Comparison of Thermal Hydraulic Simulations of Beyond Design Basis Events Using the MAAP4 and CENTS Computer Codes

Abstract

The MAAP4 computer code (Reference 1) is often used to perform thermal hydraulic simulations of severe accident sequences for nuclear power plant Probabilistic Risk Assessments (PRAs). MAAP4 can be used to simulate accidents for both Boiling Water Reactors (BWRs) as well as Pressurized Water Reactors (PWRs). This assessment employs MAAP 4.0.6a for PWRs (References 1 and 5), which incorporates explicit thermal hydraulic modeling of the Reactor Coolant System (RCS) and Steam Generators (SGs), along with a nodalized integrated containment model. In the PRA environment, MAAP4 has been used for applications such as the development of PRA Level 1 and Level 2 success criteria and human action timings. The CENTS computer code (Reference 2) is a simulation tool that is typically used to analyze non-Loss of Coolant Accident (non-LOCA) events postulated to occur in nuclear power plants incorporating Combustion Engineering (CE) and Westinghouse Nuclear Steam Supply System (NSSS) designs. It is licensed by the NRC perform design basis non-LOCA safety analyses. It is a best estimate code which uses detailed thermal hydraulic modeling of the RCS and SGs; however, it does not model the containment performance. It is used to perform a wide spectrum of licensing and best estimate non-LOCA event analysis and has the capability to simulate operator actions. The CENTS models are the basis for several full scope simulators in the industry. The purpose of the analyses described in this paper is to compare MAAP4 and CENTS predictions for the Station Blackout (SBO) and Total Loss of Feedwater (TLOFW) scenarios for a representative PWR in the Westinghouse fleet that employs a CE NSSS design. The results of this comparison are used to highlight postulated MAAP4 user challenges and assist in developing guidance on selecting MAAP4 parameters for use in these scenarios. The results of the analyses presented in this paper indicate several useful insights. Overall, this paper shows that when care is taken to normalize the MAAP4 and CENTS primary side natural circulation flowrate and SG modeling, the trends of the MAAP4 and CENTS predictions of core uncovery agree reasonably well.

Long-Term Station Blackout Accident Analyses of a PWR with RELAP5/MOD3.3

“…However, it was not in the scope of this study to treat the uncertainties. In the paper [17] describing the comparison of the short-term SBO results obtained by MAAP4 and CENTS computer codes it is indicated that the overall trends of key parameters are similar, and there are differences in the timing of significant occurrences (e.g., SG dryout, core uncovery). Also it is stated that although the timings and durations of key occurrences and actuations vary, MAAP4 predictions of core uncovery tend to be conservatively biased.…”

“…For example, as shown in Table 2 the RELAP5/MOD3.3 for scenario S0-75 depressurized to 1.57 MPa core uncovery was predicted to occur in 3 days and 17 hours, while in the case of MAAP 4.0.5 calculations in 2 days and 17 hours. Finally, in [17] it was stated that a new MAAP5 code incorporates a momentum equation to model the primary side natural circulation flow rate and a more detailed SG model to more accurately predict secondary side behavior. Therefore, using MAAP5 would provide even better basis for evaluation of MAAP conservatism with respect to RELAP5/MOD3.3 results.…”