Program Brief

Statement of Problem: Proposed Federal regulations governing the disposal of high-level waste at Yucca Mountain, Nevada, require the U.S. Department of Energy demonstrate the long-term safety of the potential repository by conducting a probabilistic performance assessment. The performance assessment must simulate the many complex physical and chemical processes that affect the ability of the natural and engineered barriers to isolate high-level waste from the biosphere and account for the effects of variability and uncertainty. To assess adequacy of the DOE’s safety assessment, the U.S. Nuclear Regulatory Commission (NRC) must have the capability to conduct independent, confirmatory performance assessments.

Approach and Accomplishments: A multidisciplinary team of scientists and engineers from CNWRA^® and NRC jointly developed the Total-system Performance Assessment (TPA) code to address this problem. Version 5.0, which is the latest in many iterations of the TPA code development, couples highly abstracted models of infiltrating precipitation, water seepage into repository tunnels, corrosion of metallic waste canisters, dissolution of the high-level waste, near- and far-field transport and decay of radionuclides in both unsaturated and saturated fractured rock, and human uptake of contaminated water through a variety of biological pathways. Natural variability in material properties and uncertainty in the abstracted models are treated using a sophisticated Monte Carlo technique based on Latin hypercube sampling. The TPA code consists of more than 70,000 lines of FORTRAN developed and tested according to a stringent, client-approved software configuration management system. The TPA code can be executed on either a Unix workstation or a PC running Windows^® NT.

Client Benefits: The TPA code produces estimates of the expected annual radiation dose for 10,000 years and can account for low probability, high consequence events, such as seismic, tectonic and volcanic disruptions. The TPA code allows NRC to selectively probe specific aspects of the DOE safety case for Yucca Mountain to assess their reasonableness.