3. POSTCLOSURE SUITABILITY EVALUATION
Postclosure suitability refers to the Yucca Mountain disposal system having the capability to meet applicable radiation protection standards in accordance with the U. S. Department of Energy (DOE) site suitability guidelines in 10 CFR Part 963 (66 FR 57298). This section of the Yucca Mountain Site Suitability Evaluation summarizes the technical and regulatory basis for the evaluation of postclosure suitability by the DOE. Specifically, this postclosure suitability evaluation examines the ability of the natural and engineered barriers to work together as multiple and diverse barriers for many thousands of years to isolate, delay transport of, and reduce concentrations of radionuclides so that radiological exposures will likely meet applicable radiation protection standards. In 10 CFR Part 63 (66 FR 55732), the U.S. Nuclear Regulatory Commission (NRC) has adopted the standards established by the U.S. Environmental Protection Agency (EPA) in 40 CFR Part 197. The DOE site suitability guidelines at 10 CFR 963.2 (66 FR 57298) have defined applicable radiation protection standards as those of the NRC in their regulations at 10 CFR 63.111(a) and (b) and 10 CFR 63.204 for the preclosure period and at 10 CFR 63.311, 10 CFR 63.321, and 10 CFR 63.331, for the postclosure period. The DOE site suitability guidelines in 10 CFR Part 963 (66 FR 57298) provide for an evaluation of postclosure suitability based on a total system performance assessment (TSPA). The TSPA forecasts disposal system performance for a compliance period of 10,000 years while taking into account uncertainties in the models, data, and potential future system conditions. For this purpose, the DOE guidelines set forth the elements of the TSPA to be conducted, as well as the analyses and analytical methods to be used (10 CFR 963.16) and the criteria to be considered in the analysis (10 CFR 963.17) to ensure a comprehensive evaluation of the performance capability of the disposal system. As illustrated in the evaluation process for postclosure suitability shown in Figure 1-2, the TSPA method involves a series of steps from the collection of data and empirical observations through the identification and screening of features, events, and processes (FEPs) to the development of models to represent these FEPs and into a suite of calculations, analyses, and studies. The method culminates in analyses using a TSPA computer model that abstracts the results of nine component models and analyses that describe the FEPs that will affect the disposal system performance (see Section 3.2.6). The TSPA computer model directly implements a subset of the computer models and uses outputs from the external process-level computer models. The TSPA method is iterative and allows for models to evolve through feedback loops to gain insights, identify data needs, refine representations and calculations, and gain confidence in the reliability of the results. Specialized analyses build on a common technical basis, and sensitivity analyses are used to examine the effects of varying specified parameters on model outcomes to account for uncertainties and variabilities in parameter values. Through this iterative process, a suite of TSPA reports and analyses has been developed. The TSPA model described in Total System Performance for the Site Recommendation (CRWMS M&O 2000b) was used in the core set of analyses for evaluating postclosure performance. After completion of these analyses, the DOE then explored the significance of unquantified uncertainties, derived insights from new scientific information and improved models, examined the effect of thermal parameters on predicted repository performance, and provided insights into the conservatism of the overall assessment of postclosure performance. This work, documented in FY01 Supplemental Science and Performance Analyses (BSC 2001a; BSC 2001b), helped in developing a more robust technical basis for the supplemental TSPA model used in the performance analyses described in Volume 2 of that report, considering alternative models, uncertainty and variability, and sensitivity of the repository system to parameter variations, including lower-temperature operating modes. This supplemental TSPA model was then modified to be consistent with provisions of the final EPA rule (40 CFR Part 197). This modified model is referred to in this evaluation as the revised supplemental TSPA model. The modifications to and results of the TSPA analyses completed using the revised supplemental TSPA model are described in Total System Performance Assessment—Analyses for Disposal of Commercial and DOE Waste Inventories at Yucca Mountain—Input to Final Environmental Impact Statement and Site Suitability Evaluation (Williams 2001a)—referred to as the "TSPA Report for Final Environmental Impact Statement and Suitability Evaluation"—and in Total System Performance Assessment Sensitivity Analyses for Final Nuclear Regulatory Commission Regulations (Williams 2001b)—referred to as the "TSPA Sensitivity Analyses for NRC Regulations." Application of the TSPA method for evaluating postclosure suitability has been documented in a suite of performance assessment reports. Table 3-1 identifies this suite of TSPA reports and analyses that support this evaluation. Since release of Yucca Mountain Preliminary Site Suitability Evaluation (DOE 2001b), the DOE has completed supplemental TSPA analyses to evaluate performance using the provisions contained in the final EPA (40 CFR Part 197) and NRC (10 CFR Part 63 [66 FR 55732]) rules. The calculations and quantitative analyses conducted against the final EPA rule are documented in the TSPA Report for Final Environmental Impact Statement and Suitability Evaluation (Williams 2001a). The revised supplemental TSPA model and analyses in that report account for changes in provisions of the EPA standards in the following areas:
3.1 POSTCLOSURE SUITABILITY EVALUATION METHOD—10 CFR 963.16(a)(1) AND (2)
The DOE regulation, 10 CFR Part 963 (66 FR 57298), provides that the TSPA method be used to evaluate whether a repository at Yucca Mountain would be likely to meet the radiation protection standards referenced in 10 CFR 963.2. Because the NRC licensing regulations prescribe a compliance period of 10,000 years, the performance attributes of the potential geologic repository must be evaluated using sophisticated computer simulation models. These simulation models are founded on an integrated set of conceptual models reflecting the relevant FEPs that would occur after permanent closure of the repository. The TSPA component models illustrated in Figure 3-1 incorporate FEPs applicable to the Yucca Mountain site. These component models, which are described in Section 4.2 of the Yucca Mountain Science and Engineering Report (DOE 2002a), Section 3.1 of Total System Performance Assessment for the Site Recommendation (CRWMS M&O 2000b), and Volume 1 of FY01 Supplemental Science and Performance Analyses (BSC 2001a) represent the scientific framework from which repository behavior has been analyzed. This report provides an evaluation of the suitability of the Yucca Mountain site as a repository for the disposal of spent nuclear fuel and high-level radioactive waste pursuant to the DOE site suitability guidelines, which are consistent with final NRC rules in 10 CFR Part 63 (66 FR 55732) (finalized in November 2001) and final EPA radiation protection standards in 40 CFR Part 197 (finalized in June 2001) as implemented in the NRC licensing rules. The methodology and parameters used to define the characteristics of the reference biosphere and the receptor that are described in earlier TSPA analyses (CRWMS M&O 2000b; BSC 2001a; BSC 2001b) are generally consistent with standards established in the final 40 CFR Part 197. Additional performance assessment analyses that were conducted using provisions of the final EPA and NRC rules are discussed in the TSPA Report for Final Environmental Impact Statement and Suitability Evaluation (Williams 2001a) and the TSPA Sensitivity Analyses for NRC Regulations (Williams 2001b). The DOE uses TSPA as the analytical method for quantifying the performance of the individual barriers and making probabilistic projections of overall system performance. System performance (in the TSPA-SR and in the supplemental TSPA model) is expressed in terms of the mean annual dose to the average member of the critical group (see Section 3.3.9 of this report) located at the point of compliance approximately 20 km (12 mi) from the potential repository site, in a manner consistent with the criteria in the NRC's proposed regulation for a repository at Yucca Mountain, 10 CFR 63.115(a) and (b) (64 FR 8640). The revised supplemental TSPA model calculates the mean annual dose to the reasonably maximally exposed individual located approximately 18 km (11 mi) from the repository, in a manner consistent with the final EPA standards at 40 CFR Part 197 and with the final NRC licensing rule at 10 CFR Part 63 (66 FR 55732). The final EPA and NRC regulations set out the characteristics of the reference biosphere and the reasonably maximally exposed individual and set numerical limits for the expected annual dose and groundwater radionuclide concentrations in the following standards, which are referenced in 10 CFR 963.2 (66 FR 57298):
3.1.1 Postclosure Suitability Evaluation Method—10 CFR 963.16(a)
10 CFR 963.16(a) (66 FR 57298) states that the DOE will evaluate postclosure suitability using the TSPA method and outlines the scope of the TSPA evaluations to be performed. The DOE guidelines outline the scope of the TSPA evaluations in two different subsections. 10 CFR 963.16(a)(1) states:DOE will conduct a total system performance assessment to evaluate the ability of the Yucca Mountain disposal system to limit radiological doses and radionuclide concentrations in the case where there is no human intrusion into the repository. DOE will model the performance of the Yucca Mountain disposal system using the method described in paragraph (b) of this section and the criteria in § 963.17. DOE will consider the performance of the system in terms of the criteria to evaluate whether the Yucca Mountain disposal system is likely to comply with the applicable radiation protection standard.10 CFR 963.16(a)(2) states:
