Development of advanced fuel pin bundle distortion analysis system for fast reactors

An advanced fuel pin bundle distortion analysis system has been developed for detailed evaluation on the distortion of fuel pin bundle and the thermal hydraulic behavior in the fast reactor fuel subassembly. In this system, the reiteration analysis (so-called 'two-way coupling') between the commercialized three-dimensional thermal hydraulic code 'STAR-CD' and the three-dimensional finite element code 'ETOILE' for fuel pin bundle distortion of liquid-metal-cooled fast reactor is executed. Therefore, we can obtain the results for the distortion behavior of fuel pin bundle and resulting change of coolant temperature distributions with high accuracy comparing with the conventional fuel pin bundle distortion analysis systems. The advanced fuel pin bundle distortion analysis system was applied to evaluate the integrity of the fuel subassembly for sodium-cooled small fast reactor 4S (Super-Safe, Small and Simple) under the BOC condition. The fuel pin bundle is considerably distorted in the upper half region of the core and the shape of coolant subchannels is deformed, resulting in the deterioration of the local cooling performance in the fuel subassembly. This leads to the occurrence of local hot spot in the coolant subchannel and the deformation of uniform temperature distributions in the fuel subassembly. The several partial grid spacers, which are '4S-specific' and have only three rows of grid cells in the periphery of fuel subassembly to reduce the pressure loss in the core, are installed in the gas plenum region. The large distortion of the fuel pin bundle occurs in the part of the gas plenum region, similarly to that observed in the core region. However, the pin to pin and/or pin to duct contact does not occur in the 4S-type fuel subassembly under the BOC condition. (author)