Calculational method for inter-subassembly heat transfer during natural circulation in fast breeder reactors; Verification based on CCTL and PLANDTL sodium tests

Kamide, Hideki ; Nishimura, Motohiko; Hayashi, Kenji; Momoi, K.; Miyake, Yasuhiro*

It is considerably effective to utilize the natural circulation on advance of reliability of the decay heat removal systems of Fast Breeder Reactors. The natural circulation dose not depend on components which needs external power sources like pumps. This increases the reliability of the decay heat removal systems. However, thermohydraulics in the core have complex characteristics under the natural circulation. Under low flow conditions, buoyancy effects and heat transfer from high temperature subassemblies to low temperature subassemblies, i.e. inter-subassembly heat transfer, will significantly modify the flow and temperature distributions in the subassemblies. Thus, development of an evaluation method for the core thermohydraulic is significant to utilize the natural circulation. A multi-subassembly analysis method using the three-dimensional thermohydraulic analysis code, AQUA, was developed to predict thermohydraulics in the subassemblies with influence of the inter-subassembly heat transfer. In this method, each subassembly is modeled in individual mesh region of multi-region model of AQUA and the staggered half-pin mesh arrangement was applied in each subassembly. The heat transfer between the subassemblies was simulated by a thermal structure model. The analysis method was applied to two sodium experiments where three or seven subassemblies were modeled in simulated cores. The experimental analyses showed that the multi-subassembly analysis method could evaluate the thermohydraulics in the subassemblies.