DOE will conduct a separate total system performance assessment to evaluate the ability of the Yucca Mountain disposal system to limit radiological doses in the case where there is a human intrusion as specified by § 63.322. DOE will model the performance of the Yucca Mountain disposal system using the method described in paragraph (b) of this section and the criteria in § 963.17. If required by applicable NRC regulations regarding a human intrusion standard, § 63.321, DOE will consider the performance of the system in terms of the criteria to evaluate whether the Yucca Mountain disposal system is likely to comply with the applicable radiation protection standard.Figure 3-2 illustrates the relationship between these two provisions of the site suitability guidelines, the TSPA evaluations, and the comparison with applicable radiation protection standards. The TSPA performed in accordance with 10 CFR 963.16(a)(1) (66 FR 57298) will provide results that will be used to evaluate the ability of the Yucca Mountain disposal system to meet the applicable radiation protection standards in the case where there is no human intrusion. The TSPA was divided into separate evaluations: (1) a nominal (or reference) scenario and (2) a disruptive scenario. The groundwater protection standard applies strictly to "undisturbed performance" (10 CFR 63.331 [66 FR 55732], as referenced in 10 CFR 963.2), which is considered equivalent to the nominal scenario. The individual protection standard applies to overall system performance. Therefore, the individual protection analyses provide for the combination of TSPA results for both the nominal and disruptive scenarios but exclude the human intrusion scenario per the provisions in 10 CFR 963.16(a)(1). The principal results of these TSPA analyses are summarized and explained in Section 3.1.2 of this report. 10 CFR 963.16(a)(2) (66 FR 57298) calls for a separate TSPA to be conducted to evaluate the ability of the Yucca Mountain disposal system to limit radiological doses in the case where there is a human intrusion in accordance with the NRC licensing regulations. The TSPA evaluation method is the same as for the case without human intrusion, except that the TSPA method for human intrusion must use prescribed assumptions about the human intrusion scenario, 10 CFR 63.322 (66 FR 55732). Further, under 963.16(a)(2), DOE will consider whether the Yucca Mountain disposal system is likely to meet the human intrusion standard only if such compliance would be required under the NRC licensing regulations. The NRC licensing regulation prescribing the human intrusion standard, 10 CFR 63.321, would require compliance with the dose limit for individual protection if the DOE determines that, within the 10,000-year regulatory compliance period, the waste packages would degrade sufficiently that a human intrusion could occur without recognition by drillers (as a result of exploratory drilling for groundwater). If the dose from a human intrusion is predicted to occur more than 10,000 years after disposal, the analysis of the human intrusion scenario must be presented in the environmental impact statement, and the dose limits for the human intrusion standard would not apply. The treatment of human intrusion is presented in Section 3.1.3. As discussed previously, the TSPA-SR model was used in the core set of analyses for the evaluation of postclosure performance. It is important to note the use of conservatisms in the assumptions chosen to assist in development of the TSPA-SR model. These conservative assumptions are utilized for several reasons, including incomplete knowledge and uncertainty. The most important conservative assumptions are presented in Appendix F of Total System Performance Assessment for the Site Recommendation (CRWMS M&O 2000b). The term "conservative" is used here to indicate that the assumption or model used may underestimate the positive contribution to the system performance of a particular part of the repository system. Alternatively, conservatism may cause negative performance to be overstated (CRWMS M&O 2000b, Section 1.1). After completion of the TSPA-SR model, DOE continued evaluating conservatisms and quantifying previously unquantified uncertainties in the TSPA-SR model. The discussion of these analyses is contained in FY01 Supplemental Science and Performance Analyses (BSC 2001a; BSC 2001b) (see Table 3-1). Volume 1, Section 2 of FY01 Supplemental Science and Performance Analyses (BSC 2001a) describes the methods and approach used in the quantification of uncertainties, updated scientific bases and analyses, and thermal sensitivity analyses that are described in the remaining sections of Volume 1. Each of the subsequent sections (Sections 3 through 14) describes the analyses conducted and the information passed to the TSPA model. This information was incorporated into the TSPA-SR model to conduct the sensitivity analyses described in Volume 2, Section 3.2 of FY01 Supplemental Science and Performance Analyses (BSC 2001b). The results of the sensitivity analyses were used to develop the supplemental TSPA model, as described in Volume 2, Section 4 of FY01 Supplemental Science and Performance Analyses (BSC 2001b). That section summarizes the results of a set of additional calculations using the supplemental TSPA model that included updated information for various model components into a single analysis. Upon issuance of the final EPA rule at 40 CFR Part 197, the supplemental TSPA model was modified to evaluate performance using provisions contained in the final EPA rule. This model (the revised supplemental TSPA model), differs slightly in configuration from the TSPA-SR and supplemental TSPA models. These differences are discussed in Section 5.2 of the TSPA Report for Final Environmental Impact Statement and Suitability Evaluation (Williams 2001a), and that report also presents results of TSPA analyses conducted in response to the final EPA rule. The final NRC rule (10 CFR Part 63 [66 FR 55732]) adopted requirements promulgated by the EPA in its final rule. The DOE conducted additional sensitivity analyses to consider certain provisions of the final NRC licensing regulations not addressed in prior analyses. These analyses are documented in the TSPA Sensitivity Analyses for NRC Regulations (Williams 2001b). The revised supplemental TSPA model was also used in these sensitivity analyses.
3.1.2 Evaluation of Repository System Performance
All scenarios used in the TSPA models are constructed of FEPs that a formal screening process examined and determined should be included in the performance assessment. A variation of the FEP screening method originally developed for the NRC (Cranwell et al. 1990) was used to screen FEPs to determine the nominal and disruptive scenarios to be used in the TSPA. This method is similar to approaches used by scientists working on repository programs in other countries (Bonano and Baca 1994). See Section 1.3.3 of this report and the discussion of the potentially disruptive scenario below for more detail on the FEP screening process. As described in detail in Section 4.3.1 of Yucca Mountain Science and Engineering Report (DOE 2002a), the methodology comprises a five-step process:3.1.2.1 Conceptual Model of Repository System Performance
The following aspects of the conceptual model of the Yucca Mountain disposal system incorporate the results of scientific investigations and explain the repository performance projected by the models:
3.1.2.2 Uncertainty
There are recognized limitations to TSPA models and their ability to forecast the future behavior of the Yucca Mountain disposal system. One of the most important uncertainties in the TSPA analyses is in projecting the long-term performance of natural and engineered barriers using data derived from short-term, multiyear tests. Also important are the inherent uncertainties in forecasting changes in climate and in other processes over the 10,000-year postclosure period. Because of the long time frames over which the disposal system must perform, the natural variability in features of a geologic repository, and limitations on the amount of data that can be collected, uncertainties will always remain. The number and detail of process models developed for the Yucca Mountain site and the complex coupling among those models make direct incorporation of all model uncertainties difficult. Model uncertainties arise from several sources, including (1) parameter values, (2) conceptual model representations, and (3) the mathematical models used to implement the conceptual models. In many cases the value of model input parameters is uncertain. Parameter value uncertainty may result from imperfect knowledge, limited data, and treatment of variability as uncertainty. Parameter value uncertainty can be addressed by developing a probability distribution that captures the full range of potential values or by using a single value that is conservative. Conceptual model uncertainty arises from incomplete understanding or characterization of FEPs that will affect a potential repository. There may be several equally plausible ways to conceptualize a specific process being modeled; that is, multiple alternative conceptual models may be considered to explain the current data equally well. Mathematical model uncertainty may be introduced by simplifications and approximations that generally are introduced to represent conceptual models to make the problem tractable and to implement the model in a computer program. Conceptual model uncertainties can also arise from an incomplete knowledge of complex physical processes. Although current conceptual models are consistent with existing knowledge, there have been questions raised by some in the technical community regarding how some processes are conceptualized in process models feeding the performance assessment. Examples of such uncertainties include detailed coupling of thermal-hydrologic processes and stability over very long periods of the passive oxide films that will develop on the Alloy 22 waste package surface. Portions of the current and planned testing and analysis are aimed at gaining a better understanding of such issues such that their representation in conceptual models is facilitated. In turn, these models can be included in the performance assessment, as appropriate. Remaining uncertainties of this type have been identified (Williams 2001c, Section 2.3) and provide a basis for part of DOE's ongoing and future testing, analysis, and modeling. These issues have been addressed in a number of ways through an uncertainty strategy that is focused on quantifying uncertainties in a defensible manner. In some cases, conservative assumptions have been made or parameter values have been bounded, creating what has been referred to as an "unquantified uncertainty." If the entire probability distribution for a parameter value is used, this is referred to as a "quantified uncertainty" because the Monte Carlo analysis used by TSPA will sample the full range of potential parameter values. If a single conservative value is used to represent a parameter value, this introduces an uncertainty of unknown magnitude into the Monte Carlo simulations, although the TSPA results are likewise believed to be conservative. More than one conceptual model may be consistent with available data, and in the absence of definitive data sets or compelling technical arguments, the most conservative model is normally used. This is another case where an uncertainty of unknown magnitude (i.e., an unquantified uncertainty) is introduced into the TSPA analyses. Efforts to address uncertainties have included enhancement of engineered barriers to provide additional defense in depth and the investigation of natural analogues to provide additional information to support the model representations used. These activities are discussed in FY01 Supplemental Science and Performance Analyses (BSC 2001a; BSC 2001b), which also explores the significance of these unquantified uncertainties. The end result is that analyses of total system performance have used a mix of probabilistic representations, single-value conservative estimates, and conservative assumptions and models. This approach has been used in other projects, but it complicates the determination of quantification of the degree of conservatism associated with the projected margin relative to the regulatory standard. In addition, the mixing of varying degrees of conservatism in models and parameter representations could be viewed as complicating the analysis. On the other hand, it increases the confidence that the results of the analysis conservatively bound any future performance. In Total System Performance Assessment for the Site Recommendation (CRWMS M&O 2000b), the critical assumptions, data limitations, and residual uncertainties in the TSPA-SR model were identified, and the significance of uncertainties included in the TSPA-SR model were evaluated. A number of activities were undertaken to accomplish this evaluation:
The TSPA-SR model included a mixture of conservative and realistic inputs. Using this approach, model results for the nominal scenario, for example, showed significant uncertainty, approximately two orders of magnitude at the time of peak dose (see Figure 3-3a). Although the DOE considered this a defensible approach, some reviewers of the work thought the approach masked information and understanding and called for a reevaluation of the TSPA-SR model. In response to these comments and observations, the model was revised and documented in FY01 Supplemental Science and Performance Analyses (BSC 2001a; BSC 2001b). The revised model used in the analyses, referred to as the supplemental TSPA model, included quantification of some important uncertainties that were previously unquantified for nominal scenario performance. The supplemental TSPA model showed wider ranges of doses at earlier times (prior to 20,000 years as compared with the TSPA-SR model results for that same time period), but this added indication of uncertainty was the result of incorporating new information into the waste package modeling and not of simply replacing single-point conservative values with distributions. For the nominal scenario, the net change in terms of dose during the period of 10,000 years after disposal was small, moving from a mean annual dose of zero (in the TSPA-SR model) to a peak mean annual dose of less than 2.0 × 10-4 mrem/yr in the supplemental TSPA model (see Figure 3-3b). This dose was a result of a few early waste package failures, which resulted from changes made to the model, taking into consideration the uncertainty associated with possible improper heat treatment of the waste package lid welds. In terms of mean annual dose beyond 10,000 years, however, additional uncertainty was introduced by changes made to parameter distributions and models that support the supplemental TSPA analyses. The net effect beyond 10,000 years was a lower, more defensible mean dose projection. Because the supplemental TSPA model incorporated additional quantified uncertainties, the projection was also associated with a wider uncertainty band.
The supplemental TSPA model also evaluated the effects on performance of operating the repository at lower temperatures. From the standpoint of the resulting calculated uncertainty distribution in the supplemental TSPA model results, both higher- and lower-temperature operating modes showed similar performance and range of uncertainty for the nominal scenario. This conclusion acknowledges that the existing conceptual models supporting the TSPA may or may not have the capacity to discern effects from the temperature or moisture differences between the operating modes. While acknowledging that the current TSPA models may not have the capability to discern all temperature effects, FY01 Supplemental Science and Performance Analyses (BSC 2001a; BSC 2001b) shows that many process models do have the capability to discern differences between the operating modes at the subsystem level. Incorporation of additional temperature-dependent processes not currently captured in the TSPA might allow for distinguishable differences at the system level between the operating modes. The supplemental TSPA model was subsequently modified upon issuance of the final EPA rule at 40 CFR Part 197. The modified model, known as the revised supplemental TSPA model, differs slightly from the supplemental TSPA model and is described in detail in the TSPA Report for Final Environmental Impact Statement and Suitability Evaluation (Williams 2001a). The report includes comparative diagrams showing the results of the TSPA-SR model, the supplemental TSPA model, and the revised supplemental TSPA model. To capture the full detail of the uncertainty and variability in the behavior of the Yucca Mountain disposal system, the reports described above display graphical representations of the results of the TSPA analyses that highlight the mean, the median, and the 5th and 95th percentiles of the distribution of results. The results explicitly incorporate uncertainty by calculating estimates of statistical measures of the output as a means to evaluate performance, thus capturing the probabilistic treatment of uncertainty in the total system performance. In the same manner described in Total System Performance Assessment for the Site Recommendation (CRWMS M&O 2000b), these statistical measures are calculated at each time step of the dose histories and include data from all realizations of the probabilistic simulations. These statistical measures—the mean, median, and 5th and 95th percentiles—are superimposed on the diagrams showing all realizations; each realization has some definite possibility of representing the actual radiological exposure from the disposal system. Because of their appearance, such diagrams showing all realizations (usually hundreds of separate realizations) are called "horsetail diagrams." The uncertainty in projecting the exposure is represented by the range of outcomes, expressed by the range of realizations produced by the TSPA model (for example, the spread between the 5th and 95th percentiles). By calculating a distribution of exposures, the models reflect the range of parameter values and models that could be appropriate, knowing that the actual exposure cannot be reasonably predicted except in probabilistic terms. Such projections of possible outcomes, given quantified uncertainties in the inputs, is a common method of displaying expected (mean) risk and associated uncertainties in probabilistic risk analyses. As an example, Figure 3-3 shows two horsetail diagrams for the nominal scenario generated by the TSPA-SR model and the supplemental TSPA model. Both diagrams represent 300 separate realizations of performance and reveal important information about forecasted performance of the modeled system. The uncertainty band in the range of realizations shown in Figure 3-3b is a function of the quantified uncertainty in the inputs to the system representation. For example, in the supplemental TSPA model, uncertainty has been quantified regarding the possibility of improper heat treatment of the waste package lid welds, which resulted in the possibility of waste package failures prior to 10,000 years. The additional quantification of uncertainty represented in Figure 3-3b results in realization of dose histories that are different from those resulting from the conservative assumptions made in the TSPA-SR model used to generate Figure 3-3a. The wider range of quantified uncertainty in the supplemental TSPA model, in this case, leads to broader uncertainty in the performance assessment results beyond 100,000 years, expressed by the range of realizations shown in Figure 3-3b. However, this added uncertainty resulted in negligible change during the period of 10,000 years after disposal. The diagrams also show that the additional quantified uncertainties and updated models in the supplemental TSPA model lead not only to a reduction in the mean peak dose beyond 10,000 years but to a broader uncertainty band in the range of the peak annual doses. An alternative way to express this result is that the conservative models of the TSPA-SR lead to a higher peak dose with a narrower range of annual doses (Williams 2001c, Section 2.2.1.1). Comparison of dose histories over a million years using the TSPA-SR model and the supplemental TSPA model shows the following two characteristics. First, the supplemental TSPA model shows significantly wider ranges of doses at a given time and of times to reach given doses. Second, except at early times, the magnitude of the mean annual dose is less for the supplemental TSPA model, and it occurs later in time. A comparison of Figures 3-3a and 3-3b shows that the supplemental TSPA model produces a broader range of annual doses or times to specific annual dose values than does the TSPA-SR model. This is represented quantitatively by the distribution of realizations at particular dose rates and at particular times. The broader range is a result of the additional uncertainties and updated models that have been incorporated into the supplemental TSPA model. In many cases, simplified or bounding models have been replaced with more physically representative models that include quantified uncertainties in their parameters. For