Analysis and Justification of MAAP4.0.7 for PRA Level 1 Mission Success Criteria

AREVA NP's U.S. EPR is a 4590-MW(thermal) evolutionary pressurized water reactor that incorporates proven technology with an innovative system architecture to provide an unprecedented level of safety. One of the measures of safety is provided by probability risk assessment (PRA). PRA Level 1 concerns the evaluation of core damage frequency based on various initiating events and the success or failure of various plant event mitigation features. Determination of this measure requires mission success criteria, which are used to build the logic that makes up the fault trees and event trees of the Level 1 PRA. Developing mission success criteria for the wide variety of accident sequences modeled in the PRA Level 1 model requires a large number of thermal-hydraulic calculations. AREVA selected the MAAP4 code to perform these calculations because of its fast computation times relative to more sophisticated thermal-hydraulic codes. This is a unique application of the MAAP4 code, which was developed specifically for severe accident and PRA Level 2 analysis. As such, a study was performed to assess MAAP4's thermal-hydraulic response capabilities against AREVA's S-RELAP5 best-estimate integral systems thermal-hydraulic analysis code.