Uncertainty analyses for HLW disposal system using probabilistic safety assessment code (GSRW-PSA): Parameter uncertainty and conceptual model uncertainty in natural barrier

In the safety assessment for a geological disposal of high-level radioactive waste, it is necessary to evaluate radionuclide migration in the disposal system and radiological consequences associated with the disposal over long time scales owing to the existence of long-lived radionuclides. It is considered that there are three kinds of uncertainties with respect to parameter values, scenario, and model in the long-term assessment. In order to achieve the more reliable safety assessment, it is essential to clarify the effects based on the quantitative estimates of the uncertainties and their factors. The Japan Atomic Energy Research Institute (JAERI) has developed the probabilistic safety assessment code for HLW disposal system (GSRW-PSA) to estimate the uncertainties in parameters and conceptual models. The features and applicability of assessment codes were indicated through the bench-mark analyses for the source term models and radionuclide migration models in a generic geosphere, which have been developed by each institute, JAERI and Japan Nuclear Cycle Development Institute (JNC). The uncertainty analyses of the parameters on the engineered and natural barriers were performed for Cs-135 and Se-79 using the GSRW-PSA code. The analyses indicate that the important parameters identified by high PRCC value are solubility of selenium, distribution coefficients of cesium and selenium in rock matrix, velocity in fracture, maximum depth for matrix diffusion and fracture aperture. The effect of the uncertainty in the difference of conceptualization on a fractured medium in geosphere was estimated from the probabilistic analyses under the assumption of two conceptual models, homogeneous pours medium model and representative fracture medium model. (author)