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 stratificatio

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 with steel melt layer. Thus, the top water flooding of the pool surface yields an essential drop of the heat flux peak acting on the vessel wall from 1.65 to ∼1.2 MW/m2 in case at 24 h after IE; (c) the heat flux peaks acting on the vessel decrease from ∼1.65 MW/m2 (at 24 h after IE) to ∼1.15 MW/m2 (at 72 h) in case when the top flooding of the corium pool is absent, and decrease from 1.2 (24 h) to ∼0.6 MW/m2 (at 72 h) when using the top flooding. In the case of the inverse corium pool, the top water flooding essentially decreases (by more than 50%) the maximal value of heat flux in upper layer of steel melt; (d) in the case of melt inversion and redistribution of total decay heat generation in the corium pool between oxide and bottom metallic layers of the pool (parameter KOxide = QOxiide/(QOxiide + QBot_Me), the dependence of maximal values of thermal load on the lateral surface of the pool depending on KOxide value is observed. Thus, the increase of power of heat generation in the bottom metallic layer of the melt from 0.2 to 0.45 (the decrease of. KOxide from 0.8 to 0.55) causes the increase of heat flux value in the bottom layer by ∼1.5 times. Taking into account the fact that in this region of RPV lower head the CHF has low values (∼0.3…0.45 MW/m2), the probability of superheat and premature failure of the vessel bottom in this field increases. The maximal values of heat flux in the oxide phase and bottom heavy metal layer of the pool are observed near the boundary separating these layers. In this region of the VVER lower head, the heat flux attains the values that may exceed the corresponding values of CHF. Because of this, there is a high probability of superheat and the reactor vessel premature failure due to worsened heat transfer and cooling conditions on the external surface of the vessel wall. This fact should be necessarily taken into account when acting on the RPV lower head the thermal loads of moderate intensity (∼0.5…0.7 MW/m2) that may cause superheat and premature failure of the reactor vessel in the case of inverse molten pool formation during the SA in VVERs.