Study on in-vessel thermohydraulics phenomena of sodium-cooled fast reactors. 2. Numerical investigations on proprieties of R/V upper plenum separation

A large-scale sodium-cooled fast breeder reactor investigated in the feasibility studies on commercialized fast reactors has a tendency of consideration of thorough simplified and compacted system designs to realize drastic economical improvements. Therefore, special attention should be paid to thermohydraulic designs for gas entrainment behavior from free surfaces, a flow-induced vibration of in-vessel components, thermal shock for various structures due to high-speed coolant flows, nonsymmetrical coolant flows, etc. in the reactor vessel. In-vessel thermohydraulic analyses were carried out using a multi-dimensional code AQUA to contribute to a planning of water scaled-model experiments for the separated upper plena of the reactor vessel. From the analysis, the following results were obtained. (1) It can be considered that there is little effect of volumetric flow rate between the upper plenum and the free surface plenum on hydrodynamic characteristics in the whole upper plenum. Because the affected area is limited in the neighborhood of gaps of hot/cold leg pipes. (2) Change in flow velocity components is limited around the outer wall of hot leg pipes if the gap width of the dipped plate and penetrating components changes. From the above results, it was concluded that the assumption seems to be valid that the free surface plenum hydrodynamic characteristics is independent of the upper plenum flows. However it is necessary to consider jet effects due to the hot leg intake flows affecting vortex concentrations in the upper plenum. (author)