Numerical study on ICI nozzle ejection of external vessel cooling reactors under severe accident conditions

This study aims to develop a safety analysis methodology through which the integrity of both the lower head and the in-core instrumentation (ICI) nozzles of an external vessel-cooling reactor in nuclear power plants can be verified through finite element analyses. The ICI was used to measure the neutron flux distribution and control in the core of the reactor. Two analysis models were implemented. One was a two-dimensional model for the lower head, and the other was a three-dimensional model for the ICI nozzle. Heat transfer and thermo-mechanical analyses were also performed. The heat transfer analysis was conducted to evaluate the heat flux caused by the high-temperature melt and the conditions caused by external vessel cooling. The thermo-mechanical analysis was performed by using thermal loading, internal pressure, and deadweight of debris. Consequently, an ejection caused by the failure of the weld zone between the nozzle and the lower head was verified. An additional analysis of the nozzle support parts was then performed. The analysis results revealed that both nozzle ejection and contaminant leakage could be prevented if the nozzle support parts have enough stiffness.