A two-dimensional thermohydraulic calculation on the core of pressurized water reactor

The design of pressurized water reactors with natural coolant circulation and passive safety features, enables one to meet increased requirements for nuclear power station safety. However, in the design there are problems associated in part with the thermohydraulic calculations on the circulation loop as a whole and on the individual parts: core, power section, overflow zone, and descending part. There are numerous programs for calculating these cores, which enable one to analyze the fuel-pin assemblies and core as a whole. Those programs have been developed mainly for the VVER reactors, and their application to other pressurized water reactors requires testing. COBSIM is a modification of the common COBRA-3C program, which was set up for calculating the cores of foreign reactors with square cross section fuel-pin assemblies. Nevertheless, it has been used for the thermohydraulic calculation on a VVER with hexagonal fuel-pin assemblies. COBSIM is intended for cellwise analysis of VVER fuel-pin assemblies. The physical models are based on the assumption that the conservation equation for the two-phase flow can be written in the same way as for a single-phase flow with coolant properties averaged over the control volume. The true bulk steam content in bulk boiling can be calculated in various ways: from the homogeneous model, with a given phase slip coefficient, by means of a modified Armand relation, or with a mass flow steam content specified by the user. The boiler during underheating can be calculated from the Levy model. In COBSIM, one can specify the coolant flow at the core inlet over the channels or the mean flow over all the channels, which can be distributed in such a way as to satisfy the condition for equal pressure differences in all channels. COBSIM can be adapted to boiling-water reactors, but one then has to consider the applicability of cellwise analysis in the thermohydraulic calculation on the entire core