Preliminary assessment of the impact of conceptual model uncertainty on site performance

Performance assessment modeling for high-level radioactive waste (HLW) disposal incorporates three different types of uncertainty. These include data and parameter uncertainty, modeling uncertainty (conceptual, mathematical, and numerical), and uncertainty associated with predicting future state of the system. In this study, the potential impact of conceptual model uncertainty on the performance of a hypothetical high-level radioactive waste disposal site in unsaturated, fractured tuff has been assessed for a given group of conceptual models. This was accomplished by taking a series of six, one-dimensional conceptual models, which differed only by the fundamental assumptions used to develop them, and conducting ground water flow and radionuclide transport simulations. Complementary cumulative distribution functions (CCDFs) of integrated radionuclide release to the water table indicate that differences in basic assumptions can have a significant impact on the performance of the site. Because each of the conceptual models employed the same mathematical and numerical models, did not consider future states of the system, changes in the CCDF could be attributed primarily to differences in conceptual modeling assumptions