Among the mitigating strategies for severe accidents, the in-vessel retention (IVR) is one of the useful remission measurements. The key point to evaluating IVR is to analyze that the final steady-state thermal load of the melt does not exceed the critical heat flux (CHF), which occurs during boiling heat transfer on the outer wall of the lower head, and the remaining wall thickness of the lower head can carry the melt to prevent the structural failure.

This study aims to analyze heat transfer of the reactor pressure vessel (RPV) lower head under severe accidents by using ASTEC code.

First of all, the composition and mass of the molten substance were assumed to be UO₂, 92 353.29 kg; Fe, 43 000 kg; Zr, 23 133.9 kg; Zr oxidation, 41.8%, for a large advanced pressurized water reactor (LAPWR). With the heavy metal oxide layer and metal layer of stable molten pool in the lower RPV of this LAPWR, the average value of core decay power and the physical properties of molten materials in RPV were input as the condition boundaries for ASTEC, the middle break accident sequence was selected for the calculation of the thermal parameters of the coolant, the outer wall CHF and the final thickness of the lower head. Then, the CHAWLA-CHAN heat transfer relationship was used to calculate the heat transfer coefficient between the melt and the inner wall of the lower head. The key safety related issues such as the heat transfer parameters of the outer wall of the lower head, the heat transfer through the lower head, and the wall thickness of the lower head were analyzed. Finally, the IVR effectiveness was estimated by the thermal properties and the structure of the lower head.

When the decay power is 21 MW and the core molten pool is divided into two layers, the average thickness of the oxide layer is 1.6 m, and the metal layer is 0.8 m. The results show that the heat exchange is more intense in the upper part of the lower head, and the maximum value of the heat flux occurs at the junction of the two melt layers, which the corresponding surface angle is 77.5°~80°. The inner wall of the lower head will be melted by the molten metal layer in the location of the minimum thickness of the lower head, and the final remaining thickness is less than 2.0 cm.