A Long-Term MAAP 3.0B Analysis of a Severe Anticipated Transient Without Scram Accident in a Boiling Water Reactor

Abstract

The long-term responses of the reactor coolant system and the containment of a boiling water reactor (BWR) during an anticipated transient without scram (ATWS) initiated by an inadvertent closure of the main steam isolation valves (MSIVs) are analyzed with the MAAP 3.0B computer code. The plant used in the analysis is the Kuosheng nuclear power station, a General Electric BWR6 reactor with a Mark III containment. A comparison of near-term system responses to an MSIV closure ATWS event shows that the MAAP 3.0B code fails to reproduce the detailed thermal-hydraulic results of the sophisticated RETRAN-02 code. The MAAP code, however, makes a reasonable prediction of the timing of major events and core power in the ATWS accident analyzed. The results of a long-term MAAP ATWS analysis show that if the standby liquid control system fails in an MSIV closure ATWS event, the ATWS event might develop into a core melt accident with the containment failing before the core melts. Among the ATWS mitigation actions specified in the emergency operating procedures, manual actuation of the automatic depressurization system (ADS) as the suppression pool temperature reaches a limit specified by the heat capacity temperature limit curve had the greatest effect on the timing of the containment failure. The reduction of the high-pressure system injection rate coupled with a delay in ADS actuation until failure of the high-pressure injection system could effectively delay the containment failure of an unmitigated ATWS event.