22 - Regulatory methodologies and the risk-informed approach

Uncertainty and sensitivity analysis in performance assessment for the proposed repository for high-level radioactive waste at Yucca Mountain, Nevada

Conceptual structure and computational organization of the 2008 performance assessment for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada

In December 1998, the U.S. Department of Energy (DOE) published the ''Viability Assessment of a Repository at Yucca Mountain'' (DOE 1998b). The Viability Assessment described a preliminary design of a potential repository at Yucca Mountain, Nevada, for disposal of spent nuclear fuel and high-level radioactive waste, and assessed the probable behavior of that repository design in the Yucca Mountain geologic setting. The report concluded that 'Yucca Mountain remains a promising site for a geologic repository and that work should proceed to support a decision in 2001 on whether to recommend the site to the President for development as a repository'. It also concluded that 'uncertainties remain about key natural processes, the preliminary design, and how the site and design would interact'. Recognizing that the design that was evaluated will be refined before a license application could be submitted, the Viability Aassesment notes that 'DOE is evaluating several design options and alternatives that could reduce existing uncertainty and improve the performance of the repository system'. During the preparation of the Viability Assessment, DOE asked the contractor for the Civilian Radioactive Waste Management Program to study alternative design concepts for a potential geologic repository for high-level radioactive waste at Yucca Mountain. The License Application Design Selection (LADS) project was initiated to conduct that study. The goal of the project was to develop and evaluate a diverse range of conceptual repository designs that work well in concert with the Yucca Mountain site and to recommend an initial design concept for the possible Site Recommendation and License Apllication. This report presents the results of the LADS project. The design process consisted of two phases. In Phase I, a series of basic design concepts (design alternatives) and components (design features) were analyzed for their potential value as elements of a repository design. In Phase II, these enhanced design alternatives were refined, screened for accepatable postclosure performance, and evaluated against a set of criteria addressing four broad aspects of the repository: performance demonstrability; flexibility; construction, operations and maintenance; and cost. Based on this evaluation, a conceptual design was recommended to DOE and could be characterized as a low thermal impact design. Included in the report is more in depth information regarding the basis of the recommendation, performance issues, benefits related to reduced uncertainties, construction/operational benefits, technical and programmatic flexibilty and cost impacts.

Uncertainty and sensitivity analysis for the early failure scenario classes in the 2008 performance assessment for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada

Expected dose for the nominal scenario class in the 2008 performance assessment for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada

