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1
INTRODUCTION
Proper management of high-level radioactive wastes, including
those resulting from the production of nuclear weapons and the operation
of nuclear electric power plants, is vital for the protection of public health
and safety. In the United States, defense wastes from the nuclear weapons
program have been accumulating for about 50 years and spent nuclear fuel
from commercial power plants has been accumulating for almost 40 years.
Together defense nuclear wastes and spent nuclear fuel have been
generated at almost 100 sites located throughout the country. At present,
high-level defense wastes are in various physical and chemical forms and
are stored much of it in underground steel tanks- in several types of
facilities, primarily at three U.S. Department of Energy (DOE) weapons-
complex locations: Hanford site, WA; Savannah River site, SC; and the
Idaho National Engineering Laboratory, ID (DOE, ~ 993a). The
commercial spent nuclear fuel is stored in water pools and in above-ground
dry-storage casks at more than 70 sites throughout the U.S.
There is therefore a need for a long-term strategy for disposal of
these wastes that limits to an acceptable level the risks that they pose to
public health and safety. By law, providing for "permanent disposal" of
high-level radioactive waste is the responsibility of the federal government.
It has been longstanding federal policy (see the Nuclear Waste Policy Act
of 1982 (Pip. 97-4251) to dispose of these wastes in an underground mined
geologic repository; the geologic disposal option has been examined and
generally endorsed by the scientific community (National Research
Council (NRC),1957, 1983, 1990b).
The responsibility for high-level radioactive waste disposal is
divided among three federal agencies. DOE is charged with the
development and eventual operation of a geologic repository. It must
locate an appropriate site; demonstrate the site's ability to meet regulatory
requirements; obtain a license from the U.S. Nuclear Regulatory
Commission (USNRC); and construct, operate, and maintain surveillance
of the repository itself. The U.S. Environmental Protection Agency (EPA)
and the USNRC share the responsibility for regulating the disposal
program to ensure adequate protection of the health and safety of the
15
-
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16
YUCCA MOUNTAIN STANDARDS
public. Operating uncler the authority of the Atomic Energy Act of 1954
(42 USC 2201(b)), EPA must establish generally applicable stanclards for
protection of the environment from offsite releases from radioactive
material in repositories (see 42 USC 1014(a), anti the Nuclear Waste Policy
Act of 1982 (Pip. 97-42511. The USNRC promulgates technical
regulations that are consistent with the standarcis and considers license
applications from DOE for any proposed repository, determining with
reasonable assurance whether the EPA standard can be met. USNRC will
have continueci regulatory responsibilities to oversee the repository
operation.
The process of selecting a deep geologic repository for high-level
radioactive waste in the Uniter! States has been going on since at least
1975, although DOE has yet to apply for a license to build such a
repository. In ~ 9X7, Congress directed} DOE's Office of Civilian
Raciioactive Waste Management to concentrate only on the Yucca
Mountain Site (Nuclear Waste Policy Act Amendments of 19871. DOE is
currently Outlying the Yucca Mountain site by a process caller} "site
characterization" to accumulate the information necessary to judge whether
it will meet the standard to be set by EPA. If the site is deemed appropriate
to be considered in the licensing process and a license application to
USNRC is approved, DOE estimates that the earliest date for possible
emplacement of high-level radioactive waste at Yucca Mountain would be
the year 2010 (C. Gertz, DOE, personal communication, May 28, 1993~.
If the site is not deemed appropriate, Congress requires, in Section 113 of
the Nuclear Waste Policy Act, recommendations from the Secretary of
DOE to assure the safe, permanent disposal of spent nuclear fuel ant! high-
leve! radioactive waste, inclucling the need for new legislative authority.
