UNCERTAINTY AND SENSITIVITY ANALYSIS RESULTS OBTAINED IN THE 1992 PERFORMANCE ASSESSMENT FOR THE WASTE ISOLATION PILOT-PLANT

Uncertainty and sensitivity analysis results obtained in the 1992 perf ormance assessment (PA) for the Waste Isolation Pilot Plant (WIPP) are presented. The primary performance measure under study is the complem entary cumulative distribution function (CCDF) used in assessing compl iance with the U.S. Environmental Protection Agency's (EPA's) standard for the geologic disposal of radioactive waste (40 CFR 191, Subpart B ). The analysis considers releases to the accessible environment initi ated by exploratory drilling for natural resources and models cuttings removal to the surface due to drilling intrusions, brine and gas flow in the vicinity of the repository and through drilling intrusions awa y from the repository, radionuclide transport by the flow of brine thr ough intruding boreholes, and brine flow and radionuclide transport in permeable formations overlying the repository (i.e., the Culebra Dolo mite). The effects of 49 imprecisely known variables are assessed with techniques based on Latin hypercube sampling and regression analysis. In addition, the effects of several alternative conceptual models for radionuclide transport in the Culebra Dolomite are investigated. Impo rtant issues identified in the analysis include (1) the importance of characterizing retardations and solubilities for individual elements, (2) the impact of assumptions involving human activities, including th e rate and properties of drilling intrusions, and (3) the need to reso lve the question of whether a single-porosity or dual-porosity transpo rt model is appropriate for use in the Culebra Dolomite.