example, a bounding solubility model for neptunium in the TSPA-SR model (CRWMS M&O 2000b, Section 3.5.5) has been replaced with a more complex model that accounts for the solubility of secondary phases that control the solubility (BSC 2001a, Section 9.3.2). The updated solubility model is believed to be more realistic, but the uncertainties in the model lead to a broader range of neptunium concentrations than the previous model. Propagation of these uncertainties, as well as those of all of the other updated process models, results in the broad ranges that are seen in results of the supplemental TSPA model. A second observation is based on a comparison of the estimates of mean performance (dose rate and time to dose) using the TSPA-SR model and the supplemental TSPA model. This comparison shows that after approximately 10,000 years, the mean annual dose using the supplemental TSPA model is always less than the mean using the TSPA-SR model. The difference between the mean estimates is one measure of the magnitude of the conservatism in the TSPA-SR model. For example, at 30,000 years, the difference between the mean estimates of annual dose is about three orders of magnitude, and at the time of peak mean dose, the difference is about one order of magnitude. During the period prior to 10,000 years, the small annual doses (less than about 2 × 10-4 mrem/yr) indicated by the supplemental TSPA nominal model exceed the zero annual dose calculated by the TSPA-SR model, and the TSPA-SR model could be interpreted as being nonconservative with respect to the supplemental TSPA model during this time. However, these small doses, resulting from the revised treatment of uncertainty regarding the potential for improper heat treatment of lid welds on waste packages, are more than a factor of 1,000 smaller than the doses forecasted for the disruptive scenario (0.1 mrem/yr). As discussed in Section 3.1.2.6, the combined nominal and disruptive scenario dose is approximated as the probability-weighted mean annual dose from the disruptive scenario. Differences between the supplemental TSPA model and the TSPA-SR model for the first 10,000 years would have essentially no impact on the mean annual dose. In its licensing regulations at 10 CFR 63.303 (66 FR 55732), the NRC states that in the case of the specific numerical requirements for individual, human intrusion, and groundwater protection, compliance with the NRC's numerical standards for licensing, referenced in the DOE site suitability guidelines, is to be based upon the mean of the distribution of projected doses of DOE's performance assessments for 10,000 years after disposal. The mean is conservative because it is sensitive to the number of realizations having zero and nonzero annual doses. Because the mean is an average of all realizations at any given point in time, if any realizations have a nonzero dose, the mean will likewise be a nonzero number. Consequently, the mean will trend towards the higher percentiles and will stay above the median (see Figure 3-3b); performing greater numbers of calculations does not materially affect the location of the mean. Because the quantitative estimates of repository performance should not be dominated by unrealistic or extreme situations or assumptions, the mean value is an appropriate measure of performance for comparison with regulatory standards because it conservatively represents hundreds of realizations that collectively capture parameter and model uncertainty. For the evaluation of site suitability, the mean values of the probabilistic simulations in the TSPA Report for Final Environmental Impact Statement and Suitability Evaluation (Williams 2001a) have been used. Despite significant efforts to reduce and quantify uncertainties in the TSPA inputs, unquantified uncertainties remain and, in most cases, are addressed through the use of conservative assumptions and bounded parameter values. Their potential implications to performance and risk have been examined, and strategies for managing them have been developed. This information and a discussion of key remaining uncertainties are presented in Sections 2 and 3 of Uncertainty Analyses and Strategy (Williams 2001c). In some cases, conceptual model process uncertainties have been identified for which gaining a better understanding is appropriate (e.g., long-term stability of passive films on the waste package surface), and testing, analysis, and modeling activities are continuing to expand and refine these models. The DOE recognizes that the results of TSPA modeling do not constitute absolute certainty with respect to future outcomes because absolute certainty is unobtainable, as both EPA and NRC explicitly acknowledge in their regulations. However, even in the presence of the remaining uncertainties the DOE is satisfied with the level of treatment and understanding of these uncertainties in the current TSPA analyses supporting the site recommendation decision process and has confidence in the overall safety of the repository. Additional confidence is gained through the analysis and investigations of anthropogenic (man-made) and natural analogues to the processes and materials—natural and engineered—used in the repository. For example, geothermal reservoirs at Yellowstone, Wyoming, and at Broadlands, New Zealand, have provided some insight into the thermal-hydrologic and chemical processes that would occur in the repository environment. Analogue systems, which have occurred over time periods of decades to millennia and over spatial scales of up to tens of kilometers, are beyond the scale of laboratory experiments such as metal corrosion tests. Therefore, careful observation and analysis of these analogue systems provide increased confidence in the performance of the Yucca Mountain disposal system. Further, in addition to the inclusion of multiple natural and engineered barriers in the disposal system and the consideration of analogues in the analyses, additional provisions are being implemented to increase confidence that the postclosure performance objectives will be met. These provisions include model validation, implementation of a rigorous quality assurance program, and continuation of a performance confirmation program. However, because uncertainty is integrated into the assessment of total system performance, the DOE does not expect that additional information will significantly change the conclusions reached in this site suitability evaluation.3.1.2.3 Nominal Scenario
For the nominal scenario, the peak mean dose calculated by the revised supplemental TSPA model over the regulatory compliance period is 1.7
10-5 mrem/yr for the higher-temperature operating mode and 1.1 10-5 mrem/yr for the lower-temperature operating mode. These doses, when combined with those projected from the disruptive scenario, were used for comparison with the NRC radiation protection standard for individual protection. The nominal scenario was also used to calculate radionuclide concentrations and releases for comparison with the groundwater protection standard as discussed later in this section. Further information on the results of the performance analyses done to support evaluations of the nominal scenario, as well as the methodology used in the evaluations, is described in the paragraphs that follow.
The nominal scenario for the TSPA is composed of the set of expected FEPs, as determined by a formal screening procedure. The FEP screening basis and decisions are summarized in Appendix B of Total System Performance Assessment for the Site Recommendation (CRWMS M&O 2000b). Section 4.3 of Yucca Mountain Science and Engineering Report (DOE 2002a) describes the steps of the screening procedure. Eleven FEPs were added in an update of the FEP database (Freeze et al. 2001, Section 5.4) subsequent to the preparation of Total System Performance Assessment for the Site Recommendation (CRWMS M&O 2000b). These FEPs were not expected to significantly alter the dose results, so they were not included in supplemental TSPA analyses conducted after completion of Total System Performance Assessment for the Site Recommendation (CRWMS M&O 2000b).
The nominal scenario analyzed by all the TSPA models incorporates the important effects and system perturbations caused by climate change, seismic activity, and repository heating that are projected to occur over the 10,000-year compliance period. Although identified as a disruptive event in 10 CFR 963.17(b)(2) (66 FR 57298), certain seismic events of uncertain magnitude are likely to occur at Yucca Mountain in the future. Among these likely events, vibratory ground motion is treated in the TSPA with the nominal scenario (CRWMS M&O 2000b, Section 4.5.4). The technical basis for the conceptualization of the nominal scenario is summarized in Section 4.2 of Yucca Mountain Science and Engineering Report (DOE 2002a) and extensively documented in a set of process model reports (see list in Section 3.2.1.2 of this report) and numerous supporting analysis model reports.
The model results for the nominal scenario from the TSPA-SR model, the supplemental TSPA model, and the revised supplemental TSPA model (higher-temperature operating mode only) are shown in Figure 3-4. TSPA results for the nominal scenario based on these models are summarized as follows:
Both the supplemental and revised supplemental TSPA models calculated doses before 10,000 years; the TSPA-SR model forecasted no dose in the 10,000-year compliance period. The difference between the results over this period is because not all mechanisms that could lead to early failure of waste packages, such as improper heat treatment in the closure weld area, were included in the TSPA-SR model. This was based on the low probability and the planned use of administrative controls to further reduce the probability of some of the mechanisms that could lead to early failure. In reevaluating the potential of early failure mechanisms and their potential consequences, a more conservative approach assumed improper heat treatment and subsequent failure of a few waste packages in the supplemental and revised supplemental TSPA model analyses (BSC 2001b, Section 3.2.5). The early waste package failure assumes concurrent failure of both the inner and outer Alloy 22 lids and the stainless steel inner lid. To ensure that the potential consequence of early waste package failures is treated conservatively, it was included in the nominal scenario, not as a sensitivity analysis, for both the supplemental and revised supplemental TSPA model analyses (BSC 2001a; BSC 2001b).
