Introduction of accidental procedures in the Event Trees of the 900 MW PWR PRA

In France a probabilistic risk assessment (PRA) is presently in progress in the Safety Assessment Department of CEA for a standardized 900 MW PWR plant. Indeed, the French safety approach for severe accidents management is the implementation of a set of special emergency procedures which identify the optimal actions even for out of design situations. So by accounting for these procedures (correctly or incorrectly applied) an important set of operator actions can be included in the PRA. This approach implies the introduction of the notion of procedure failure at the level of the Event Trees. Quantification of the corresponding probabilities leads to several problems as physical efficiency of the procedures, systems availability (including instrumentation), and human behavior. Among these problems the physics of the sequence is a primary question, both to state the efficiency of the procedure to prevent a core damage, and to assess the allowable time for the operator actions. So a set of thermohydraulic studies was initiated for various accident sequences and procedures. The authors present the example of the small LOCA Event Trees and the studies related to the introduction of procedure actions in case of HPSI failure. The results illustrate the interest of the approach and its significant impact on the PRA