This report deals with only one aspect ofthis long an(l complicated
process the standard that must be set to protect public health. The
stanclard-setting process itself has extende(i over a perioc] of nearly twenty
years. EPA promulgated its first standard! for deep geologic disposal of
high-level radioactive waste (40 CFR 191) in 1985, after about a ciecade of
study. Consistent with the directive of its authorizing statute, EPA
intended this standard to be generally applicable to any creep geologic
disposal site. At the time, several repository sites were being considered
for spent nuclear fuel and defense high-level waste, and the Waste Isolation
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INTRODUCTION
17
Pilot Plant (WIPP) near CarIsbad, New Mexico, was being clesignec] to
accept transuranic waste from the defense nuclear program. ~
Challenged by interveners anti state agencies, the stanciard was
judicially reviewed, and in 1987 the U.S. Court of Appeals for the First
Circuit remanded the standarc! to EPA for reconsideration of several of its
provisions. Before EPA promulgated a new stanciard, however, Congress
enacted the Energy Policy Act of 1992 (Pip. 1 02-4X6), which mandated a
separate process for setting a standard specifically for the proposed
repository at Yucca Mountain, Nevada. Through Section 801 of the Act,
Congress severer] the Yucca Mountain standard from coverage under the
generally applicable stanciard in 40 CFR 191 ant! the Atomic Energy Act
of 1954. In December 1993, EPA issuer! a final regulation (as 40 CFR
191) responding to the issues raised in the 19X7 court remand, but this
revised regulation does not apply to the proposer! repository at Yucca
Mountain.
In Section 801, Congress man`datec! that EPA arrange for an
analysis by the National Academy of Sciences of the scientific basis for
standarcis to be applier! at the Yucca Mountain site and directed the
agency," based upon anti consistent with the findings and recommendations
of the National Academy of Sciences, [to] promulgate, by rule, public
health ant! safety standards for protection of the public from releases from
radioactive materials stored in or disposeci of in the repository at the Yucca
Mountain site." The first paragraph of Section SOlfa) provides that the
stanciard prescribe the maximum annual effective dose equivalent to
individual members of the public from releases to the accessible
environment. These standards will be the only ones for high-level
radioactive waste disposal applicable to the Yucca Mountain site, and are
to be promulgated within one year after the Academy submits its study.
USNRC then has one year to issue its specific regulations, requirements,
and criteria to be consistent with the EPA Yucca Mountain standarci.
This report responds to the charge made explicit in Section
SOl~a)~2), ant} in particular to the three questions that it posed:
According to the definition provided in 40 CFR 191, "transuranic waste" is
waste that is contaminated with alpha-emitting radionuclides with atomic
numbers greater than that of uranium (92), half-lives greater than 20 years, and
concentrations greater than 1 ten-millionth of a curie per gram of waste.
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18
YUCCA MOUNTAIN STANDARDS
1.
Whether a health-basec! standard based upon closes to
incliviclual members of the public from releases to the
accessible environment . . . will provide a reasonable
standard! for the protection of the health and safety of the
general public.
Whether it is reasonable to assume that a system for post-
closure oversight of the repository can be developed, based
upon active institutional controls, that will prevent an
unreasonable risk of breaching the repository's engineered
barriers or increasing the exposure of individual members of
the public to radiation beyond allowable limits.
3. Whether it is possible to make scientifically supportable
predictions of the probability that a repository's engineered
or geologic barriers will be breached as a result of human
intrusion over a period} of 10,000 years.
The conference report accompanying Section 801 makes clear that
Congress does not intend for our report to "establish specific standards for
protection of the public but rather to provide expert scientific guidance on
the issues involved in establishing those standards." (See Congressional
Record, Oct. 8, 1992, pp. SI7555 an(l H11399.) Furthermore, the
conference report ant} subsequent correspondence, ciated May 20, 1993,
from the Chairman ofthe Senate Energy and Natural Resources Committee
point out that our study is not precluded from addressing additional issues.
(See Appendix B for the language of P.L. 102-486, the accompanying
conference report, and the correspondence.) Accordingly, the scope of this
report embraces a range of scientific questions about the Yucca Mountain
stanciards ant! the process of demonstrating compliance with the standard.
SCOPE OF THE STUDY
The disposal of high-level radioactive waste in a geologic
repository initially requires placing radionuclicles in the repository at
concentrations far in excess of natural levels. Some radionuclides decay
quickly: for example cesium-137 has a half-life of 30 years ant! strontium-
90 has a half-life of about 29 years. But some of the radionuclides have
long half-lives: for example, the half-life of carbon-14 is 5,730 years and
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INTRODUCTION
19
the half-life of iocline-129 is 17 million years. Others produce decay
products that in turn persist for very long periods. The half-lives of
plutonium-239 and neptunium-237 are 24,360 years and 2.2 million years,
respectively.
