Assessment of the possibility of core melt containment in a BBER-640 vessel

Abstract

Melting of a reactor core is a multilevel process that can proceed along more than one path. Investigation of this process should be oriented toward studying the properties and quantitative ratios of phases forming conglomerates and melts in the core space. Data on the properties of the phases and their quantitative ratio can form a base for calculating the properties of conglomerates and melts over a wide range of concentrations. The correctness of estimating the properties of complicated alloys on the basis of a knowledge of the properties of individual components and the adequacy of the model can be checked experimentally by comparing the measured and computed properties of reference alloys of a complicated composition.The postulated scenarios of hypothetical serious accidents at a nuclear power plant with a BBER reactor which are accompanied by destruction and melting of the core include a stage at which a melt of the core elements is present at the bottom of the reactor vessel. The possibility of containing the melt inside the vessel largely determines the further scenario and strategy for controlling the consequences of the accident. From this standpoint, an important technical problem, which on account of its peculiar nature presupposes mainly a computational-theoretic justification, is to preserve the integrity of the reactor vessel during prolonged contact with the melt. A probability analysis of core melt containment at the bottom of the reactor vessel performed for the Lovis nuclear power plant in Finland and the prospective AP600 design in the USA showed that with external cooling of the vessel by freely circulating water the destruction of the vessel can be regarded as physically unrealistic [1]. However, the adequacy of the simplified balance relations and empirical correlations employed in so doing for the distribution of the heat fluxes on different surfaces requires a detailed justification.The main results of the numerical and experimental modeling of the natural circulation of the heat-releasing liquid in volumes of different configuration, as published in an extensive literature (for example, [2][3][4][5][6][7]), indicates its unstable character and a substantial nonuniformity of the heat fluxes on different surfaces, which depends on the shape of the surfaces and the boundary conditions employed. Depending on the preceding scenario of a serious accident (duration of the stage of destruction of the core, the fraction of oxidized metallic materials, the degree of the yield of fission products, and so on), the bottom of the vessel can be filled with the melt, in which the liquid is likely to become separated into intermetallic and oxide regions with a different distribution of the residual heat release and degree of chemical corrosiveness with respect to the vessel steel. In this connection, it is necessary to perform computational estimates of the residual thickness of the reactor vessel and its carrying capacity during the interaction with the melt under conditions of passive external cooling ...