The objective of this paper is comparative study on the effect of thermal boundary condition in contact area between debris and lower head of nuclear reactor in severe core melting accident. It is focused on the structural integrity of lower head and ICI(In-Core Instrumentation) nozzle under pressure and thermal loadings in severe accident scenario.
The core debris in severe core melting accident may be relocated to and accumulated in the reactor pressure vessel lower head. In case of insufficient cooling, the excessive heat would drive the overheating and melting of lower head and ICI nozzles, and hence govern the vessel failure mode and timing. The recent idea in the management of severe accident is that the relocation of molten could be arrested at the inside of lower head by external flooding at the outside of lower head.
The boiling heat transfer was applied in the outside of lower head to the cavity water. It was suggested three kinds of thermal boundary conditions in contact area between debris and inside of lower head:
(1) applying the heat flux by natural convection of the heat-generated pool.
(2) applying the perfect contact area by assuming one body.
(3) applying the thermal resistance by assuming gap between solidified debris and lower head.
The steady state and transient heat transfer analysis were carried out to calculate the temperature of lower head applied boundary conditions. The two dimensional axisymmetric models were used to verify the integrity of the lower head. Meanwhile, the three dimensional models were used to focus on the integrity of the ICI nozzles.
It was found that the head temperature at the contact area heat up to melting temperature and the head causes damage.
The standard of safety evaluation for structures in high temperature condition is using the strain. The strain value of result for structure is not exceed allowable strain.
The analysis of according to cooling water temperature for two-dimensional heat flux model and contact resistance model is performed to compare with heat flux of outer wall in each temperature condition. This result is indicative of similar in each location.
And the result of tree-dimensional structure analysis is performed for heat flux model and contact resistance model. The boundary condition is same with two-dimensional analysis and also the result of structure analysis is not exceed allowable strain.
The creep effect should be considered under the high temperature Therefore, the creep effect is included in three-dimensional analysis. The strain value of result for creep analysis is larger than model without creep creep strain is approximately 4~6%. The result of creep analysis is evaluated safely, because the strain value is not exceed allowable strain value.