The specific procedures and equipment for heat-treating a waste package and verifying proper heat treatment will be developed. With the use of administrative (procedural) controls, engineering controls, and multiple checks included as part of the development of the induction annealing process, the probability of improper heat treatment will be reduced even more. However, until these mitigating measures can be fully analyzed, the inclusion of early failures has built conservatism into the nominal scenario simulated by both the supplemental and revised supplemental TSPA models. This leads to early failure of a small number of waste packages; for the nominal scenario, these early failures are the only contributor to the early dose that begins at approximately 1,000 to 2,000 years and extends out beyond the period of NRC regulatory compliance in the supplemental and revised supplemental TSPA model evaluations of nominal performance, as shown in Figure 3-4 (BSC 2001b, Section 5.1; Williams 2001a, Section 5.2.4.2). The difference in calculated dose between the TSPA-SR model and both supplemental and revised supplemental TSPA models for the nominal case after 10,000 years is consistent with the efforts to quantify uncertainties and address conservatism found in the TSPA-SR model. The drop in peak annual dose for the supplemental and revised supplemental TSPA models after 10,000 years is largely due to updated treatments of waste package degradation and the more realistic treatment of radionuclide solubilities, particularly neptunium, thorium, and plutonium (BSC 2001b, Section 4.1.1). The increase in peak annual dose after 10,000 years from the supplemental TSPA model compared to the revised supplemental TSPA model is primarily due to the exclusion of the consideration of temperature dependence in Alloy 22 corrosion rates. The revised supplemental TSPA model used biosphere dose conversion factors consistent with the reasonably maximally exposed individual defined in DOE's site suitability guidelines at 10 CFR 963.2, whereas the TSPA-SR model and the supplemental TSPA model used biosphere dose conversion factors based on the average member of the critical group. Reasonably maximally exposed individual biosphere dose conversion factors are lower than those for the average member of the critical group. As a result, doses calculated by the TSPA-SR model and supplemental TSPA model are higher than those calculated by the revised supplemental TSPA model (Williams 2001a, Section 5.2.5, Table 5-1). Evaluations of higher- and lower-temperature operating modes in the supplemental and revised supplemental TSPA models have provided insight into nominal performance of the repository at the subsystem level over a range of thermal operating modes. While some subsystem models indicated significant differences, results from changing the thermal operating mode showed only a minor impact on overall system-level performance. For nominal performance, the lower-temperature operating mode yields mean annual dose estimates that are generally slightly less than those for the higher-temperature operating mode (BSC 2001b, Section 4.1; Williams 2001a, Table 6-1).3.1.2.4 Disruptive Scenario
For the combined volcanic disruptive scenario (igneous intrusion and volcanic eruption) during the 10,000-year regulatory compliance period, the probability-weighted peak mean annual dose calculated by the revised supplemental TSPA model is 0.1 mrem/yr for both the higher- and lower-temperature operating modes (Williams 2001a, Section 6.3 and Table 6-1). These doses, when combined with those calculated from the nominal scenario, were used for comparison with the radiation protection standard for individual protection. Detailed information on the results of the performance analyses done to support evaluation of the disruptive scenario, as well as the methodology used in the evaluations, is described in the paragraphs that follow. Volcanic activity has been identified as the only disruptive event that has the potential to affect disposal system performance during the 10,000-year regulatory compliance period. Other disruptive FEPs, such as seismic perturbations of the water table and nuclear criticality, were analyzed and found either to be unsupported by scientific evidence or to have probabilities less than the inclusion threshold of one in 10,000 in 10,000 years as set forth in the DOE suitability guidelines at 10 CFR 963.16(b)(4) (66 FR 57298). As a result, these disruptive FEPs were not incorporated into the TSPA models through the formal FEP screening procedure (BSC 2001h; (CRWMS M&O 2000m, Table 2-2; DOE 2002a, Section 4.3). Potential seismic effects on the underground facilities and waste packages were screened out (i.e., excluded from consideration) because the waste packages would not be adversely damaged by design basis rockfalls or vibratory ground motion (CRWMS M&O 2000ac, Section 6.2). However, because vibratory ground motion from seismic events might damage spent nuclear fuel cladding, the effect of this potential damage to cladding by a discrete seismic event was analyzed as part of the nominal scenario (CRWMS M&O 2000b, Section 3.5.4). The disruptive scenario considers two distinct types of volcanic activity: (1) eruptive volcanism at the repository location and (2) igneous intrusion (or magmatic flooding) of some of the emplacement drifts in the repository. Disruptive Events Process Model Report (CRWMS M&O 2000m) documents the geological basis and data for these scenario conceptualizations. The disruptive scenario assumed that the eruptive event consisted of a magmatic penetration of the repository facility after permanent closure. The conceptualization of the eruptive event assumes that the magma flow intersects and destroys waste packages, bringing waste to the surface through one or more eruptive conduits. The igneous intrusion event assumes that a hypothetical igneous dike intersects drifts of the repository and that the associated waste packages are damaged, exposing the waste within to percolating water. In the eruptive event, the TSPA models analyzed the atmospheric transport of radionuclides bound in the particles of volcanic ash that were then dispersed downwind and ultimately deposited on the ground at the receptor location. In the igneous intrusion event, the TSPA models accounted for the additional waste package failures and analyzed the transport of radionuclides through the groundwater pathway to the location of the receptor. A probabilistic volcanic hazards assessment study (CRWMS M&O 1996a) focused on the task of examining available geologic data for the Yucca Mountain region and estimated the annual probability for the scenario of the repository footprint being intersected by a basaltic dike. A group of about 30 earth scientists from such organizations as the U.S. Geological Survey (USGS); the University of Nevada, Las Vegas; the Center for Nuclear Waste Regulatory Analyses; and Los Alamos National Laboratory contributed to this study. After this study, a formal elicitation of expert judgment was conducted in accordance with the NRC guidance on elicitation procedures (Kotra et al. 1996). The expert panel used in the formal elicitation was composed of ten internationally recognized volcanism experts. The probability estimate obtained from the expert panel was 1.5 × 10-8 per year (or a probability of 1.5 × 10-4 of occurring over 10,000 years) (CRWMS M&O 1996a, Section 4). The probability estimate for the igneous intrusion event was subsequently recalculated based on a change in the configuration of the repository layout. The more recent probability estimate for the igneous intrusion event is 1.6 × 10-8 per year (CRWMS M&O 2000ad, Section 6.5.3.1). If a basaltic dike does intersect the repository, there is about a 77 percent chance that a volcano will form at the surface with magma flowing through a portion of the repository (the eruptive event) (CRWMS M&O 2000ad, Section 7.1.3 and Table 13a). This translates to approximately one chance in 8,000 of a volcano forming above the repository during the first 10,000 years. The annualized probabilities for each of the disruptive events are just slightly greater than the probability cutoff of 10-8 per year in 10 CFR 963.16(b)(4) (66 FR 57298). Therefore, both intrusive and eruptive events have been included in the TSPA model analyses of the disruptive scenario. The expected probability-weighted dose histories calculated for the disruptive scenario by the TSPA-SR model and the revised supplemental TSPA model are shown in Figure 3-5 (Williams 2001a, Figure 6-10c). Figure 3-6 is a horsetail diagram showing the results of the revised supplemental TSPA model analyses for the disruptive scenario for the higher-temperature operating mode (Williams 2001a, Figure 6-10a). The diagram shows the results for 500 of 5,000 realizations calculated by the revised supplemental TSPA model; the 500 include the realization with the highest probability-weighted dose within the regulatory period. In addition, statistical measures (i.e., the mean, median, and 5th and 95th percentile dose curves), based on all 5,000 realizations, are also shown. The time histories are associated with a random occurrence of the disruptive events in the compliance period. The mean annual probability of an igneous event disrupting the potential repository is 1.6 × 10-8 or approximately 1 in 60 million per year. TSPA results for the disruptive scenario based on the TSPA-SR model, the supplemental TSPA model, and the revised supplemental TSPA model are summarized as follows:
3.1.2.5 Nominal Scenario and the Groundwater Protection Standard
