Study on natural circulation evaluation method for a large FBR

A study has been conducted to evaluate thermal-hydraulics during the total blackout transient of a large top-entry type fast breeder reactor equipped with a direct reactor auxiliary cooling system (DRACS) for the decay heat removal. Development has been made for a three-dimensional thermal-hydraulic model to evaluate the radial heat transfer in the core region, a three-dimensional porous body model for the time dependent temperature distribution in the fuel subassemblies and a one-dimensional natural circulation analysis model considering the thermal-hydraulic phenomena in the core region. The models have been applied to the total blackout transient analysis. It has been clarified that the secondary peak temperature at the fuel outlet decreases by 30degC by the consideration of the three-dimensional radial heat transfer involving the inter-wrapper-tube gap flow, and the third peak temperature decreased by 50degC by intentionally leading the cold sodium from the direct heat exchanger (DHX) outlet into the core barrel. The temperature peaking factor in the fuel subassemblies drops to 1.07 at the secondary peak temperature from the value of 1.16 at the rated power and to 1.04 at the third peak. These results show the importance of the models studied in this paper. (author)