is negligible is a policy judgment. We
suggest the risk equivalent of the negligible incremental dose
recommended by the NCRP as a reasonable starting point for cleveloping
consensus in a rulemaking process. For example, the average dose to a
member of the global population from exposure to i4C from the repository
9 Where authors use "negligible dose" or "negligible risk" the teens should be
understood as increments to the unavoidable background radiation. In life, there
is no zero dose and no zero risk.
id Note that this is equivalent to an annual risk of fatal cancer of about 1 .SxlO~7/yr.
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PROTECTING HUMAN HEALTH
61
is estimated to be about 3 x 10-9 Sv/yr, corresponding to a risk of fatal
cancer of I.5 x 10~~°/yr or about 1~8 per lifetime. As indicated earlier,
NCRP has recommended a negligible incremental dose that corresponds to
a risk of 5 x 10~7/yr (NCRP, 1993~. Therefore, if the NCRP
recommendation were aclopted, the effects of gaseous ~4C releases on
individuals in the global population would be considered negligible.
PROTECTING LOCAL POPULATIONS
Persons in some populations outside the critical group might be
exposed to risk from repository releases in excess of the level of negligible
incremental risk. As individuals, these persons would be (by definition and
in practice) exposed to less risk than the risk limit establisher! by the
stanciarc! for the critical group. If many persons were exposed to this
incliviclual risk, however, the total number of health effects that couIci occur
might be relatively large, particularly if integrated over a very long period
of time.
We know of no analysis that has addressed the spatial distribution
of radiation closes and risks near Yucca Mountain at the distant future times
when individual doses and risks would be at their maximum. It should be
feasible to determine a spatial distribution of potential concentrations in
ground water or air and a spatial distribution of individual doses ant! risks,
employing the same types of exposure assumptions used for calculating
doses ant} risks to members of a critical group (see Chapter 3~. However,
the total number of fatal cancers cannot be known without knowledge of
the number of future persons residing in the Yucca Mountain vicinity. This
number is obviously unknowable. Even if EPA were to define it arbitrarily
through a rulemaking process, comparing the total population risk against
some cleaned figure-of-merit in order thereby to decide on whether to
accept or reject a repository seems too arbitrary to be useful.
Population-Risk Standard
As an example of the difficulty of framing an absolute population-
risk standard, we considered normalizing the population risks as a means
to avoid the difficulty of not having a technical basis for knowing the total
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YUCCA MOUNTAIN STANDARDS
population at risk. Such a regulatory scheme might require that the
integrated population risk over a given period (one generation, for
example) be limited to some fractional risk in the affected population. A
specific hypothetical example would be to require that the integrated
population risk must produce fewer than x health effects per N people
during a defined interval of time.
Framed this way, however, the standard looks very much like an
individual-protection standard: each person outside the critical group
wouIci have an indiviclual lifetime risk limited to x/N. As a matter of
policy, it is certainly legitimate to (lesire to protect a smaller group (the
critical group) by limiting indiviclual risk to a certain level, ant! also to
protect a larger group (the nearby population) with a different but still
meaningful risk limit. However, this approach is not a collective-risk
protection scheme - it is merely a two-tierec] inclividual-risk protection
scheme.
Spatial Gradient in Risk
-An alternative approach that does have a technical basis is
consideration of the spatial distribution of individual risks near the critical
group, at the distant future time when the critical-group risk is highest.
Such a spatial distribution has a technical significance because it depends
on the characteristics not only of the Yucca Mountain physical site but also
of the waste form and the engineered and geologic barriers of the
repository (design.
Furthermore, a risk distribution with a steep spatial gradient-that
is, a distribution in which the indiviclual risks become smaller relatively
quickly with increasing distance from the location of the highest individual
risks seems obviously preferable to a distribution with a more gradual
spatial gradient, all other things being equal. This is because a steeper
spatial gradient implies smaller integrated population risks than does a
more gradual gradient for the same spatial distribution of population.
This observation cannot provide information for discriminating
between an "acceptable" repository and an "unacceptable" one without an
acceptable level of risk for comparison purposes. However, we have not
been able to identify a technically based figure-of-merit that could be used
to judge the compliance acceptability of a given spatial risk gradient. To
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PROTECTING HUMAN HEALTH
63
use the gradient in an absolute sense, one is facet) with not only selecting
a time interval of concern, which is arbitrary, but also defining the future
nearby population. For the simpler task of adequately characterizing the
exposure scenarios leacling to calculation of risks to a critical group, we
have concluded that a feasible procedure can be developed using known
distributions of physical and chemical parameters ant! defensible
assumptions on lifestyles; in other worcis, there is a reasonable technical
basis for a critical-group calculation. For identifying the size, the
distribution and the varied lifestyles of a larger population, more
assumptions of greater uncertainty would! be required. The resulting data
for a risk assessment would become so arbitrary that no adequate decision
basis would result. We therefore conclude that there is no technical basis
for establishing a population-risk standard that would limit the risk to the
nearby population for a Yucca Mountain repository.
