The Timing of Reactor Vessel Failure during Severe Accident Progression

摘要

One of the key objectives of severe accident management is to maintain the integrity of the reactor pressure vessel (RPV) during a severe accident. The timing of an RPV lower head failure has a controlling effect on the subsequent accident progression and is an important aspect for operator recovery actions. In this paper, a sensitivity study on a lower head failure was performed to obtain insight into the operator recovery actions. Analyses on a severe accident progression were performed for a station blackout (SBO) and small break LOCA (SBLOCA), initiating severe accidents, using MELCOR 1.8.6. Operator recovery actions for decay heat removal and inventory make up using a depressurization system and safety injection pump were simulated in parallel with a simulation of the accident progression. Sensitivity studies on the timing of operator actions and variations of heat transfer coefficients from the debris to the penetrations and to the lower head were performed to investigate the changes in the timing of the vessel failure. The results show that lower head failure time is sensitive to the heat transfer from debris to the penetration and to the lower head. The change in the timing of the lower head failure was more prominent in the case of an SBLOCA initiated accident in comparison with SBO. It was also found that the formation of a molten pool plays an important role in the failure of a lower head when heat transfer from the debris to the penetration and lower head is very low.