Distribution of trace substances in the coolant of a BWR fuel element

Abstract

A transport model was developed and numerically solved for trace substances dissolved in the coolant flowing through a complete BWR fuel element. The integration of the trace substance transport equations is carried out on the same computational grid used by a two-phase three-field subchannel code, the output of which provides the necessary coolant mass flow data, including field exchange terms. The results consist of complete, field specific trace substance distributions within the domain of the simulation and with the same spatial resolution as used by the subchannel code. Downstream the onset of annular flow, it was observed that the trace substance concentrations in liquid films on heated rods are significantly higher than the concentrations in droplets and in films on unheated surfaces. A mechanistic model for disturbance waves was further introduced to assess the additional trace substance concentration increase in the time period of base film evaporation between two successive disturbance waves. The highest trace substance concentrations were found next to part-length rods in the corner of the considered fuel assembly just upstream of the two uppermost spacer grids, which approximately corresponds to the location of trace substance deposition observed during fuel inspections at a Swiss nuclear power plant.