The waste forms under consideration for disposal in the proposed repository at Yucca Mountain contain scores of radionuclides (Attachments V and VI). It would be impractical and highly inefficient to model all of these radionuclides in a total system performance assessment (TSPA). Thus, the purpose of this radionuclide screening analysis is to remove from further consideration (screen out) radionuclides that are unlikely to significantly contribute to radiation dose to the public from the proposed nuclear waste repository at Yucca Mountain. The remaining nuclides (those screened in) are recommended for consideration in TSPA modeling for license application. This analysis also covers radionuclides that are not screened in based on dose, but need to be included in TSPA modeling for other reasons. For example, U.S. Environmental Protection Agency (EPA) and U.S. Nuclear Regulatory Commission (NRC) regulations require consideration of the combined activity of Ra-226 and Ra-228 in groundwater (40 CFR 197.30, 10 CFR 63.331). Also, Cm-245, Pu-241, and U-235 decay indirectly to potentially important radionuclides, and are not identified by the screening analysis as important. The radionuclide screening analysis separately considers two different postclosure time periods: the 10,000-y regulatory period for the proposed repository at Yucca Mountain and the period after 10,000 y up to 1 million y after emplacement. The incremental effect of extending the screening for the regulatory period to 20,000 y is also addressed. Four release scenarios are considered: (1) the nominal scenario, which entails long-term degradation of disposal containers and waste forms, (2) a human-intrusion scenario, (3) an intrusive igneous event, and (4) an eruptive igneous event. Because the first three scenarios require groundwater transport, they are called groundwater scenarios below. The screening analysis considers the following waste forms: spent boiling water reactor (BWR) fuel, spent pressurized water reactor (PWR) fuel, U.S. Department of Energy (DOE) spent nuclear fuel (DSNF), and high-level waste (HLW). Average and outlying (high burnup, high initial enrichment, low age, or otherwise exceptional) forms of each waste-form type are considered. This analysis has been prepared in accordance with a technical work plan (BSC 2002c). In a review of Revision 00 of this radionuclide screening analysis, the NRC found that ''processes that affect transport in the biosphere, such as uptake by plants and bioaccumulation are not accounted for'' and that ''the direct exposure pathway is not accounted for'' (Beckman 2001, Section 5.3.2.1). The NRC also found that the solubility and sorption classes were too broadly defined, noting, for example, that Se is in the same solubility and sorptivity groups as Np and U, yet is ''more soluble than Np and U by several orders of magnitude'' (Beckman 2001, Section 5.3.2.1). This revision seeks to build upon the strengths of the earlier screening method while responding to the specific concerns raised by the NRC and other reviewers. In place of simple inhalation and ingestion dose conversion factors, the revised radionuclide screening uses screening factors that also take into account soil accumulation, uptake by plants, exposure to contaminated ground, and other features of the biosphere that were neglected in the previous screening. Whereas the previous screening analysis allowed only two solubility classes (soluble and insoluble), the revised screening introduces an intermediate solubility class to better segregate the radionuclides into transport groups.

Yucca Mountain is currently being studied as a potential site for an underground repository for high-level radioactive waste. One aspect of the site characterization studies is an evaluation o the resource potential at Yucca Mountain. Geochemical and isotopic signatures of past alteration of the welded tuffs that underlie Yucca Mountain provide a means of assessing the probability of hydrothermal ore deposits being present within Yucca Mountain. In this preliminary report, geochemical and isotopic measurements of altered Tiva Canyon Tuff and Topopah Spring Tuff collected from fault zones exposed on the east flank of Yucca Mountain and from one drill core are compared to their unaltered equivalents sampled both in outcrop and drill core. The geochemistry and isotopic compositions of unaltered Tiva Canyon Tuff and Topopah Spring Tuff (high-silica rhyolite portions) are fairly uniform; these data provide a good baseline for comparisons with the altered samples. Geochemical analyses indicate that the brecciated tuffs are characterized by addition of calcium carbonate and opaline silica; this resulted in additions of calcium and strontium,increases in oxygen-18 content, and some redistribution of trace elements. After leaching the samples to remove authigenic carbonate, no differences in strontium or lead isotope compositions between altered and unaltered sections were observed. These data show that although localized alteration of the tuffs has occurred and affected their geochemistry, there is no indication of additions of exotic components. The lack of evidence for exotic strontium and lead in the most severely altered tuff samples at Yucca Mountain strongly implies a similar lack of exotic base or precious metals.

In June 2008, the DOE Office of Civilian Radioactive Waste Management (OCRWM) submitted an application to the US Nuclear Regulatory Commission (NRC) for a licence to construct a geologic repository for high-level nuclear waste and used nuclear fuel at Yucca Mountain, Nevada. The application was prepared by the OCRWM. In September 2008, the NRC accepted the application for review. Upon completion of a comprehensive technical review, the NRC will conduct formal adjudicatory hearings. Based on the results of the licensing review and the formal hearings, the NRC will decide whether to authorise construction of the Yucca Mountain repository.

