BWR-4 ATWS modeling with RELAP5-S3K

Abstract

Anticipated Transients without Scram (ATWS) are evaluated to demonstrate the capability of the barriers to prevent fission product release. Swiss nuclear authority guidelines allow the use of best-estimate codes and methodologies for beyond design basis events analyses, such as ATWS, to simulate more realistic plant responses. Thus, realistic initial and boundary conditions may be applied so that all systems unaffected by the event are credited and no independent single failures need to be assumed.The RELAP5-S3K coupled thermal-hydraulics/neutronics code was used for KKM (Mühleberg BWR-4 NPP) ATWS analyses. The KKM RELAP5-S3K model was demonstrated to be valid for a wide range of transients including Loss of Coolant Accident (LOCA) and ATWS. Two ATWS cases, a main steam isolation valve (MSIV) closure and a loss of offsite power (LOOP), were simulated to demonstrate the KKM plant capability. The MSIV closure is a limiting ATWS event for reactor pressure vessel (RPV) overpressure and torus pressure and temperature. After closure of the MSIV, all RPV outlet flow is directed to the torus through the safety relief valves resulting in limiting torus pressures and temperatures comparable to other events. The LOOP event can be a limiting ATWS event for peak cladding temperature (PCT) if insufficient makeup water is available.The simulation results showed that the ATWS acceptance criteria for PCT, reactor peak pressure, and torus temperature and pressure are satisfied. The RELAP5-S3K results have been compared to the ATWS results from KKM’s fuel vendor, General Electric-Hitachi (GEH), using their TRACG code (TRACG being perhaps the most advanced system code available for BWRs). The plant response predicted by the RELAP5-S3K model is similar to that reported by GEH. Variations between the two are attributed to (1) code differences in modeling of the physical phenomena and (2) different levels of detail in modeling the KKM plant and core parameters, e.g. cross-sections.Considering that an independently developed model using the RELAP5-S3K code predicts plant behavior during an ATWS similar to the state-of-the-art code TRACG, it is concluded that the KKM ATWS analysis with RELAP5-S3K provides trustworthy information for comparison against safety criteria.