シビアアクシデント発生時におけるBWRプラント挙動の基礎的研究(1): （主蒸気系/再循環系 LOCA＋ECCS 失敗）事故

Abstract

In this paper, we report on the results using the BWR plant simulator about the plant behavior under the condition of the severe accident that LOCA occurs but ECCS fails the water irrigation into the reactor core. The simulation experiments were carried out for the cases that LOCA has occurred in the main steam piping or in the recirculation piping, respectively. As for the results about the relationship between the LOCA area and the time from LOCA occurs until the fuel temperature rise start, the effect that RCIC operated was extremely big for LOCA area of up to 100cm2 for both type LOCA. In the case of main steam system LOCA, the core water level suddenly decreased for large LOCA of 2000cm2 area, however, if the irrigation into the reactor core was carried out 30min after LOCA occurrence, the core had little damage. In addition, the H2 concentration in the containment vessel did not exceed both limits of H2 explosion nor detonation. The pressure of the containment vessel was around 3kg/cm2 of design value, so the soundness of the containment vessel was confirmed. On the other hand, for the recirculation system LOCA of 2000cm2 area, a drop of the core water level was extremely in comparison with main steam system LOCA, and the fuel assemblies were completely exposed during up to 30min. to the irrigation from approximately 100sec. after LOCA occurrence. Therefore, the fuel temperature during the irrigation had reached approximately 1900 C. Thus, the fuel cladding were damaged approximately less than 10%, and H2 concentration in the containment vessel was approximately 9% which did not exceed H2 detonation limit of 13% but exceeded H2 explosion limit of 4%. However, the containment vessel internal pressure was settled around design pressure value of containment vessel. As the results, some core damage could not be avoided, but soundness of the containment vessel, which should take the role of "confine", was found to be secured.