Probabilistic and Deterministic Safety Analysis: Complementary Use for Safety Performance Evaluation

Deterministic Safety Assessment (DSA) and Probabilistic Safety Assessment (PSA) are both used for regulatory purposes. In general, PSA and DSA groups work independently even though the main objective of their analyses is common: demonstration of the safety of the concerned installation. The objective of the present paper is to propose an exchange of information between DSA and PSA with a sample application. The proposed approach was tested by performing Large Break Loss of Coolant Accident (LBLOCA) analysis for a VVER-1000 with a double-ended break. The RELAP5 code was used for the analysis. Besides the classical LBLOCA assumptions (e.g. instantaneous guillotine break occurrence), PSA provides information on the most probable Emergency Core Cooling (ECC) system failure that needs to be considered. This approach substitutes for the assumption made in the traditional PSA approach on the sequencing of events, which is generally based either on engineering judgment or derived from a set of preliminary deterministic calculations by simply applying a brute force method. In this view, best estimate analysis is conducted with boundary conditions derived by not only accounting for plant design (e.g. single failure assumption) but also by performing systematic analysis, which is accepted by the scientific community and by regulatory bodies. The paper summarizes the complementary use of DSA and PSA and their relationship for Level 1 analysis.