Performance assessment in support of the 1996 compliance certification application for the waste isolation pilot plant

The conceptual and computational structure of a performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP) is described. Important parts o f this structure are (1) maintenance of a separation between stochastic (i. e., aleatory) and subjective (i.e., epistemic) uncertainty, with stochastic uncertainty arising from the many possible disruptions that could occur ov er the 10,000-year regulatory period that applies to the WIPP, and subjecti ve uncertainty arising from the imprecision with which many of the quantiti es required in the analysis are known, (2) use of Latin hypercube sampling to incorporate the effects of subjective uncertainty, (3) use of Monte Carl o (i.e., random) sampling to incorporate the effects of stochastic uncertai nty, and (4) efficient use of the necessarily limited number of mechanistic calculations that can be performed to support the analysis. The WIPP is un der development by the U.S. Department of Energy (DOE) for the geologic (i. e., deep underground) disposal of transuranic (TRU) waste, with the indicat ed PA supporting a Compliance Certification Application (CCA) by the DOE to the U.S. Environmental Protection Agency (EPA) in October 1996 for the nec essary certifications for the WIPP to begin operation. The EPA certified th e WIPP for the disposal of TRU waste in May 1998, with the result that the WIPP will be the first operational facility in the United States for the ge ologic disposal of radioactive waste.