The purpose of (leep geologic disposal is to provide long-term
barriers to the escape of these radionuclicles into the biospheres. Most of
the original radioactive material placer! in a repository is expected to have
clecayec! to natural background levels while these barriers are effective.
However, some of the longer-lived radionuclicies involved will ultimately
enter the biosphere, although it might take tens to huncirecis of thousands
of years or longer to do so. These releases will be "acceptable" in a
regulatory sense if the adverse consequences for public health are
sufficiently low. The health standard to be set by EPA and compliance
with the standard will, in principle, determine whether the resiclual risks are
acceptable.
Implicit in setting such a stanclard, and in (lemonstrating
compliance with it, is the assumption that EPA, USNRC, and DOE can,
with some degree of confidence, assess the future performance of a
repository system for time scales that are so long that experimental
methods cannot be used to confirm directly predictions of the behavior of
the system or even of its components. This premise raises the basic issue
of whether scientifically justifiable analyses of repository behavior over
many thousands of years in the future can be macle. Based on our
evaluation of this issue and the state of scientific ant! technical
understanding, we conclude that such analyses are indeed possible within
limitations noted in this report. In such cases, these analyses can provide
useful guidance for assessing compliance with required health standards,
as Chapter 3 of this report will describe.
Even when scientifically useful analysis is possible, assessments
of repository performance must contend with substantial uncertainties in
information about, and understanding of, the basic physical processes that
are important to judging the effectiveness of the repository system to
2 In this report, "biosphere" refers to the region of the earth in which
environmental pathways for transfer of radionuclides to living organisms are
located and by which radionuclides in air, ground water, and soil can reach
humans to be inhaled, ingested, or absorbed through skin. Humans can also be
exposed to direct irradiation from radionuclides in the environment.
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20
YUCCA MOUNTAIN STANDARDS
isolate wastes. Although some of these uncertainties can be resolved by
further research, not all of them can be. Some areas projecting the
behavior of human society over very long periods, for example-are
beyond the limits of scientific analysis. For these reasons, we have
attempted to be candid about the limits of scientific analysis in supporting
the standard-setting process. We have made explicit those instances where,
because there is no aciequate scientific basis for an analysis, policy
judgments are required.
Additionally, setting and assessing compliance with a stanciarci
must rely on informed judgments en c} reasonable assumptions based on
scientific expertise when uncertainties and unknowns otherwise stand in
the way of determinative analysis. There are no alternatives to relying on
policy judgments and informer} assumptions since some aspects of
standard-setting and compliance analysis are not amenable to scientific
analysis.
The processes of setting a standard and licensing a repository also
raise social, political, and economic issues that would be difficult to
resolve even if the scientific challenges were less formidable. Some of
these issues might have more effect on the repository program than the
health and safety standard itself. Although we have taken a broac] view of
our charge as related to the scientific basis for the standard, we have not
addressed these other, potentially important, issues. The following
discussion describes eight issues that we have not addressed.
We have not recommended what levels of risk are
acceptable. A standard} that serves as an objective for
protection of public health must be states] in terms of some
quantitative limit, such as acceptable dose, health effects, or
risk. The specific level of acceptable risk cannot be
identified by scientific analysis, but must rather be the result
of a societal decisionmaking process. Because we have no
particular authority or expertise for judging the outcome of
a properly constructed social decisionmaking process on
acceptable risk, we have not attempted to make
recommendations on this important question. However,
many domestic and international bodies have reached
carefully considered conclusions on this ant! related
questions. We discuss these instances in Chapter 2 and note
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INTRODUCTION
21
the cases where we believe that existing scientific,
regulatory, ant! other expert opinions establish ranges within
which lie useful starting points for consistent regulatory
proposals.
We have not considered whether the clevelopment of a
permanent repository shou1~dproceed at this time. A central
objective of the DOE program is to license and operate a
repository as soon as possible. As individuals, we hoIc!
differing views on the urgency of meeting this objective.
