Uncertainties in geochemical models of natural systems: Implications for performance assessments

Licensing a geologic repository for nuclear waste will be based primarily upon the results of a performance assessment (PA), which evaluates compliance of the repository with radiation exposure limits by modeling the engineered and natural barrier systems over extended periods (10, to 10, years). The properly completed PA must include an analysis of the sources of uncertainty, as well as the range of input parameter values, in order to determine whether the analysis is "robust", "realistic" or "conservative". This paper examines the sources of uncertainty that may be encountered in source-term and near-field models in a performance assessment of a repository for spent nuclear fuel by examining the uncertainties in modeling the geochemical behavior of the Bangombe natural reactor in Gabon.