Experimental investigation of the coupling interaction between nucleate boiling and boric acid migration

During the operation of pressurized water reactors (PWRs), boron hideout within Corrosion Related Unidentified Deposition (CRUD) results in the crud-induced power shift (CIPS). CIPS occurrence correlates with bubble formation rate and boron acid concentration on the fuel rod surface. The underlying mechanism of boron acid concentration migration and bubble evolution interactions during the bubble growth is unclear. In this paper, a single bubble is controlled to generate at a microfabricated cavity on a polished titanium wafer, and the bubble dynamics and associated concentration field of aqueous boron are studied under pool boiling conditions by the high-speed camera and the double-sensor conductivity probe. Experimental results reveal two stages in boron concentration distribution during bubble growth. In the initial stage, as the small bubble generates at the nucleation point, jet entrainment and vortex-shedding motion determine the boron concentration distribution near the heater surface, resulting in a plume of highly concentrated boron below the bubble. As the bubble size increases, the vorticity and range of the bubble wake decrease, and the evaporation of water and the diffusion of boric acid concentration play a significant role in the boron concentration distribution near the bubble-liquid interface. The bubble parameters, such as bubble departure diameter, departure frequency, growing time and waiting time, exhibit a nonlinear relationship with boric acid concentration under constant thermodynamic conditions. Marangoni convection induced by concentration gradients and temperature gradients plays a crucial role in the evolution of bubble parameters under pool boiling conditions. Variation of surface tension and Marangoni force with boron concentration leads to a first increase and then a decrease in bubble detachment diameter. Furthermore, the experimental data is used to support the development and validation of the bubble departure diameter and bubble frequency model. The comparison results demonstrate that the models can predict the bubble departure diameter and bubble departure frequency with accuracies of 10% and 30%, respectively. This research offers theoretical and empirical insights for developing strategies to mitigate CIPS in nuclear power plants.