The methodology and parameters used to calculate postclosure releases for the nominal scenario are consistent with 10 CFR Part 963 (66 FR 57298). For purposes of comparison with the groundwater protection standard (nominal scenario), the revised supplemental TSPA model calculated groundwater concentration activities, as well as the dose to critical organs, in a manner consistent with the EPA's groundwater protection standards and the NRC standard at 10 CFR 63.331 (66 FR 55732). The NRC licensing standard, as referenced in DOE's site suitability guidelines at 10 CFR 963.2, specifies concentration limits for combined radium-226 and radium-228, gross alpha activity, and combined beta- and photon-emitting radionuclides based on a representative volume of 3,000 acre-ft of groundwater that would be withdrawn and used annually at a location within the accessible environment. The accessible environment includes any location outside of the controlled area. Groundwater protection is analyzed at the point above the highest concentration of radionuclides in the plume where the plume crosses the southernmost boundary of the controlled area (at a latitude of 36° 40' 13.6661" North) and reaches the accessible environment. This is also the location of the reasonably maximally exposed individual. The repository footprint is somewhat larger for the lower-temperature operating mode than for the higher-temperature operating mode. The location at which the reasonably maximally exposed individual is located and the point where groundwater protection is analyzed is the same, approximately 18 km (11 mi) from within the potential repository footprint (Williams 2001a, Section 5.2.3). As shown in Figures 3-7 and 3-8, the peak mean annual activity concentration calculated by the revised supplemental TSPA model for combined radium-226 and radium-228 over the 10,000-year compliance period is less than 10-10 pCi/L for both the higher-temperature and lower-temperature operating modes (not including background radiation) (Williams 2001a, Table 6-3). Figures 3-7 and 3-8 also show that the calculated peak mean activity concentration for alpha-emitting radionuclides for the 10,000-year compliance period is 1.8 × 10-8 pCi/L (not including background radiation) for the higher-temperature operating mode and 3.3 × 10-8 pCi/L (not including background radiation) for the lower-temperature operating mode (Williams 2001a, Table 6-2). Figures 3-9 and 3-10 show horsetail diagrams for gross alpha activity and total radium activity concentrations, respectively, for the regulatory period and for the time period out to 1 million years for the lower-temperature operating mode. It is apparent from these figures that the magnitude of the calculated gross alpha activity concentration, total radium activity concentration, and the uncertainty in the calculated values for both increase significantly after approximately 70,000 years. A cumulative distribution function and histogram of gross alpha and total radium activity concentrations at the time of peak mean dose during the regulatory period showed that over 75 percent of the realizations have a zero activity concentration value. This is because no waste packages have failed in those realizations during the regulatory period. Data taken from a Nevada Department of Transportation well approximately 20 km (12 mi) from the potential repository indicate that gross alpha background concentration is 0.4 pCi/L ± 0.7, for a possible maximum of 1.1 pCi/L; total radium background concentration is no greater than 1.04 pCi/L (CRWMS M&O 2000b, Section 4.1.5). Because the calculated gross alpha activity and total radium activity concentrations are orders of magnitude lower than their natural background concentration, the combined background and calculated concentrations, when rounded, are the same as the natural background. Therefore, natural background for both gross alpha and total radium is used for comparison with limits for groundwater protection (see Table 4-2). Consistent with applicable radiation protection standards referenced in the DOE suitability guidelines at 10 CFR 963.2 (66 FR 57298), concentrations of combined radium-226 and radium-228 are limited to 5 pCi/L, and those of gross alpha activity (including radium-226, but excluding radon and uranium) are limited to 15 pCi/L. The maximum mean annual dose to any critical organ from combined beta- and photon-emitting radionuclides calculated by the revised supplemental TSPA model is 2.3 × 10-5 mrem/yr for the higher-temperature operating mode and 1.3 × 10-5 mrem/yr for the lower-temperature operating mode (Williams 2001a, Table 6-4). The limit from beta- and photon-emitting radionuclides, based on drinking 2.0 L (0.53 gal) of water per day from the representative volume (3,000 acre-ft), must be less than 4 mrem/yr. Background radiation is not required in the organ dose calculation. Figure 3-11 shows a horsetail diagram with the mean, median, and 5th and 95th percentiles for the annual dose to a critical organ (thyroid) from iodine-129, one of the predominant contributors to organ dose (Williams 2001a, Figure 6-20; (CRWMS M&O 2000b, Section 4.1.5). This figure shows a significant increase in the values of annual dose from iodine-129 after approximately 70,000 years and an increase in the magnitude of the uncertainty in those values. A cumulative distribution function and histogram of this information at the time of peak mean dose during the 10,000-year regulatory compliance period showed that over 75 percent of the realizations have a zero critical organ dose value from iodine-129. This is because no waste packages have failed in those realizations during the regulatory period. The maximum critical organ dose from iodine-129 calculated during the regulatory period is 1.4 × 10-3 mrem/yr; results for technetium-99 are similar to iodine-129. The maximum values calculated from the realizations are below the regulatory limit. Results of the TSPA analyses show that the calculated peak mean radionuclide activity concentrations and maximum mean annual doses are below the applicable radiation protection standards for groundwater protection. Radionuclide concentrations and critical organ doses calculated by the revised supplemental TSPA model are discussed in more detail in Section 6.6 of the TSPA Report for Final Environmental Impact Statement and Suitability Evaluation (Williams 2001a). Earlier TSPA analyses for the evaluation of groundwater protection were conducted in a manner consistent with proposed regulations. These TSPA results are summarized as follows:
The final NRC rule (10 CFR Part 63 [66 FR 55732]) adopted requirements promulgated by the EPA in its final rule (40 CFR Part 197) regarding limits on the conduct of performance assessment analyses (10 CFR 63.342). The NRC licensing regulation would require exclusion of unlikely natural processes and events in the performance assessment evaluation for the human intrusion and groundwater protection standard. However, the NRC did not specify a numerical limit for defining the term "unlikely." For purposes of analysis, DOE conducted a sensitivity analysis to consider an unlikely event (igneous intrusion) in the evaluation of the groundwater standard, as described in the TSPA Sensitivity Analyses for NRC Regulations (Williams 2001b, Section 6.1).
The report presents estimates of activity concentrations and dose estimates from combining releases from the nominal scenario with those from an unlikely event (igneous intrusion scenario) for the evaluation of groundwater protection. Because the nominal scenario results are negligible compared to the results calculated for an igneous intrusion, the activity concentrations and dose estimates for groundwater protection considering igneous intrusion are approximated as those calculated for an igneous intrusion. Gross alpha concentration for the igneous intrusion scenario was calculated as 2.9 × 10-2 pCi/L for the higher-temperature operating mode and 5.5 × 10-2 pCi/L for the lower-temperature operating mode. These concentrations are about 10 percent of natural background radiation. Total radium concentration was 5 × 10-6 pCi/L for the higher-temperature operating mode and 5.2 × 10-6 pCi/L for the lower-temperature operating mode. These concentrations are orders of magnitude lower than natural background radiation, and both concentrations are below radiation protection standards for groundwater protection. The calculated maximum mean dose to any critical organ for the igneous intrusion scenario was 2.5 × 10-1 mrem/yr for the higher-temperature operating mode and 2.4 × 10-1 mrem/yr for the lower-temperature operating mode (Williams 2001b, Tables 6-1, 6-2, and 6-3). These doses, which in this case are not required to include natural background, are also below the applicable radiation protection standard. It can be concluded that all the calculated TSPA model activity concentrations and doses for comparison to the groundwater protection standard, as referenced in DOE's site suitability guidelines at 10 CFR 963.2 (66 FR 57298), are below the limits specified in the radiation protection standards for groundwater protection.3.1.2.6 Combined Releases from the Nominal and Disruptive Scenarios and the Individual Protection Standard
The NRC has provided guidance on the method for combining the expected annual doses from the nominal and disruptive scenarios (NRC 2000b, Section 4.4.1). That guidance states that the dose for each scenario is weighted by the scenario probability, so the summed expected annual dose includes both consequence and probability and, therefore, represents the expected risk for the repository. For comparison to the individual protection standard, DOE calculated the annual dose to the reasonably maximally exposed individual as a result of releases from the Yucca Mountain disposal system (considering nominal and disruptive scenarios) that are projected to occur during the 10,000-year compliance period. The revised supplemental TSPA model calculations supporting the evaluation of individual protection were performed within a probabilistic framework combining the most likely ranges of behavior for the various component models, processes, and corresponding parameters in the process models describing repository performance. The calculations provide data with which to evaluate the repository performance against the applicable radiation protection standards. The TSPA analyses evaluated both the nominal scenario (70,000 MTHM) and the disruptive scenario (igneous activity) under both the higher- and lower-temperature operating modes (Williams 2001a). The nominal scenario was analyzed by running the revised supplemental TSPA model for 300 realizations (see Figure 3-3b), and the disruptive scenario was analyzed using 5,000 realizations (see Figure 3-6) (Williams 2001a, Sections 6.2.1 and 6.3). Figure 3-12 graphically represents the disruptive scenario results (Figure 3-6) with the individual realizations removed and the statistical measures—the mean and the 5th and 95th percentiles—displayed separately. This representation emphasizes the general features of the diagram without all of the details. Examining the annual dose histories from the standpoint of the probability distribution of realizations provides insights into the aggregate or system-level significance of uncertainties of the inputs. For instance, the horsetail diagrams can be sliced vertically, at particular times, or horizontally, at particular dose rates, to reveal details of the distributions. A variant of this approach is to produce a slice at the time of the peak of the mean of the realizations. Figure 3-13 is an example of this type of diagram for the realizations shown in Figure 3-6. Figure 3-13 includes a cumulative distribution function created by summing the number of realizations at particular probability-weighted annual doses (Figure 3-13a) and a histogram created by summing the number of realizations within various dose-rate increments (Figure 3-13b). These types of plots provide additional insights into the details of the TSPA results. The cumulative distribution functions and histograms are most useful for comparisons between different TSPA models. For instance, the slope of the cumulative distribution function is a function of the amount of uncertainty captured in the TSPA calculation. Comparison of cumulative