HR: 1340h
AN: H53F-1492    [Abstracts]
TI: Complex Unsaturated Zone Flow and Thermohydrologic Processes in a Regulatory Environment: A Perspective on Uncertainty
AU: * Fedors, R W
EM: rwf@nrc.gov
AF: U.S. Nuclear Regulatory Commission, Mail Stop EBB-2-B2, Washington, DC 20555-0001, United States
AU: Manepally, C
EM: cmanepally@swri.edu
AF: CNWRA - Southwest Research Institute, 6220 Culebra Rd, San Antonio, TX 78238, United States
AU: Justus, P S
EM: psj@nrc.gov
AF: U.S. Nuclear Regulatory Commission, Mail Stop EBB-2-B2, Washington, DC 20555-0001, United States
AU: Basagaoglu, H
EM: hbasagaoglu@swri.edu
AF: CNWRA - Southwest Research Institute, 6220 Culebra Rd, San Antonio, TX 78238, United States
AU: Pensado, O
EM: opensado@swri.edu
AF: CNWRA - Southwest Research Institute, 6220 Culebra Rd, San Antonio, TX 78238, United States
AU: Dubreuilh, P
EM: pdubreuilh@swri.edu
AF: CNWRA - Southwest Research Institute, 6220 Culebra Rd, San Antonio, TX 78238, United States
AB: An important part of a risk-informed, performance-based regulatory review of a potential license application for disposal of high-level radioactive waste at Yucca Mountain, Nevada, is the consideration of alternative interpretations and models of risk significant physical processes. The Nuclear Regulatory Commission (NRC) expects that simplified models will be abstracted from complex process-level models to conduct total-system performance assessments. There are several phases or steps to developing an abstracted model and its supporting basis from more detailed and complicated models for each area of the total system. For complex ambient and thermally perturbed flow in fractured tuffs of the unsaturated zone at Yucca Mountain, these steps c,an be summarized as (i) site characterization and observation, (ii) field and laboratory tests, (iii) conceptual model development, (iv) process-level numerical modeling, and (v) abstraction development. Each step is affected by uncertainty in (i) assessing parameters for models and (ii) conceptualization and understanding of governing processes. Because of the complexity and uncertainty, alternative interpretations and models become important aspects in the regulatory environment. NRC staff gain confidence in performance assessment model results through understanding the uncertainty in the various models. An example of a complex process in the unsaturated zone is seepage into drifts, which leads to liquid water potentially contacting waste packages. Seepage is a risk-important process for the unsaturated zone at Yucca Mountain because of its potential effect on waste package integrity and trainsport of potentially released radionuclides. Complexities for seepage include (i) characterization of fractures that carry flow, (ii) effect of small to intermediate scale structural features on flow, (iii) consideration of the diverse flow regimes (rivulets, film flow, capillarity) in fractures, (iv) effect of vapor transport associated with convection along drifts and air flow through fractures, (v) effect of the thermal perturbation caused by radioactive waste, and (vi) consideration of drift stability and climate change because of the long time periods of interest-I 0,000 to a million years. This poster tracks the available information starting from site characterization and continuing on to abstractions used in performance assessment models. At each step, multiple interpretations or alternatives can be identified based on fundamental observations and measurements. From a regulatory perspective, confidence is gained when alternative models or abstractions lead to similar results from a performance assessment; otherwise uncertainty from alternative explanations should be considered in terms of a defendable review. This abstract is an independent product of the CNWRA and does not necessarily reflect the views or regulatory positions of the NRC. The NRC staff views expressed herein are preliminary and do not constitute a final judgment or determination of the matters addressed or of the acceptability of a license application for a geologic repository at Yucca Mountain.
DE: 1873 Uncertainty assessment (3275)
DE: 1875 Vadose zone
DE: 6309 Decision making under uncertainty
DE: 6324 Legislation and regulations (6615)
SC: Hydrology [H]
MN: 2007 Fall Meeting