Nuclear energy is one of the easiest and the most economical energy resources to use. However, nuclear energy that can be used easily and inexpensively also has various risks, one of which is nuclear accident.
This study aims to develop a safety analysis methodology through which the integrity of both the lower head and the in-core instrumentation (ICI) tubes can be verified by applying finite element analyses. The ICI is used to measure the neutron flux distribution and to control in the core of the reactor.
Two analysis models were implemented. One was a two-dimensional model for verifying integrity of structures of the lower head, and the other was a three-dimensional model for verifying ejection of ICI tube.
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 boundary conditions caused by external vessel cooling. The thermo-mechanical analysis was performed by using thermal loading, internal pressure, and deadweight of debris.
The results showed that the melting and breakage of the ICI tube occurred because of the high temperature, but the ejection of the ICI tube was delayed because of the contact between the ICI tube and the lower head. However, the ICI tube was eventually ejected, thereby confirming that the structural combination of the reactor head and tube could not prevent the ejection.
Substructure analysis was performed to propose the method that can be applied to prevent ICI tube failure. Consequently, the analysis result confirmed that the ICI tube could be prevented from ejection if the lower structure has sufficient stiffness.
However, if these proposed preventive measures are not properly applied in situation of unexpected exception, the inner melt may be ejected and contact with the cooling water may cause an external steam explosion. An external steam explosion analysis was performed because the steam explosion may influence on the reactor vessel. As a result of the analysis, failure of the reactor vessel was confirmed.