Application of the Integrated TH-PA Method to the Westinghouse AP600 Design

This paper presents an application of a thermal-hydraulic and probabilistic assessment (TH-PA) method to the Westinghouse Advanced Passive 600-MW(electric) (AP600) design. The Westinghouse probabilistic risk assessment (PRA) was used for screening probabilistically important scenarios only. Corresponding integrated-behavior-logic diagrams (IBLDs) were constructed, and accident trajectories were developed. Small cold-leg break accidents and direct vessel injection line break accidents were analyzed. Scenarios in which TH phenomena may play a major role, and which have the highest frequency of core uncovery and heatup were identified. Important insights were obtained. Application of this method to the AP600 has shown its value in prioritizing safety issues. Its application to the AP600 has been further ensured by (a) relying on the Westinghouse PRA to model functional/logical relationships of the components/systems, (b) using a peer review approach to validate the most important IBLDs, and (c) comparing accident trajectories with the U.S. Nuclear Regulatory Commission-sponsored AP600 experimental results. While this application has demonstrated effectiveness of the method in accident scenario screening for the AP600, the TH-PA method has broader potential.