Development of thermal-hydraulics model for two-phase flow in the horizontal rectangular finned channel

For mitigation of severe accident with core melting, several types of core-catcher have been developed to catch and cool molten core. The core-catcher developed by Toshiba for Advanced Boiling Water Reactor was designed to be installed under Reactor Pressure Vessel (RPV) and catch molten core in basin with thermal-resistant material. It also has a flow path consisting of risers, coolant channels and downcomers to generate natural circulation flow of cooling water. On the other hand, there is not enough space to install it in the existing plants due to the height of the inclined coolant channels. Then, we have been developing a flat core-catcher with flat coolant channels for the existing plants to reduce total height of the structure. Finned channels will be adopted to increase heat transfer rate by increasing heat transfer area. However, the thermal-hydraulics characteristics has not been clarified due to the specific configuration, that is, the horizontal rectangular finned channel with the heated surface from upper side. We conducted experiments with the test section of coolant channel to investigate natural circulation characteristics and heat transfer behavior. In the experiments, pressure drop, flow rate and temperature were measured by changing heat flux with the test section of coolant channel with 3.5m length, which is the same as the internal radius of RPV pedestal. From the experiment results, we concluded that the flat core-catcher has enough heat removal performance to ensure structural soundness of RPV. On the other hands, it is needed to predict the maximum temperature for various plant configuration. In this study, we developed the thermal-hydraulics model to estimate maximum temperature. One of the important subject to establish method is to estimate pressure loss in the horizontal rectangular finned channel. The other subject is to estimate heat transfer rate from the heat source of debris to the water in the horizontal rectangular finned channel. By developing these two models based on the experimental results, we achieved the maximum temperature estimation method in accordance with the change of heat flux as input condition. The calculated pressure loss and temperature were compared with the test results. It was confirmed that the both calculation results agreed well with the experimental data within about plus 15 percent. It can be concluded that the heat removal performance estimation method for the flat coolant channel core-catcher was established. (author)