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Two-phase Boiling Heat and Mass Transfer Characteristic within PWR CRUD Deposition

LIU Yan;LIU Xiaojing;HE Hui;School of Nuclear Science and Engineering, Shanghai Jiao Tong University;  
The porous chalk river unidentified deposit(CRUD) depositions not only increase the thermal resistance and lead to the deterioration of heat transfer, but also change the power distribution as a result of boron hideout. Therefore, accurate prediction of heat and mass transfer characteristics in the CRUD depositions is of great significance to the economy and safety of the reactor. In this paper, multiple physical phenomena such as heat transfer, capillary flow, solute transport and chemical reaction within the CRUD depositions were fully coupled. Coolant entered porous media under capillary action, and evaporated at the surface of chimney. The solute diffused into the porous media with capillary flow and got concentrated, while the chemical equilibrium between the solutes was always maintained. The solute transport and chemical reaction were coupled to the heat transfer and capillary flow through the saturated temperature, which was a function of dissolved solute concentration. Based on the law of conservation of energy, Darcy's law and the law of conservation of mass, the governing equations of temperature, pressure, flow velocity and solute concentrations were derived, and the boundary conditions were also determined. The finite volume method was used to discretely solve the equation in cylindrical coordinates. The maximum relative error value of temperature, pressure and each solute concentration was used as the convergence condition, and the maximum allowable value should be less than 10~(-7). The above method can reasonably predict the distributions of temperature, pressure, flow velocity and solute concentration, which relative error is within ±0.83% compared with Haq et al. Evaporation at the surface of CRUD chimney has an important influence on heat and mass transfer. The stronger the evaporation is, the smaller the maximum temperature difference is, the larger the maximum pressure drop is, and the larger the boric acid concentration is. Also the porosity, thickness, chimney radius and chimney diameter of the CRUD depositions are selected to evaluate the influence of the morphology parameters on heat and mass transfer. With the increase of thickness, the existing form of boron element changes from soluble boron species to precipitation reaction product Li_2B_4O_7. And the porosity, chimney diameter and chimney density that affect the heat and mass transfer in CRUD all have positive correlations with SNB, and have negative, positive and positive correlations with boron concentration, respectively. This study is conducive to further understanding of the boron adsorption phenomenon in CRUD and can provide technical support for the prediction of the CIPS phenomenon.
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