Considerations for severe accident management under extended station blackout conditions in nuclear power plants

Primary objective of accident management in nuclear power plants is to minimize fission product release into environment by cooling the core and containment at the same time. However, under extreme condition of extended station black-out (ESBO), as in Fukushima, electric power is totally unavailable to cool the core and the containment. Thus, too early opening relief valves for depressurizing the primary system would generate large amount of steam and ironically over-pressurize the containment prior to severe core melting and subsequent reactor vessel failure. Therefore, knowledge of plant behavior considering this conflicting issue is essential under ESBO for safer accident management. Thus, in this paper, the effects of primary system depressurization and external water injection on the behaviors of the core and the containment of a pressurized water reactor under ESBO are analyzed using the code, MAAP4, which is generally used for integral analysis of nuclear power plant behavior under core-melting severe accidents. Diverse scenario cases of depressurization and external water injection starting times and flow rates are analyzed to derive positive and negative effects with respect to the integrity of a reactor vessel and a containment as well as the fission product release. The results show that too early depressurization has not only a positive effect of delaying reactor vessel failure but also a negative effect of expediting containment failure due to over-pressurization. With respect to fission product release, total cesium iodine (CsI) release for three days is significantly larger for this earlier depressurization. Reactor vessel and containment failure times are not very sensitive to the external water injection starting time when we provide an injection flow comparable to on-site safety injection pump capacity, whereas CsI release into the containment is much larger than for limited flow rates. On the other hand, with limited injection flow rates, the containment failure occurs much earlier than the reactor vessel failure since much hotter steam released from the core pressurizes the containment. These results show that optimal accident management of opening and closing primary relief valves is necessary to cope with ESBO in a safer manner.