EFFECT OF ALTERNATIVE CONCEPTUAL MODELS IN A PRELIMINARY PERFORMANCE ASSESSMENT FOR THE WASTE ISOLATION PILOT-PLANT

The most appropriate conceptual model for performance assessment (PA) at the waste isolation pilot plant (WIPP) is believed to include gas g eneration resulting from corrosion and microbial action in the reposit ory, and a dual-porosity (matrix and fracture porosity) representation for the solute transport in the Culebra dolomite member of the Rustle r formation. Under these assumptions, complementary cumulative distrib ution functions (CCDFs) which summarize the radionuclide releases to t he accessible environment, resulting from both cuttings removal and gr oundwater transport, fall substantially below the release limits promu lgated by the US Environmental Protection Agency (EPA), with the relea ses being dominated by cuttings removal. To provide additional views, the following alternative conceptual models were considered as part of a preliminary PA for the WIPP: (1) no gas generation in the repositor y and a dual-porosity transport model in the Culebra; (2) gas generati on in the repository and a single-porosity (fracture porosity) transpo rt model in the Culebra; (3) no gas generation in the repository and a single-porosity transport model in the Culebra; (4) gas generation in the repository and a dual-porosity transport model in the Culebra, wi thout chemical retardation; (5) gas generation in the repository, chem ical retardation in the Culebra, and extremes of climatic variation. T hese variations relate to groundwater transport, so do not affect the releases resulting from cuttings removal. Several of these variations substantially increase the importance of releases resulting from groun dwater transport relative to releases resulting from cuttings removal. However, the total amount of releases generally remained small, with the CCDFs which summarize the releases to the accessible environment f alling below the EPA release limits.