BEPU Methodologies and Plant Technical Specifications

The Technical Specifications (TS) of a nuclear power plant define the conditions for a safe normal operation. With such an objective, the TS set limits on operational parameters of the plant and give surveillance requirements for the observation of such bounds. The values of TS limits are obtained from the safety analyses of the plant. In fact, the traditional conservative methodologies of deterministic safety analysis (DSA) have been profusely used in this task. Nevertheless, in recent years realistic (also termed BEPU) methodologies have started to replace the conservative ones. This new methodologies use realistic models and assumptions and implement techniques for performing uncertainty analysis of their results. Many of them are statistical, with a probabilistic representation of uncertainty, and based on the random sampling of uncertain inputs and uncertainty propagation to the outputs. In this paper the relation between BEPU safety analyses and TS is analyzed. The authors have a deep regulatory experience in the evaluation and licensing of DSA methodologies. Safety analyses are aimed at showing that the real operation of the plant is safe, but they have a stronger goal: to prove that the allowed operation of the plant is safe. BEPU methodologies are not fitted for the estimation of TS bounds. They rather are used to prove the coherence of the safety analysis with the preestablished TS. Procedures for proving such coherence, with different degree of strictness, are discussed in the case of Monte Carlo-based methodologies.