Analytical Study on Failure Mode Map for Lower Formed Head of Reactor Pressure Vessel Under BDBE

Abstract

Some of the important lessons learned from Fukushima Daiichi nuclear power plant accident are that mitigation of failure consequences and prevention of catastrophic failure are essential to combat severe accidents (SA) and excessive earthquake conditions that correspond to design extension conditions (DEC). To improve mitigation measures and accident management, clarification of failure behaviors depending on locations is premised under DEC such as SA and earthquakes. Design extension conditions induce some failure modes that are different from those in design conditions. The best estimation for these failure modes is necessary in order to prepare countermeasures and management. A prerequisite for conducting best estimation is to clarify the failure modes with the ultimate structural strength under extreme loads due to very high temperatures, pressure, and great earthquakes. The authors attempt to clarify unclear failure mechanisms caused by extreme loading under DEC using numerical simulation. In this paper, the relationships between failure modes and extreme loading were studied through numerical simulation using the cylinder and half-spherical model that assumes the bottom of the reactor pressure vessel (RPV) (e.g. Lower Formed Head, Instrument Tube, Gide Tube, Nozzle). This bottom structure of RPV is estimated to be under high temperature and pressure conditions due to the relocation of the molten corium. This heat loading causes major deformation of the bottom head due to creep, which leads to RPV failure. On the other hand, there is a possibility that structural discontinuities (e.g. Gide Tube Nozzle) may fail in advance. In order to recognize actual failure modes, the authors had to study the basic relationship between failure modes and load conditions in the failure mode map. This failure mode map is being considered for use as initial simple structural in beyond design basis events (BDBE) and DEC.