We were not asked and we did not attempt to acidress
whether a repository is needed in the near future; nor did we
compare the risks and benefits of proceeding with a
repository now as opposer! to those that might be realized by
continued reliance on surface storage well into the next
century. Accordingly, this report should not be interpreted
as a recommendation for or against the development of a
Yucca Mountain repository or even a judgment on whether
any cteep geologic repository should or should] not be built at
this time.
We have not made a judgment about the suitability of Yucca
Mountain as a repository site, or on whether the proposed
repository there would meet requirements of any standard
consistent with our recommendations to EPA. Within our
scope, we have not producer! new scientific or technical data
or made calculations that wouIc! add to the continuing
assessment of the suitability of the site. Although we have
reviewer] the assessments currently underway, we have not
evaluated either the quality or the results of the assessment
program in a detailed, rigorous way. Finally, the question of
site acceptability raises a variety of social, political, ant!
economic issues that we have not examined because such
issues are not within our mandate.
4. We horse not considered the effects of our recommendations
on the future of nuclear power. It has been argued that
unless and until means for long-term clisposal of spent fuels
from commercial nuclear power plants are available, the
future of nuclear power is in question. Some states and some
foreign countries require by law or regulation that a means
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22
YUCCA MOUNTAIN STANDARDS
for disposing of waste be in place before additional plants
are licensed. We die] not, however, consider the effect on the
future of nuclear power on the fecleral program for managing
spent fuel from commercial nuclear power plants.
We have not compared the basis for regulating high-level
radioactive waste with the basis for regulating
nonradioactive long-lived toxic substances, such as lead or
cadmium. Radioactive wastes are sometimes regulates} on
more stringent bases than nonradioactive wastes even though
some nonradioactive substances are more persistent and can
pose a greater hazard than many radionucli(les. However, it
is consistent with our charge in Section 801 to concern
ourselves only with the radioactive constituents of the waste.
6. We have not evaluated the standards applicable to the
operational phase of the repository program. This phase
refers to the time before the approved repository is closed
and includes the transportation of waste to the repository site
en cl the steps taken at the site to prepare and emplace the
waste in the repository. These operations are closely
analogous to other nuclear activities regulated by EPA and
USNRC. Even though some would argue that the health risk
associated with these relatively transitory activities might be
greater than those associated with the repository over
geologic time, we have not addresseci the issues because the
clear intent of Section 801 is that our report should focus on
the post-operational performance of the repository over very
long time-periocis. Furthermore, the basis for regulating
operating nuclear facilities is considerably better established.
7. We have not considered! the potential effects of the repository
on nonhuman biota and ecosystem functions. These effects
might deserve attention, but the clear charge in Section 801
to focus on protection of public health has deterrer] us from
going further. We are aware, of course, and have
considered, that human health can be affecter! by exposure to
radionuclides taken up by other organisms such as food
crops.
8. We have not considered the potentialfor chain reactions of
Missile materials as part of a standard. The possibility
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INTRODUCTION
23
theoretically exists that circumstances might ultimately arise
in which radioactive wastes containing fissile materials
could undergo a chain reaction in a geologic repository. The
potential is an important concern for engineering design that
ultimately is likely to be the subject of regulation, perhaps by
USNRC. This topic, however, requires specialized analysis
that is sufficiently far from our primary focus that we left it
for the consideration of others.
BACKGROUND AND APPROACH
A general description of the repository system, and of the ways that
it may release raclionuclicles into the accessible environment, is essential
background information for understanding our approach to this assignment.
This description appears below, ant} is followed by discussions of the
major issues to be considered in setting a health and safety standarci, and
of their implications for the study. A map showing the location of the
Yucca Mountain region is shown in Figure I.1. A schematic cross section
of the potential Yucca Mountain repository is shown in Figure ~ .2.
The Repository System
DOE plans to achieve containment anti isolation of high-level
radioactive waste in a proposed repository by using an engineered barrier
system ant! locating the repository in the geologic setting of Yucca
Mountain. The general repository design suggests that the waste wouIc] be
emplaced in drifts (tunnels) about 300 meters (1,000 feet) beneath the land
surface but above the water table of the uppermost aquifer, that is, in the
unsaturated or vadose zone. By law the repository is conceptually designeci
to hoIct 70,000 metric tons of high-level radioactive waste. Under current
policy, about 90% of this amount (63,000 metric tons) would be spent
commercial fuel and the rest wouIc! be defense high-level waste. Up to 100
years abler emplacement operations begin, the repository wouIc} be sealed
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24
YUCCA MOUNTAIN STANDARDS
Figure 1.1 Map showing location of Yucca Mountain region
adjacent to the Nevada Test Site in southern Nebula.
