Effect of alternative conceptual models in a preliminary performance assessment for the waste isolation pilot plant

Abstract The most appropriate conceptual model for performance assessment (PA) at the waste isolation pilot plant (WIPP) is believed to include gas generation resulting from corrosion and microbial action in the repository, and a dual-porosity (matrix and fracture porosity) representation for the solute transport in the Culebra dolomite member of the Rustler formation. Under these assumptions, complementary cumulative distribution functions (CCDFs) which summarize the radionuclide releases to the accessible environment, resulting from both cuttings removal and groundwater transport, fall substantially below the release limits promulgated by the US Environmental Protection Agency (EPA), with the releases 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 repository and a dual-porosity transport model in the Culebra; (2) gas generation in the repository and a single-porosity (fracture porosity) transport 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, without chemical retardation; (5) gas generation in the repository, chemical retardation in the Culebra, and extremes of climatic variation. These 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 groundwater 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 falling below the EPA release limits.