distribution functions constructed for different TSPA models, such as the TSPA-SR model and the supplemental TSPA model, will immediately reveal differences in the degree of uncertainty quantification because of differences in the slopes of the cumulative distribution functions. Examination of the histogram reveals that the results at the time of the peak mean dose have an approximately normal distribution when the results are plotted on a logarithmic axis. Additional insights into uncertainty can be obtained by comparing cumulative distribution functions and histograms generated for the various TSPA models (Williams 2001c, Section 2). Using the final NRC rule at 10 CFR 63.303 (66 FR 55732) as guidance, performance is evaluated with reference to the mean value of the output of the performance assessment model. A comparison of the mean annual dose from the results of the model simulations using the revised supplemental TSPA model, as presented in the TSPA Report for Final Environmental Impact Statement and Suitability Evaluation (Williams 2001a, Figure 6-14), shows that the dose is below applicable radiation protection standards (15 mrem/yr for individual protection). Table 3-3 summarizes the results of these simulations and shows the peak mean annual dose for the nominal and disruptive scenarios for both higher- and lower-temperature operating modes over the 10,000-year regulatory compliance period. Table 3-3 presents the peak 95th percentile dose for the multiple realizations for the same cases. These data provide confidence in the expected repository performance because peak values of the 95th percentile data are also below the applicable limits, meaning that 95 percent of the realizations yielded peak doses that are less than the values shown on the table. For example, the 10,000-year peak 95th percentile value for the disruptive scenario (igneous-activity case), higher-temperature operating mode, is 4.1 × 10-1 mrem/yr, indicating that 4,750 of the 5,000 realizations for that case had a peak value less than 4.1 × 10-1 mrem/yr. Results shown in Table 3-3 are summarized from Table 6-1 of the TSPA Report for Final Environmental Impact Statement and Suitability Evaluation (Williams 2001a), which also presents results for other time frames. Further, examination of the plots presented in the TSPA Report for Final Environmental Impact Statement and Suitability Evaluation (Williams 2001a, Figures 6-10a and 6-11a) shows that all doses for all realizations are less than the applicable radiation protection standard. For purposes of comparison to the individual protection standard (combined nominal and disruptive scenario), the revised supplemental TSPA model calculated doses in a manner consistent with the final EPA rule at 40 CFR 197.21 as discussed in the TSPA Report for Final Environmental Impact Statement and Suitability Evaluation (Williams 2001a). The final EPA rule provides a definition of the boundary between the controlled area and the accessible environment and mandates the location at which releases and doses must be assessed. Dose is calculated to the reasonably maximally exposed individual, a hypothetical person, located at the point above the highest concentration of radionuclides in the simulated plume of contamination where the plume crosses the southernmost boundary of the controlled area (at a latitude of 36° 40' 13.6661" North) and reaches the accessible environment. This distance is approximately 18 km (11 mi), compared to the original distance of approximately 20 km (12 mi) used in the saturated zone transport modeling calculations supporting the TSPA-SR model and supplemental TSPA model analyses. The revised supplemental TSPA model utilized a slice-of-the-plume method to calculate the dose to the reasonably maximally exposed individual based on an average annual water demand of approximately 2,000 acre-ft of groundwater. Using this method, 100 percent of the released radionuclides reaching the accessible environment are contained in that groundwater volume which represents the amount of water that would be withdrawn. This method minimizes the effects of natural dilution processes that would occur along the flow path and bounds potential consequences. This volume of water is an average volume necessary to operate 15 to 25 farms, representing a range of annual groundwater volumes from 887 to 3,367 acre-ft, consistent with the proposed NRC licensing rule 10 CFR 63.115 (64 FR 8640). In addition, the revised supplemental TSPA model, consistent with 40 CFR 197.21, used a daily average groundwater consumption of 2.0 L (0.53 gal), compared to the daily average groundwater consumption volume of 2.1 L (0.55 gal) in the earlier TSPA models. The calculated peak mean annual doses over the 10,000-year regulatory compliance period for the nominal scenario using the revised supplemental TSPA model are thousands of times smaller than the probability-weighted peak mean annual doses for the disruptive scenario. Therefore, the combined peak mean annual dose for comparison with individual protection standards as calculated by the revised supplemental TSPA model is approximated as the probability-weighted peak mean annual dose from the disruptive scenario, or 0.1 mrem/yr, during the 10,000-year regulatory compliance period (Williams 2001a, Section 6.2 and Table 6-1). This is the calculated dose for both the higher- and lower-temperature operating modes, as shown in Figure 3-14. This dose is below the limits of the applicable radiation protection standard for individual protection (15 mrem/yr) as specified in 10 CFR 963.2 (66 FR 57298). The final NRC individual protection standard, as referenced in DOE's site suitability guidelines, states that the dose to the reasonably maximally exposed individual should be calculated using an annual water demand of 3,000 acre-ft (10 CFR 63.312(c) [66 FR 55732]). An average annual water demand of approximately 2,000 acre-ft was used in the TSPA analyses. The TSPA analyses assumed that 100 percent of the released radionuclides reaching the accessible environment is contained in that groundwater volume for use and consumption. The TSPA mean doses calculated for the evaluation of individual protection, therefore, represent conservative (higher) estimates; calculated values of dose for groundwater-pathway-dominated scenarios (e.g., igneous intrusion scenario and nominal scenario) would be reduced by approximately one-third using the NRC-specified annual water demand of 3,000 acre-ft. The effects of the change in annual water demand are discussed further in the TSPA Sensitivity Analyses for NRC Regulations (Williams 2001b, Section 6.3). Earlier TSPA analyses (TSPA-SR model and supplemental TSPA model) for the evaluation of individual protection (combined nominal and disruptive scenarios), were based on provisions contained in the proposed regulations. These TSPA results are summarized as follows:
It is concluded that all the projected TSPA model doses for comparison to the NRC individual protection standard, as referenced in 10 CFR 963.2 (66 FR 57298), are below the limits specified in the radiation protection standards (15 mrem/yr) for individual protection.
3.1.3 Evaluation of Repository System Performance for the Human Intrusion Scenario
The TSPA method set forth by the DOE guidelines at 10 CFR 963.16 (66 FR 57298) calls for evaluation of postclosure performance of the Yucca Mountain disposal system where there is a human intrusion, in accordance with the NRC licensing regulations. The TSPA method for human intrusion will use prescribed assumptions about the human intrusion scenario given in 10 CFR 63.322 (66 FR 55732). Further, under 10 CFR 963.16(a)(2), the DOE will consider whether the Yucca Mountain disposal system is likely to comply with the human intrusion standard only if such compliance would be required under the NRC licensing regulations. The NRC licensing regulations establishing the human intrusion standard, 10 CFR 63.321, would require compliance in licensing with the 15-mrem dose limit for individual protection if the DOE determines that, within the 10,000-year regulatory compliance period, the waste packages would degrade sufficiently that a human intrusion could occur without recognition by the driller. If the DOE determines that exposure from a human intrusion is projected to occur more than 10,000 years after disposal, the analysis of the human intrusion scenario would be presented in the environmental impact statement, and the dose limits for the human intrusion standard would not apply. The first failure of the waste package material, Alloy 22, occurs after approximately 30,000 years due to general corrosion. The DOE has, therefore, determined that the earliest time a human intrusion could occur without recognition by a driller is 30,000 years. This determination was based on analyses presented in Volume 1, Appendix A of FY01 Supplemental Science and Performance Analyses (BSC 2001a). Additional information supporting the timing of the human intrusion (at 30,000 years) is presented in the TSPA Sensitivity Analyses for NRC Regulations (Williams 2001b). The compressive strength and ductility of the metals from which the drip shields and waste packages are fabricated differ significantly from the rock that would surround them. Drillers would notice these differences. For example, the drilling assembly would buckle and bend when the bit attempts to penetrate the titanium drip shield and waste package (drill bits that are designed for rock do not easily penetrate metal, particularly titanium). The drillers should, therefore, recognize that they have attempted to drill into some material other than rock for at least as long as the drip shield or waste packages are intact. Analyses predict a 95 percent probability that the waste packages will remain intact for about 30,000 years. Because the dose from the human intrusion is expected to occur after the 10,000-year regulatory compliance period, the analysis of the human intrusion scenario (at 30,000 years) is presented in Final Environmental Impact Statement for a Geologic Repository for the Disposal of Spent Nuclear Fuel and High-Level Radioactive Waste at Yucca Mountain, Nye County, Nevada (DOE 2002c), and the dose limits for the human intrusion standard do not apply for purposes of the site suitability evaluation under 10 CFR Part 963 (66 FR 57298). The analysis is documented in the TSPA Report for Final Environmental Impact Statement and Suitability Evaluation (Williams 2001a, Section 6.4). The human intrusion scenario described in 10 CFR 63.322 (66 FR 55732) and referenced in 10 CFR 963.16(a)(2) (66 FR 55732) considers an "intruder" to be someone drilling a land-surface borehole using a drilling apparatus (under the common techniques and practices that are currently employed in exploratory drilling for groundwater in the region around Yucca Mountain). In the scenario, the intruder drills directly through a degraded waste package and subsequently into the uppermost aquifer underlying