PREFERRED FORM OF THE STANDARD
Although we have coucher! the discussion of the last two sections
in terms of an individual-risk standard, we noted in an earlier section of
this report that there are several possible forms of standard that could be
used. We end this chapter by explaining why we conclude that the
indiviclual-risk form has scientific advantages over the others.
Release Limits. It is possible to state the standard in terms of a
limitation on the amount of radionuclides crossing an imaginary boundary
that encloses the repository. The limit generally would be placed on
cumulative release over a specified time period. This is the approach used
by EPA in 40 CFR 191, which relies primarily on a table of maximum
allowable cumulative radioactive releases to the accessible environment for
a period of 10,000 years.
A release limit has the appearance of simplicity because it focuses
on the amount of radionuclides released from the repository across some
specified boundary. This form of standard does not provide any
information about how these releases affect public health, however, ant! so
is incomplete unless coupled with a calculation of inclividual (or
population) risk (or dose or health effects). If one is interested! in this
information on public health for a specific site, it is good scientific practice
to incorporate specific data about the site into the calculation. If that is
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YUCCA MOUNTAIN STANDARDS
done, essentially all of the calculations clescribed in Chapter 3 are required.
The advantage of our recommendation is that these calculations are to be
clone using a methoclology approver! by a rulemaking, with all calculations
explicit to the public. Hence, we conclude that a release limit for a site-
specific standarci fines not reduce scientific complexity or uncertainty.
Without calculations of dose or risk, a release standard appears arbitrary.
Other than the appearance of simplicity, there seem to be no other
advantages to a release-limit form of the standards. It floes not produce
information that is easy to understand or to compare with other risks. Note
that no other standard listen! in either Table 2-3 or 2-4 is expresser! as a
release limit.
A population standards, such as the one that appears to be the
basis for the release limit in 40 CFR 191, establishes a total number of
health effects permitted over some time period 1,000 in 10,000 years,
in the case of 40 CFR 191. This form of standard does not provide a basis
for assessing the risk to the individuals in the critical group, or for local
populations nearby. Therefore, a population standard alone is insufficient
to protect the population most at risk and, probably for this reason, 40 CFR
191 contains a parallel individual standard.
Also, as discussed earlier in this chapter, assessing compliance
with a standard designed to protect the global population involves highly
uncertain calculations because of the extremely low incremental doses to
which large numbers of persons may be exposed. We have recommended
the use of the concept of negligible incremental risk to inclividuals as a
preferable way of dealing with these uncertainties at the outset.
An indivicI?val standarat is needed, however, and the issue is
whether to state it in terms of dose, health effects, or risk. In Section 801,
Congress directs EPA to use individual close. As mentioned above, we
recommend using the risk form for the following reasons:
A risk-based standard would not have to be revised In
subsequent rulemaking if advances in scientific knowledge
reveal that the dose-response relationship is different from
that envisaged today. Such changes have occurred
frequently in the past, ant! can be expected to occur in the
future. For example, ongoing revisions in estimates of the
Or, equivalently, a cumulative dose standard.
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PROTECTING HUMAN HEALTH
65
radiation doses received by atomic bomb survivors of
Hiroshima and Nagasaki may significantly modify the
2.
apparent dose-response relationships for carcinogenic effects
in this population, as have previous revisions in dosimetr
(see Straume et al., 19921.
Risks to human health from different sources, such as
nuclear power plants, waste repositories, or toxic chemicals,
can be compared in reasonably understandable terms. Doses
or releases have to be states] in radiation units Sieverts or
Becquerels that are not easily understood by the general
public and that can only be compared conveniently with
other sources of radiation or radioactivity.
Although we recommend a risk-basec! standard rather than the
dose-basec! standard in Section 801, they are closely related. We define
risk as the expected value of the probabilistic distribution of health effects.
The distribution of health effects is derived from a distribution of dose and
the expected health effects per unit dose.
Consequently, in answer to congressional question No. 1, we
believe that a health-based individual standard will provide a reasonable
standard! for protection of the general public. However, we recommenc!
that this be a risk-based, rather than a dose-based standard.
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Representative terms from entire chapter:
ground water
