Modeling heat transfer through chimney-structured porous deposit formed in pressurized water reactors

The model was developed to predict the heat transfer within the Chalk River Unidentified Deposit (CRUD), which is a kind of fouling found on the fuel rods of pressurized water reactors (PWR). The CRUD tends to develop steam chimneys that separate the liquid from the vapor phase. Therefore, the model describes the CRUD as a porous medium with steam chimneys. Unlike the previous approaches that assume that the evaporation takes place at the lateral surfaces of the chimneys, in the present study it is postulated that the vapor is generated by the bubble nucleation at the CRUD-clad interface, as observed via the visualization study for the chimney-structured porous medium. The generated bubble escapes through the steam chimney. The heat transfer in the CRUD can be described by three mechanisms of heat removal, which are nucleate boiling, liquid convection in the CRUD, and forced convective heat removal from the surface of the CRUD. The predicted CRUD-clad interface temperatures and overall heat transfer coefficients were compared to the experimental results, which were produced under the simulated PWR conditions (approximately 15 MPa, 300 °C). The prediction data presented better agreement with the experimental data; the normalized Root Mean Square Error (RMSE) of the present model is 18.6% in contrast with the 42.4% obtained with the Cohen model. After the validation with the experimental data, the prediction of temperature in the model was used to investigate how the heat transfer characteristics tended to change within the CRUD. Furthermore, the parametric study regarding the CRUD properties revealed that the effect of permeability on heat transfer is not significant in the nucleate boiling regime.