Features of heat and deformation behavior of a VVER-600 reactor pressure vessel under conditions of inverse stratification of corium pool and worsened external vessel cooling during the severe accident. Part 1. The effect of the inverse melt stratification and in-vessel top cooling of corium pool on the thermal loads acting on VVER-600’s reactor pressure vessel during a severe accident

•Simulations to determine a thermal state in two-layer and inverse molten corium in VVER-600.•Top flooding of the corium decreases by more than 40% the heat flux peak in two-layer pool.•For inverse corium pool, top flooding decreases by more than 50% the heat flux peak in steel layer.•In inverse corium pool, the heat flux values in the bottom heavy layer may exceed the CHF.•Comparison of the results for two-layer and inverse corium pool has been presented. The problems touched on in this work are closely associated with the realization of in-vessel melt retention strategy through the external reactor vessel cooling and cooling of the molten corium pool inside the medium- power reactor VVER-600 (thermal power is ∼1600 MW) in the course of the SA. The general objective of the research was to determine a thermal state in two- and inverse three-layer molten corium pools, which can be formed in the reactor vessel during the SA. The second task was to estimate the efficiency of the top water flooding of corium pool for its cooling in SA by comparing the new results with those obtained in the previous investigation of the authors. Compositions and mass of the corium pools for the two-layer and inverse three-layer pool structures are analyzed and presented in the paper. Simulation of heat transfer in the molten pool was performed for time values 10, 24 and 72 h from the initiating event (IE) in the SA. To estimate the influence of decay heat generation in the bottom metallic layer of the inverse molten pool on the thermal state of molten pool, a series of model SA scenarios was considered in the work. To simplify the simulations the computation domain was bounded by only the pool with taking corresponding boundary conditions. Simulations of thermal state of the molten pool were carried out by means of the NARAL/FEM computer code in which the turbulent convection at the high-Rayleigh numbers was used through the use of the effective heat conduction properties of the corium materials. The numerical results obtained for two-layer corium pool brought out a series of features: (a) the top water flooding of the melt pool resulted in temperature decrease by ∼150 K only in the upper melt steel layer and had no effect on an essential temperature change in the oxide phase of the corium; (b) top flooding of the corium results in an essential decrease (by more than 40%) of maximal values of heat flux acting on the reactor vessel in the region of contact of the vessel wall wit