Experimental investigation of local boron concentration in a 2 x 2 rod bundle channel under subcooled boiling flow

During the operation of pressured water reactors (PWRs), subcooled boiling occurring at the upper -span region of the fuel assembly causes migration of the boron concentration in the coolant. The distribution of boron concentration in the coolant is essential to predict the boron hideout within the Corrosion Related Unidentified Deposition (CRUD) and crud -induced power shift (CIPS). However, experimental data on local boron concentration of rod bundle geometries in subcooled boiling conditions are not available. In this paper, a vertical test channel is constructed to determine the distribution of the local boron concentration of a 2 x 2 rod bundle channel under subcooled boiling conditions. The bubble -boron concentration simultaneous measurement device is developed and calibrated to measure the local two-phase flow parameters and boron concentrations under the subcooled boiling conditions. The experimental results indicate that the boron concentration is highest near the narrowest gap and decreases toward the subchannel center or subchannel wall. The boron concentration increases linearly with void fraction in the central subchannel, while the boron concentration first increases rapidly and then slowly in the wall channel. The underlying mechanism for the boron concentration distribution is revealed by considering the boron enrichment on the bubble - liquid interface, diffusion of the boron concentration, boron mixing induced by bubble wake, and boron dilution caused by bubble condensation. On the basis of that, a boron equilibrium model considering the evaporation and condensation of bubbles is developed. The experimental data is used to support the development and validation of the boron equilibrium model. The comparison results show that the boron equilibrium model can predict the performance of the boron concentration in the rod bundle channel with an accuracy of 20 %.