When a reactor pressure vessel (RPV) wall embrittled owing to exposure to neutrons is cooled rapidly by the emergency core cooling system, cracks, if they exist, could propagate in the wall because of induced thermal stresses. This event is called pressurized thermal shock (PTS). Looking ahead to the aging of nuclear power plants due to extended operation, the best estimate of the RPV integrity is essential. Depending on the accident scenario, the flow regime in the downcomer can be single-phase mixed convection, wherein forced and natural convection is equally prominent. The reactor integrity assessment method based on JEAC4206-2007, used for regulatory safety examination with respect to PTS events in Japan, uses the Jackson and Fewster correlation to evaluate the heat transfer coefficient at the downcomer wall required to get the thermal stress in the wall. Since this equation was developed based on experimental data with water flowing in a heated vertical tube with a diameter of about 10 cm, it was criticized for its applicability to an actual plant in a court case seeking an injunction to grant an extension of the operating period.
 In this study, existing heat transfer correlations for opposing mixed convection were reviewed, and the prediction performance of the correlations was evaluated by comparing the correlations with several existing experimental data. It was confirmed that heat transfer correlations using the hydraulic-equivalent diameter as a characteristic length could be used for predictions regardless of channel-geometry differences. Furthermore, correlations based on nondimensional dominant parameters could be used for predictions regardless of the differences in working fluids.

This study is partly result of the contract research entrusted by the Secretariat of Nuclear Regulation Authority of Japan, and this study was referred to in the court case.