Source: Wilson et al., 1994.
N evade
Reno
\ Yucca ountain
~ \ Nevada ~st Site
\ \~
1 ~ l
`~ L
_ _
_ _
Las Vegas
_ ~
0 50 100150km _
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25
LL
o\ Rae
-I -
~ ~ --a
/
I/
~ \
Is
ee - ~
o
En
Ace'
~1
&.
be
~ 1
\
_
.C~
C) ~
o ~ - ^ o
~ ~ ~ .
·3
JO ~ ~ ..
A SO ~ ~
.° ~ ~ ~ ~
~ ~ Be, ~ o
o ~ V
_ `} = ~ Z
.e "C ~ ID p
° ~ U) U.
~ ~ ~ ~ -
c~ ~ - ~ :'
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- 3 4,, - A,
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o-, ,~ = = 0
C) Cal ~ ~ o
(Q o A Pa ~
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OCR for page 26
26
YUCCA MOUNTAIN STANDARDS
by backfilling the cirifts, closing the opening to each emplacement drift,
ant} sealing the entrance ramps ant] shafts.
The engineered barrier system would include the waste form (for
example, reactor-fuel assemblies or high-level defense waste embedded! in
a glass matrix), internal stabilizers, the canister in which the waste is
placed, and backfill between the canister and the adjacent host rock. The
spent fuel assemblies include naturally radioactive uranium oxicle
containing fission products, as well as fuel cIadcling and support hardware,
both of which will be radioactive due to activation or contamination. The
defense waste consists of products resulting from physical ant! chemical
processes associates! with the separation of fissionable materials in
weapons manufacture.
The engineered barrier system would be placed beneath Yucca
Mountain in the unsaturated zone, which consists of layered units of
welder! and non-welclec! tuffs3. Some of these units are highly fractured
a characteristic that may influence the flow of water underground. The
water table at Yucca Mountain occurs at depths of 600 meters to 800
meters below land surface, which would correspond to clepths of 300 to
500 meters below the repository. The volume of rock below the water
table contains two principal aquifer systems, one in the volcanic tuff ant!
another at greater depth in carbonate rock. In the Yucca Mountain region,
the regional ground water in the upper aquifer appears to flow generally
southerly, from higher elevations north of the mountain to the Death Valley
region to the southwest where it emerges at the surface (NRC, 19921.
Radionuclide releases from an undisturbed repository into the
geologic environs can occur through the following sequence: degradation
and failure of the waste canister through corrosion, relatively quick release
of substances from the more mobile components of the radionuclicle
inventory, slow release of substances from the less soluble or less mobile
components of the inventory, anti movement of radionuclides from the
waste package to the air and water in the pores and fissures of the host rock
by gas phase and aqueous phase. Radionuclicles can enter the environment
accessible to humans by traveling clown through the unsaturated zone and
into the aquifer (the saturated zone), then through the aquifer to wells or
springs where the water might be used for purposes such as drinking or
agricultural irrigation. Releases might also occur in gaseous form,
3 Tuff is consolidated volcanic ash.
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INTRODUCTION
27
transported upward or laterally from the waste package through the rock
to the atmosphere. Other pathways might develop if the site is disturbed,
for example, by human intrusion or earthquakes.
More detailed information on the proposed repository and the
inventory of radionuclides in the waste is presented in the 1993 total-
system performance assessments for Yucca Mountain that were prepared
for DOE (Andrews et al., 1994; Wilson et al., 1994~.
Issues to Be Considered in Approaching the Study
The aim of this study is to provide guidance on the scientific basis
for a standard that would protect the public health from the adverse effects
of releases from a proposed repository for high-level radioactive waste at
Yucca Mountain. There are two major considerations in providing this
guidance. The first is how to make the best use of the scientific knowledge
that is now or might soon be available. The second is how to make
decisions when the scientific basis is deficient. We present below several
examples that illustrate these two considerations, anti then describe how we
have addressed them in our approach to the study.
