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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~ ~ - ~ :' ~ ~ ~ o ~ - 3 4,, - A, ~U] ~ ~ =.-~ ~ C_ ~ =~ ~ L- . ~ ~ ~C, .^ o-, ,~ = = 0 C) Cal ~ ~ o (Q o A Pa ~ . ~ ~ a ~ ._ ._ ~

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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.

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