As mandated in the Nuclear Waste Policy Act of 1982, the U.S. Department of Energy (DOE) has been investigating a candidate site at Yucca Mountain, Nevada, to determine whether it is suitable for development of the nation's first repository for permanent geologic disposal of spent nuclear fuel (SNF) and high-level radioactive waste (HLW). The Nuclear Waste Policy Amendments Act of 1987 directed that only Yucca Mountain be characterized to evaluate the site's suitability. Three main components of the DOE site characterization program are testing, design, and performance assessment. These program components consist of: Investigation of natural features and processes by analyzing data collected from field tests conducted above and below ground and from laboratory tests of rock, gas, and water samples Design of a repository and waste packages tailored to the site features, supported by laboratory testing of candidate materials for waste packages and design related testing in the underground tunnels where waste would be emplaced Quantitative estimates of the performance of the total repository system, over a range of possible conditions and for different repository configurations, by means of computer modeling techniques that are based on site and materials testing data and accepted principles of physics and chemistry. To date, DOE has completed and documented four major iterations of total system performance assessment (TSPA) for the Yucca Mountain site: TSPA-91 (Barnard et al. 1992), TSPA-93 (Wilson et al. 1994; CRWMS M and O 1994), TSPA-95 (CRWMS M and O 1995), and the Total System Performance Assessment-Viability Assessment (TSPA-VA) (DOE 1998a, Volume 3). Each successive TSPA iteration has advanced the technical understanding of the performance attributes of the natural features and processes and enhanced engineering designs. The next major iteration of TSPA is to be conducted in support of the next major programmatic milestone for the DOE, namely the Site Recommendation (SR). The Total System Performance Assessment-Site Recommendation (TSPA-SR) will present a compliance evaluation of overall system performance against the guidelines and requirements in the revision of the DOE siting guidelines to be promulgated at 10 CFR 963, U.S. Nuclear Regulatory Commission (NRC) regulation for HLW disposal at proposed 10 CFR 63 (the proposed rule has been published at 64 FR 8640), and U.S. Environmental Protection Agency (EPA) environmental radiation protection standard to be promulgated at 40 CFR 197. At present, the NRC has issued the proposed 10 CFR 63 (64 FR 8640) for public comment whereas the EPA standard and the revised DOE siting guidelines are currently being developed. EPA has announced the release of 40 CFR 197 proposed rule on its website (www.epa.gov/radiation/yucca/rule.qui.htm) and the Federal Register announcement (which initiates the 90 day public comment period) is expected by the end of August, 1999. The purpose of this document is to present the overall goals, objectives, scope, methods, approach, and assumptions to be used in the development of the TSPA-SR. This document will serve as a communication tool for coordinating the DOE TSPA activities and for keeping the NRC staff informed of the TSPA activities for the SR.

Overview of total system model used for the 2008 performance assessment for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada

Analysis of colloids erosion from the bentonite barrier of a high level radioactive waste repository and implications in safety assessment

Use of replicated Latin hypercube sampling to estimate sampling variance in uncertainty and sensitivity analysis results for the geologic disposal of radioactive waste

Temporal damping effect of the Yucca Mountain fractured unsaturated rock on transient infiltration pulses

The saturated zone at Yucca Mountain: an overview of the characterization and assessment of the saturated zone as a barrier to potential radionuclide migration

This Analysis/Model Report (AMR) documents an analysis that was performed to estimate climatic variables for the next 10,000 years by forecasting the timing and nature of climate change at Yucca Mountain (YM), Nevada (Figure l), the site of a potential repository for high-level radioactive waste. The future-climate estimates are based on an analysis of past-climate data from analog meteorological stations, and this AMR provides the rationale for the selection of these analog stations. The stations selected provide an upper and a lower climate bound for each future climate, and the data from those sites will provide input to the infiltration model (USGS 2000) and for the total system performance assessment for the Site Recommendation (TSPA-SR) at YM. Forecasting long-term future climates, especially for the next 10,000 years, is highly speculative and rarely attempted. A very limited literature exists concerning the subject, largely from the British radioactive waste disposal effort. The discussion presented here is one method, among many, of establishing upper and lower bounds for future climate estimates. The method used here involves selecting a particular past climate from many past climates, as an analog for future climate. Other studies might develop a different rationale or select other past climatesmore » resulting in a different future climate analog.« less