Development and application of an external cooling model for pressure vessels based on the ISAA program

Abstract

The integrity of the pressure vessel is crucial to the safety of a reactor. During severe accidents in nuclear power plants, the IVR-ERVC (In-Vessel Retention-External Reactor Vessel Cooling) strategy is commonly adopted to prevent lower head failure. The heat transfer coefficient of the external coolant directly determines the success of the IVR-ERVC strategy. This study examines the IVR-ERVC strategy using the Integrated Severe Accident Analysis Program (ISAA). The ISAA program originally considers only three heat transfer modes for external cooling of the reactor lower head, with an overly conservative criterion for transition boiling. To improve the assessment of lower head failure, we developed a more accurate external cooling model. The model includes all external heat transfer modes. Its accuracy was validated by comparing the results with the ULPU-LIKE experimental data, with the maximum error in lower head wall temperature not exceeding 5 %. Finally, we applied the improved ISAA program to analyze a severe accident triggered by a Station Blackout (SBO) in a nuclear power plant, investigating the effectiveness of the IVR strategy for second-generation pressurized water reactors. The results indicate that in the event of an SBO accident, penetration component failure occurs at the 0° position on the lower head bottom after 8.03 h.