Risk-Assessment Methodology for Disposal of Low-Level Radioactive Waste
Project Description and Significance
Sandia has developed a methodology for the Nuclear Regulatory Commission (NRC) to use in evaluating the postclosure performance of near-surface facilities for the disposal of low-level radioactive waste. The initial development of the methodology consisted of identifying important postclosure pathways from the disposal system to man, identifying models that can be used in analyzing those pathways, identifying potential future processes that might affect the overall performance of the disposal system, and identifying and implementing computer codes that contain appropriate models for use in the methodology. Subsequent work is directed toward expanding and improving the initial methodology, and toward building confidence that use of the methodology will lead to defensible results. The methodology under development at Sandia is intended to provide the NRC and agreement states with the tools to conduct performance assessment analyses of low-level waste sites. The work is funded under programs with both research and licensing branches of the NRC.
Sandia's Contribution
In 1990, Sandia completed an initial version of a comprehensive methodology for the postclosure performance assessment of low-level waste sites in the United States. This initial work included (1) identifying and developing models and computer codes for simulating release and migration of radionuclides from the disposal facility to humans; (2) developing a structured, generic methodology for assessing the risk associated with low-level waste sites, and (3) implementing these codes and approaches within the methodology. The initial version of the methodology has been transferred to NRC and to state regulatory authorities through a series of workshops.
Subsequent work performed at Sandia has focused on expanding and improving this initial methodology, and on developing confidence in low-level waste performance assessments. As part of this work, additional modeling capabilities have been identified or developed. In addition, the program has led to fundamental restructuring of low-level waste performance assessment in several areas:
Areas have been identified in which significant regulatory guidance is needed, and technical support has been provided to NRC to resolve these issues
Approaches have been introduced for treating uncertainty in models and their associated parameters
The relationship between performance assessment results and confidence in decision making has been elucidated.
Sandia has also initiated the development of an automated decision-support system to execute the methodology and provide the NRC, as well as other federal agencies, with a consistent technical basis for regulatory analysis of proposed sites.
Confidence is being built in the methodology through participation in international programs on model validation and intercomparison. One of these programs, INTRAVAL, focuses on validation of models for groundwater flow and transport. Another, NSARS, is an International Atomic Energy Agency (IAEA) Coordinated Research Program on near-surface disposal. This program stresses comparing international approaches for performance assessment modeling for overall system performance.
Confidence is also being built by updating the methodology to include, as appropriate, new models and modeling methods under development at Sandia, other NRC-funded agencies, or other independent researchers. Key NRC-funded research is being performed at the Idaho National Engineering Laboratory, Pacific Northwest Laboratory, and Brookhaven National Laboratory. Sandia has the role of integrating results from these areas of research into a comprehensive and functional methodology.
The IAEA has adopted the methodology as a standard near-surface disposal performance assessment methodology and is actively promoting its use in developing countries.
Future Work
A number of unresolved issues exist at present, but are expected to be addressed through the efforts of the project. The project has several primary goals:
Identify a comprehensive set of conditions to which the methodology may be applied
Identify the types of analyses needed to evaluate the conditions
Determine the adequacy of the performance assessment methodology to make these analyses
Identify the validation needs of the methodology
Evaluate the important assumptions of the methodology using validation problems
Provide ongoing updates to the NRC of the methodology as changes and improvements are implemented
Develop confidence in the methodology by participation in the IAEA intercomparison project
Provide training on the methodology to NRC and state staff
Provide short-term technical assistance upon request by the NRC program manager
Implicit in these goals is the identification and resolution of additional unresolved issues.
For further information, contact:
Paul A. Davis
Sandia National Laboratories, MS-1345
Albuquerque, NM 87185-1345
Phone: (505) 848-0754
e-mail: padavis@sandia.gov
Submitted October 1995 Layout design by Wanda Mar.
Submitted October 1995