Thermal-hydraulic investigation on core and fuel assembly of several fast reactors design concepts

The feasibility study (Phase I) has been carried out at JNC to build up new design concepts of commercialized fast reactors from the viewpoint of economy, safety, effective use of resources, reduction of environmental burden and nuclear non-proliferation. This report describes the results of the investigation related to core/fuel-assembly thermal-hydraulics that was performed in fiscal 2000 as a part of the feasibility study. A numerical analysis method was developed for the coated-particle-type fuel assembly in the helium-gas-cooled fast reactor and a parametric study was performed using it. It revealed that with proper form pressure losses at inlet and outlet surfaces of the fuel region it is possible to control flow distribution under the rated power operation condition and that the decay heat removal may fail if the natural circulation is driven only by heat generation in the fuel region. A detailed numerical analysis of local fuel region was also carried out. The characteristics of coolant flow/temperature fields, particle-surface temperature distribution and the maximum temperature in the fuel particle were grasped and the applicability of the pressure drop correlation to such porous media was confirmed. A subchannel analysis code ASFRE was applied to calculations of flow and temperature fields in a fuel assembly with inner duct in sodium cooled reactors, which is examined for re-criticality elimination. The calculation results showed that the peak coolant temperature was higher than that of the normal fuel assembly (without inner duct) under the same power-to-flow ratio condition and its temperature difference becomes much larger as the number of fuel pins decreases. The same tendency was observed in the case of lateral skew power profile in the fuel assembly. In this case, the difference of the peak temperatures between fuel assemblies with/without inner duct is almost proportional to the peaking factor. A parametric analysis was carried out for an example design of large ductless cores sodium-cooled reactors. It revealed that the flow through the gap between fuel assemblies has much influence on the temperature field in the core and that the peaking factor can be decreased if the gap flow is reduced by grid spacers. With respect to the minor-actinide-loaded MOX fuel assembly that has heat generation even before initial loading, the structure temperature distribution was evaluated under the storage condition in the air atmosphere. In the case of vertical storage, the calculation results showed that the fuel assembly with 5% minor-actinide loading was coolable by natural convection heat removal. In the case of horizontal storage, however, the clad surface temperature of fuel assembly with at most 1.2% minor-actinide loading exceeded the temperature limitation under the conservative assumption neglecting natural convection along the fuel pins. (author)