the Yucca Mountain repository. The intrusion then causes the subsequent compromise and release to groundwater of the contaminated waste in the penetrated waste package. Two human intrusion scenarios were simulated and discussed in the TSPA Report for Final Environmental Impact Statement and Suitability Evaluation (Williams 2001a, Section 6.4): one intrusion at 100 years after repository closure and the other at 30,000 years, the earliest time after disposal that the waste package would degrade sufficiently that a human intrusion could occur without recognition by the drillers. The results of the simulations for a human intrusion at 100 years after closure show a peak mean dose of 4.8 × 10-3 mrem/yr over the period of regulatory compliance. The results of the 30,000-year simulation analyses are included in the Final Environmental Impact Statement for a Geologic Repository for the Disposal of Spent Nuclear Fuel and High-Level Radioactive Waste at Yucca Mountain, Nye County, Nevada (DOE 2002c). Again, because the DOE has determined that a dose from the human intrusion could not occur at or before 10,000 years, the dose limits for the human intrusion do not apply in this suitability evaluation. Although some uncertainty exists about the timing of when penetrating a waste package would not be detected, the results above, based upon an assumption of penetration at 100 years, show that the resulting dose is orders of magitude less than the applicable radiation protection standard. Therefore, even if penetration were much earlier than expected, the resulting doses are of little consequence for the suitability evaluation. The final NRC rule (10 CFR Part 63 [66 FR 55732]) adopted requirements promulgated by the EPA in its final rule (40 CFR Part 197) regarding limits on the conduct of performance assessment analyses (10 CFR 63.342). The NRC licensing regulation would require exclusion of unlikely natural processes and events in the performance assessment for the human intrusion and groundwater protection standard. However, the NRC did not numerically define the term "unlikely." For purposes of analysis, the DOE conducted a sensitivity analysis to consider an igneous intrusion in the evaluation of the human intrusion scenario and documented it in the TSPA Sensitivity Analyses for NRC Regulations (Williams 2001b, Section 6.2). The report discusses the scenario of a human intrusion preceded by an igneous intrusion event, even though the mean annual probability of an igneous intrusion at the location of the repository is 1.6 × 10-8 (CRWMS M&O 2000b, Table 3.10-5). In the analysis, it was determined that the mean annual dose due to a human intrusion following an igneous intrusion can be approximated by the product of the conditional dose of a human intrusion event, the probability of the preceding igneous intrusion event, and the probability of the drillers not detecting the waste package (assumed to be equal to one if the drilling is preceded by an igneous intrusion event). The conditional human intrusion dose would be a function of when the preceding igneous intrusion occurs. The worst case would be if the intrusion occurs immediately following an assumed loss of institutional controls. This calculation is presented in the TSPA Report for Final Environmental Impact Statement and Suitability Evaluation (Williams 2001a) for a human intrusion at 100 years. The resultant maximum mean dose is 4.8 × 10-3 mrem/yr over the period of regulatory compliance (Williams 2001a, Table 6-1). Considering the probability of the igneous initiating event occurring sometime in 30,000 years (the earliest time after disposal that drillers would not recognize they had penetrated a waste package), the approximate peak mean dose due to a human intrusion following an igneous intrusion is the product of the probability of the intrusion and the conditional mean dose, or 2.3 × 10-6 mrem/yr. This peak mean dose is much lower than the peak mean dose due to the igneous intrusion alone (Williams 2001b, Section 6.2). These analyses show that the dose to the reasonably maximally exposed individual from a human intrusion following an igneous intrusion, as modeled in the revised supplemental TSPA-SR model, is below the applicable radiation protection standard for human intrusion (15 mrem/yr).3.2 POSTCLOSURE SUITABILITY EVALUATION METHOD—10 CFR 963.16(b)(1) THROUGH (12)
As part of the TSPA, the DOE implemented the postclosure suitability evaluation method at 10 CFR 963.16(b)(1) through (12) (66 FR 57298) (see Table 1-2 in Section 1) to provide a comprehensive evaluation of postclosure suitability. Section 3.2 presents the rationale and basis for the evaluation of each of the method guidelines.3.2.1 Data Related to Suitability Criteria
3.2.1.1 Statement of Guidelines
The postclosure suitability guideline in 10 CFR 963.16(b)(1) (66 FR 57298) states that in conducting a TSPA the DOE will:Include data related to the suitability criteria in § 963.17.The guidelines at 10 CFR 963.17 (66 FR 57298) include 12 guidelines for data and information to be included in the TSPA. Nine criteria, 10 CFR 963.17(a)(1) through (9), apply to expected processes that comprise a nominal (or reference) scenario and three criteria, 10 CFR 963.17(b)(1) through (3), are used by the DOE to consider disruptive processes and events important to the total system performance at the geological repository. Both the nominal scenario and the disruptive scenario include FEPs that, consistent with DOE guidelines at 10 CFR 963.16(b)(4), have at least one chance in 10,000 of occurring in 10,000 years. The expected processes are discussed here and in Sections 3.3.1 through 3.3.9 [3.3.1, 3.3.2, 3.3.3, 3.3.4, 3.3.5, 3.3.6, 3.3.7, 3.3.8, 3.3.9], and the disruptive processes and events are discussed in Section 3.3.10. The expected processes are pertinent to understanding the performance of a repository at Yucca Mountain and form the basis for the numerical TSPA models, which in turn forms the basis for the suitability criteria in 10 CFR 963.17(a)(1) through (9) (66 FR 57298). They are the physical processes of water falling on Yucca Mountain as rain and snow, infiltrating into the mountain, flowing down through the unsaturated zone to the potential repository level, flowing from the repository level through the unsaturated zone to the saturated zone, and flowing from the saturated zone to the outside environment. At the repository level, the water would be affected by the physical processes associated with the repository and with the waste packages and waste forms. Eventually the water, which could transport radionuclides after interaction with the waste forms, could flow out of the repository horizon and further downward through the unsaturated zone. It could subsequently move into the saturated zone, where it could eventually reach a point where the reasonably maximally exposed individual could be exposed to any radionuclides. The suitability criteria in 10 CFR 963.17(a)(1) through (9) (66 FR 57298) are derived from these expected physical processes. The sequence in which the suitability criteria are addressed in the following sections generally corresponds to the process of water flow and radionuclide transport described earlier. Above the potential repository horizon, flow processes are considered; below the repository horizon, both flow and transport processes are considered because at this point, following possible interaction with waste forms, water could be transporting radionuclides. The following sections evaluate whether the TSPA method for including data conforms to the guidelines method in 10 CFR 963.16(b)(1) (66 FR 57298). They are organized according to how a drop of water could move through the mountain and eventually reach the reasonably maximally exposed individual (i.e., a hypothetical receptor), whose location is specified in the final NRC and EPA rules (10 CFR Part 63 [66 FR 55732] and 40 CFR Part 197, respectively) and adopted by reference in the DOE guidelines in the definition of reasonably maximally exposed individual (10 CFR 963.2).
3.2.1.2 Basis for Consideration
The site characterization program conducted over the past two decades has produced a comprehensive body of site data, process models, and design information for the potential geologic repository at Yucca Mountain. These data and information are documented in many scientific and technical reports issued by the participating organizations, which include the previous and current management and operating contractors for the Yucca Mountain Site Characterization Project, the USGS, and DOE national laboratories, including Lawrence Berkeley National Laboratory, Sandia National Laboratories, Los Alamos National Laboratory, Lawrence Livermore National Laboratory, and many other organizations. Over 120 analysis model reports have been developed to support the TSPA analyses by reporting the results of field and laboratory data, together with analysis and modeling results. Interpretations and summaries of the available information from these process models are presented in nine process models, which are summarized Section 1.3.3. Volume 1, Section 1.1 of FY01 Supplemental Science and Performance Analyses (BSC 2001a) describes supplemental analyses that have been grouped into three broad categories: (1) consideration of uncertainties that were not fully quantified in Total System Performance Assessment for the Site Recommendation (CRWMS M&O 2000b), (2) consideration of scientific information developed since completion of the TSPA-SR, and (3) evaluation of the performance of the disposal system for a range of thermal operating modes. Data and information extracted from these and other technical reports were used as input to the TSPA-SR model and the supplemental and revised supplemental TSPA models. Laboratory and field data compiled for the Yucca Mountain site have been included in the postclosure suitability evaluation in the following ways:
Process models included in the revised supplemental TSPA model are generally consistent with the process models described in Total System Performance Assessment for the Site Recommendation (CRWMS M&O 2000b) and in Volume 1 of FY01 Supplemental Science and Performance Analyses (BSC 2001a). The model configuration for the long-term performance evaluations presented in the TSPA Report for Final Environmental Impact Statement and Suitability Evaluation (Williams 2001a) was modified in a manner consistent with the final EPA rule at 40 CFR Part 197. Differences in the model configuration used for these analyses from the earlier TSPA models include the following:
Conceptual Model Development—Site-specific data and design information have been translated into the formulation of conceptual models for various FEPs that are expected to occur after closure and that will, therefore, determine the evolution of the repository system. These conceptual models are presented in the nine process model reports and implemented in comprehensive process level models (DOE 2002a