Large but improbable doses
It is important to define the standard in such a way that it is a
useful measure of the degree to which the public is to be protected from
releases from a repository. The nature of geologic disposal is to
concentrate and isolate high-level radioactive wastes in a small area for a
very long time. It is always possible to conceive of some circumstance
that, however unlikely it may be, will result in someone at some time being
exposed to an unacceptable radiation dose. Some of these scenarios are
common to all geologic repositories; for example, it is always possible that
a person will drill or otherwise intrude into any repository in such a way as
to bring to the surface some amount of radioactive waste. Other such
scenarios are dependent upon the characteristics of the repository site. In
the case of Yucca Mountain, human ingestion of radionuclides in ground
water drawn from a well is an example of a site-specif~c scenario that,
because of the limited amounts of water in a relatively isolated hydrologic
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28
YUCCA MOUNTAIN STANDARDS
basin, potentially could lead to radiation closes of a relatively high level to
a few persons. The possibility that future volcanic activity in the region
might seriously compromise the integrity of a repository at Yucca
Mountain must also be evaluatecI. The challenge is to define a standard
that specifies a high level of protection but that does not rule out an
adequately sited and well-`iesignec! repository because of highly
improbable events.
Demonstration' of compliance
The feasibility of assessing compliance with the standard is another
key issue. Quantitative performance assessment is the too} generally
proposed for use in evaluating whether a repository is likely to meet the
standard with a given level of assurance. Performance assessment requires
analyzing the processes by which raclionuclicies might be releaser} from the
repository, the processes by which people might be exposed to them, and
the health consequences of exposure. The first steps in the analysis are to
mode! the degradation of waste packages and the migration of
raclionuclides through the engineered and geologic barriers of the
repository ant! the adjacent host rock. Although this analysis involves
important uncertainties, they can, in principle, be addressee! by scientific
methocis. More difficult is the identification of the pathways through the
biosphere that would result in exposure to humans. There are countless
possible pathways for radionuclides but only a limited number of them
need to be analyzed, that is, the ones most likely to yiel~i the highest closes.
Moreover, in principle, pathway and exposure analyses require specifying
the state of human society many thousands of years into the future
where people might live, what they will eat ant] firing, what technologies
will be available to detect and avoici ra`dionuclicles, and other factors.
These difficulties cannot be ignored in setting a practical health-basec!
standard, but dealing with them can clepend as much, or perhaps more, on
assumptions and informed judgment as on testable scientific hypotheses.
The scientific basis for performance assessment thus varies considerably
among the steps in the analysis.
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INTRODUCTION
Fundamental vs. elegized standar~ls
29
To avoid explicitly using uncertain assumptions in compliance
assessment, a ~ierive~i standard is sometimes proposed rather than a
fundamental one. A fundamental standard uses as its criterion the endpoint
that the stanciard is intended to control. Thus, when adverse health effects
are the outcome to be controlled, a fundamental standard! wouIc] be stated
in terms of limiting the number of adverse effects, the risks of developing
an Diverse health effect, or of some closely related parameter such as a
close rate. A derives! standard translates the fundamental criterion into
some other unit of measure, such as the total flux of radionuclides across
a repository boundary, expresser} for example in the cumulative amount of
radioactivity released over a specified period of time.
The difference between the two is that the ~ierivec} standard
subsumes into its clefinition various assumptions, such as specifying the
particular sets of pathways to human exposure, and a close-response
relationship, that would otherwise have to be made in compliance
assessment for a fundamental standard. Because a derived standard might
eliminate from the licensing process some of the calculations involved in
specifying these pathways, it has the advantage of a simpler licensing
(recision (M. Federline, USNRC, personal communication, May 27, 19931.
In choosing between a fundamental or a derived standard, a balance must
be struck between clarity of purpose in the stanciarci ant] complexity of the
licensing process on the one hancI, and complexity in the standard, but a
clearer focus in the licensing process on the other.
