Probabilistic Performance Assessment Models for Low-Level Radioactive Waste Disposal Sites at the Nevada Test Site. B. Crowe, P. Black, V. Yucel, A. Colarusso, P. Black, and R. Ryti, Los Alamos National Laboratory, Neptune and Company, Bechtel Nevada, NNSA/Nevada Operations Office
The NNSA, Nevada Operations Office operates and maintains two active facilities on the Nevada Test Site (NTS) that dispose DOE-generated low-level radioactive (LLW), mixed radioactive, and classified waste in shallow trenches and pits. The operation and maintenance of the LLW disposal sites are self-regulated by the Department of Energy under Order 435.1. This Order requires formal review of a performance assessment (PA) and composite analysis (CA; assessment of all interacting radiological sources) for each LLW disposal system. The Low-Level Federal Review Group has conditionally approved, using the deterministic performance objectives of Order 435.1, the performance assessments and composite analyses for both LLW sites. These sites are now in a transitional phase under a formal maintenance program that requires testing of conceptual models, reducing uncertainty, implementing confirmatory and long-term background monitoring, all leading to eventual closure of the disposal sites. To streamline the cost of this maintenance program, we are converting the deterministic PAs to probabilistic PAs following an iterative stepped approach that includes: 1) conversion to probabilistic PAs using a probabilistic simulation computer code (GoldSim) while preserving the previous conceptual models and structure of the parameter/variable inputs; 2) benchmarking the probabilistic PAs against the deterministic PAs; 3) performing sensitivity/uncertainty analysis of the output of the probabilistic PAs to identify significant uncertainty components; 4) conducting value-of-information analysis to establish cost-benefit of uncertainty reduction; and 5) revising model probability distributions to incorporate new site information. The final step will use Bayesian updating combined with cost-benefit analysis to determine if stopping criteria for the maintenance program have been satisfied. This provides the decision basis for transitioning from maintenance to routine monitoring. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. Government.
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