Numerical Simulation of the Penetration Tube Ejection using PENTAP plus

The penetrations are considered as the most vulnerable parts with respect to the reactor vessel failure when a severe accident like the Fukushima accident occurs, since the melted core material (corium) relocated to the lower plenum of the reactor pressure vessel. Therefore, the determination of the failure modes and the timing at the lower head, is an important task under a given sever accident condition. The penetration tube failure modes and mechanisms were identified by J. L. Rempe et.al. Penetration tube failure can be divided into the two categories: tube ejection out of the vessel lower head and rupture of the penetration tube outside the vessel. Tube rupture assumes that the debris bed has melted the instrument tube inside the reactor and melt migrates down into the tube to a location outside the vessel wall where a pressure rupture can occur, thus breaching the pressure boundary. Tube ejection begins with degrading the penetration tube weld strength to zero when the weld is exposed to higher temperatures that range up to melting and then overcoming any binding force in a reactor vessel wall-penetration tube interface which results from differential thermal expansion of the tube and the reactor vessel. It is observed that if the melt migrates down into the tube to a location outside the vessel wall, the tube ejection does not occur although it is highly possible to lead to tube rupture. Also, the penetration tube ejection is very sensitive to the temperature distribution