Performance Assessment Model for Japanese High-Level Waste Disposal System

Program Brief

Statement of Problem:  The Japanese High-Level Radioactive Waste Disposal program is presently evaluating several sites for a potential deep geologic repository. Performance assessments are used to support feasibility analyses. CNWRA^® has been conducting a research and development program for IHI to support technology transfer in performance and risk assessment. The overall goal is to assist IHI to become a contending organization in areas related to repository construction and operation in Japan.

Approach and Accomplishments:  A model for diffusive transport in buffer material was developed to evaluate the relevance of processes such as retardation and precipitation to delay radionuclide release to the geosphere. A clay material that expands when wet, bentonite, may be used as buffer material to enclose waste packages, drive the system towards anoxic conditions, and delay the release of radionuclides to the geosphere in case of waste package failure. The buffer material could delay release of radioactive contaminants by thousands of years.

CNWRA^® developed performance assessment models of a hypothetical repository concept to estimate dose consequences to a critical group. The models account for source terms, diffusive transport of radionuclides in buffer material, and advective-dispersive transport in the geosphere. An exposure model was developed considering diverse pathways (e.g., ingestion, inhalation, direct exposure) to estimate doses resulting from release of radioactive material into surface waters. The models considered transport models developed for the potential repository at Yucca Mountain. A stochastic performance assessment model was constructed in GoldSim® (commercial software for stochastic and dynamic modeling) to account for uncertainty and variability in the contaminant release and transport processes. The GoldSim performance assessment model was presented at an international conference on the nuclear fuel cycle in Japan.

Client Benefits:  CNWRA^® has promoted transfer of corporate expertise in the development and interpretation of complex stochastic performance assessment models to support client interest in active involvement in development of a repository system in Japan.
 

Hypothetical Repository Concept