Time scale
A final issue involves the time scale over which compliance with
the standard] shouIc! apply. The repository could release radionuclides over
hundreds of thousands of years or more, but as performance assessments
are extender! into the future, the uncertainties in some of the calculations
that might be required could render further calculation scientifically
meaningless. On the other hand, analyses that are uncertain at one time
might not be so uncertain at a later time; for example, the uncertainties
about cumulative releases to the biosphere that depend on the rate of failure
of the waste packages are large in the near term but are smaller later, when
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30
YUCCA MOUNTAIN STANDARDS
enough time has passed that all of the packages will have faileci. Selection
of a time scale for the standard must therefore take into account the
scientific basis for the performance assessment itself. Selection of a time
scale also involves policy considerations. (For example, the level of
protection that the standard affords to future generations is an important
ethical question that must be consiclered. Limiting the time period covered
by the standard could be inconsistent with a policy on long-term
intergenerational equity.)
The remanded! EPA standard} anti the recently promulgated
standard for radioactive waste repositories other than the proposed Yucca
Mountain repository places a time limit on performance assessment of
10,000 years. This time limit makes some aspects of the analysis more
tractable by eliminating from consideration the uncertainties that increase
at times beyond 10,000 years. In the case of Yucca Mountain, however,
recent performance assessment calculations (Ancirews et al., 1994) indicate
that the likely time for some radionuclides, such as technetium-99, to reach
the biosphere is longer than 10,000 years. If that time limit were to apply
at the Yucca Mountain site, potential exposures occurring beyond 10,000
years would be excludeci from the compliance analysis. The problem of
the cumulative uncertainties must therefore be weighed against the need to
consider the exposures when they actually are calculated to occur.
Choices Affecting the Bases of the Standard
The foregoing issues illustrate two considerations that we have had
to balance in reaching our conclusions and recommendations. First, is the
Semi to choose among the available options (for example, alternative forms
of the standard! and time scales) in a way that makes the best use of the
scientific information that is available. For example, it might be intuitively
attractive to state a standard in terms of risk to human health. But as noted
earlier, the demonstration of compliance with such a stanciar~i requires a
mode} of the radionuclides and their pathways from the repository to the
biosphere that is scientifically challenging to develop. This difficulty can
be avoided by abandoning a health-based standard in favor of a limitation
on releases from the repository, but doing so would obscure crucial
information about the potential of the radionuclide releases for causing
health effects. Similarly, selecting a time scale for analysis involves
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INTRODUCTION
31
weighing how the scientific basis for analysis changes with time against the
timing at which more numerous future health effects are likely to occur.
We have trier! to clear explicitly with these choices en c} to arrive at a basis
for judging the form of standard that is best supported by the available
scientific information taken as a whole.
The second consideration is how to provide, within the regulatory
process, a system for making those choices for which scientific information
is unavailable or insufficient. The regulatory process involves the two
major steps of rulemaking ant! licensing. The rulemaking procedure allows
extensive public participation and considerable administrative discretion
in weighing anti assimilating alternative points of view. Licensing is a
quasijudicial process that benefits from having clear-cut limits against
which to judge an applicant's proposals. It is for the latter reason that
several members of the USNRC staff have pointed out their reluctance to
leave any speculation about the future of human society for the licensing
process (which USNRC administers).
There are several choices to be macie in designing the standard} for
which science cannot provide all the necessary guidance ~ cleaning the
critical group to be protected or the radionuclide pathways to them through
the biosphere, for example. Since these choices must be made, even in the
absence of clear-cut scientific information, we recommend that such issues
should be treated as part of the rulemaking process, since this process, as
inclicateci earlier, allows a broader scope for discussing and weighing
alternatives.
In the course of this study, we analyze~i separately the scientific
bases for setting a health-basec! stantiarci, conducting compliance
assessment, ant! dealing with human intrusion ant} episodic geologic
processes, such as volcanoes and earthquakes. We adoptecl this procedure
to help us uncierstand the choices involved among these different aspects
of the problem, and to clarify where the scientific basis for choice was
insufficient. We then weighed these consicierations in making our final
findings and recommendations, which are presented in the remaining
chapters of our report.
OCR for page 32
Representative terms from entire chapter:
radioactive waste
