Numerical modelling of two-phase flow in the upper plenum of a BWR by a three dimensional two-fluid model

High moisture contents in the steam to the turbines have been detected in the nuclear power plants at Forsmark, Sweden. The origin of the problem has been located to the steam separators, which work adequately only for a fairly narrow range of moisture contents. Thus, a rather complete information of the two-phase flow field in the upper plenum is needed in order to determine the performance of each individual steam separator. A viable alternative to obtain the necessary information is a numerical study. Therefore, a three-dimensional numerical model of the upper plenum has been developed. The model is complemented with empirical correlations based on experimental data to simulate the behaviour of the steam separators. The necessary inlet boundary conditions are given by the results obtained with the POLCA code for simulation of core dynamics. The fluid dynamics of the two-phase mixture in the upper plenum is modelled using the two-fluid model, with steam as continuous phase and water droplets as dispersed. The corresponding equations of motion have been solved by means of the PHOENICS code. As a result of the calculations, the different mass flow rate to the separators are obtained. The carry-over from each separator is then obtained form the mass flow rates and empirical correlations for the separation performance. The present results show a tendency of the phase to separate, giving an increased concentration of water at the periphery, with a corresponding increase of the void at the central region of the moderator tank. This effect produces a shift in the moisture content of the steam to the separators, away from the mean nominal value, deteriorating their performance. It is concluded that separators with better characteristics are needed in order to lower the moisture content of the steam to the turbines