Abstract: Background: The study on the transient ablation of the pressure vessel wall, acted by the detained melt core, is very important for damage prediction after occurrence of serious loss of coolant accident (LOCA), and design of accident mitigation scheme. Purpose: This paper attempts to study the ablation process of downward part of pressure vessel of AP600, in condition of losing all coolant. Methods: A two-layer of melt material model was used to simulate the structure of melt core, and the heat flux on the vessel wall was computed. The unsteady 2-D heat conduction model was developed for the downward part of pressure vessel, and water boiling in containment was considered as cooling of outer wall. A moving boundary model was used to simulate the decreasing wall thickness on some point in the ablation process. The variation of wall temperature and thickness with time, from occurrence of the accident, till 3000s after the accident, was computed in detail. Results and Conclusion: Simulation results shows that heat flux from the molten UO2 on the vessel wall grows up with increasing inclination angle, and heat flux from the molten metal is a little small. Temperature on internal wall grows up rapidly with time, which causes the ablation firstly appearing at interface of the two molten layers, and the ablation zone tends to deepen and widen after that. The heat balance between molten material, vessel wall and containment pool will come 9000s after occurrence of accident, and temperature and ablation zone would not change after 9000s.

Key words: Reactor, Coolant loss, Core melt, Pressure vessel, Wall ablation