A Study of Nuclear Reactor Pressure Vessel Lower Head Penetration using Numerical Analysis

Heat flux from the molten core heats up the pressure vessel inside and melts the inside penetrations and the welding zones. When a welding zone is melted, penetration tube ejection or rupture can occur. In this situation, the only mechanism that held the penetration fixed is the friction force between penetration tube and vessel hole. In that case, the bonding force due to the contact is influenced by the friction force between the tube and the lower vessel. This is due to the differences in thermal expansion experienced in the high temperature environment after the re-location of the molten core. In this study, a numerical analysis using ANSYS 17.1 Release 17.1 was performed to investigate the contact status due to thermal expansion between the penetration tube and the lower vessel. Three major nuclear accidents, i.e., Large Break Loss-of-Coolant Accident (LBLOCA), Station Black Out (SBO), and Total Loss of Feed-Water (TLOFW), were considered as part of the study. Results indicated that when a severe accident occurs, the inside of a penetration is initially heated up. Later, the upper part of inner vessel and the welding zone of tube No.4 are heated up. Because of the heat input and thermal expansion, contacts between the vessel and tubes occur. This contact was found to increase over time. It is probable that the tubes do not eject because of the friction forces.