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 2. Creep deformation and failure of the reactor pressure vessel

Abstract

In the present manuscript the features of deformation of medium-power VVER-600 reactor pressure vessel (RPV) during a hypothetical severe accident (SA) in the case of worsened regimes of external vessel cooling are considered and analyzed. The analysis of thermal and deformation behavior of the RPV was performed for thermal loads on the reactor vessel that were defined before. The thermal loads acting on the reactor vessel from the corium melt were defined for two types of corium melt structures. A two-layer stratified melt, when a less dense layer of molten steel is located over the oxide heat-generating phase of the corium, was considered as the first structure of the molten pool. The inverse three-layer corium melt was considered as the second type of the melt structure. The numerical simulation of heating and creep deformation processes of the reactor vessel was performed with allowance for creep effect and failure of the RPV while varying both the value of in-vessel overpressure (from 0.2 to 0.8 MPa) and the conditions of external cooling of the RPV in the SA. The external cooling of the RPV were simulated by giving corresponding values of heat transfer coefficients (HTC) on the external surface of the vessel wall. The values of HTCs on the external surfaces of both cylindrical part of the vessel and the vessel bottom varied from 350 to 900 W/(m2K). The performed analysis resulted in the fact that under the conditions of worsened external cooling of the RPV in the SA, significant deformations of the RPV structure are observed. Particularly, during the SA the vertical displacement of the RPV lower head (LH) until the moment of its failure may attain 500 mm and more. Such considerable creep deformations of the reactor vessel structure are observed in the case of forming the inverse structure of the corium pool. Here, the LH deformations make up the bulk of total RPV deformation. Such a model of the vessel deformation may lead to partial or complete blockage of cooling gaps used for external cooling of the reactor vessel in the SA. In turn, the disturbance of the regime of external reactor vessel cooling may cause its overheating and premature RPV failure. The dependences of RPV failure time and the value of vertical displacement of the RPV structure on the value of in-vessel overpressure were obtained for the considered group of SA scenarios.