An approach to the conversion of regulatory requirements into a conceptual and computational structure that permits meaningful uncertainty and sensitivity analyses is descibed. This approach is predicated on the description of the desired analysis in terms of three basic entities: (i) a probability space characterizing aleatory uncertainty, (ii) a probability space characterizing epistemic uncertainty, and (iii) a model that predicts system behavior. The presented approach is illustrated with results from the 2008 performance assessment for the proposed repository for high-level radioactive waste at Yucca Mountain, Nevada.