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

Uncertainty and sensitivity analysis results obtained in a preliminary performance assessment for the Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico are presented. The most appropriate conceptual model for performance assessment at the WIPP is believed to include g as generation due to corrosion and microbial action in the repository and a dual-porosity (matrix and fracture porosity) representation for solute transport in the Culebra Dolomite Member of the Rustler Formati on. Under these assumptions, complementary cumulative distribution fun ctions (CCDFS) summarizing radionuclide releases to the accessible env ironment due to both cuttings removal and ground-water transport fall substantially below the release limits promulgated by the U.S. Environ mental Protection Agency (EPA). This is the case even when the current estimates of the uncertainty in analysis inputs are included. Perform ance assessment results are dominated by cuttings removal. The release s to the accessible environment due to groundwater transport make very small contributions to the total release. The variability in the dist ribution of CCDFs that must be considered in comparisons with the EPA release limits is dominated by the rate constant in the Poisson model for drilling intrusions. The variability in releases to the accessible environment due to cuttings removal is dominated by drill bit diamete r. For a single borehole, whether or not a release due to groundwater transport from the repository to the Culebra occurs is controlled by S alado halite permeability, with no releases for small values (i.e., <5 x 10(-21) m2). Further, releases that do reach the Culebra for larger values of halite permeability are small and usually do not reach the accessible environment. A potentially important scenario for the WIPP involves two or more boreholes through the same waste panel, of which at least one penetrates a pressurized brine pocket and at least one do es not. For these scenarios, the uncertainty in release to the Culebra due to groundwater transport is dominated by borehole permeability, b rine pocket pressure, and the solubilities for individual elements (i. e., americium, neptunium, plutonium, thorium, uranium). Once a releas e reaches the Culebra, matrix distribution coefficients for the indivi dual elements are important, with releases to the Culebra often failin g to reach the accessible environment over the 10 000-yr period specif ied in the EPA regulations.