Ablation and thermal stress analysis of RPV bottom head under the heating of core melt

After the reactor core melts and collapses suddenly, the melting core deposits on the wall surfaces of the bottom head of RPV, causing severe thermal ablation and thermal stress, which endangers the safety of the bottom head of the RPV. In this paper, a 1000 MW pressurized water reactor is taken as an example to study the heat transfer ablation and thermal stress of the RPV bottom head after the core collapses using numerical simulation method. The two-dimensional polar coordinate thermal model was used to analyze the coupling heat transfer among the wall surface of RPV, the two-layer melting core pool and the outer water chamber. The transient 2 D temperature and ablation of the bottom head wall surface were calculated. The deformation and stress of the surface were performed by finite element analysis program. The results show that: (1) the volume and height of the melting core pool increase with the deposition time of the core melts in the RPV bottom head. For a statistic situation, the melting pool had a volume of 22.9 m³, a height of 3.24 m, in which volume of melted uranium oxide was 8.9 m³ and volume of melted steel in the core, RPV wall and lower support plate was 14 m³; (2) the existence of a thin layer of condensate shell among the uranium oxide melting layer, the bottom head wall surface and the metal melting layer. The thickness of the upper condensate shell was 0.01 m while the thickness of the side of the condensing shell becomes thinner with increasing polar angle. The thickness of condensate shell at the top of the melting layer was 0.01 m, the thickness of condensate shell at the bottom of the melting layer was 0.12 m; (3) After the core melts and collapses, the wall temperature of the submerged RPV under the melting pool raised with time and at 200 second after the collapse. Then the thickness of the wall was straight down until 2500 second, reaching a minimum of 0.04 m. Then the thinnest thickness no longer changed, but the melting area expanded further (4) The bottom head of RPV wall surfaces temperature distributions of the molten zone were the same shape between 4000 second and 5000 second, the molten zone along the inner wall of the head formed a lancet shape distribution. (5) The inner wall surfaces of the bottom head had a larger endothermic heat flux than the exothermic heat flux of the outer wall surfaces. The heat fluxes at the inner and outer surfaces of the two layers were maximized at 750 kW/m2 and 250 kW/m2. The heat flux on the inner surface of the molten metal pool dropped rapidly and remained constant at 400 kW/m2. While the outer surface heat flow decreases continuously with the polar angle. (authors)