Study on in-vessel Thermohydralics phenomena of sodium-cooled fast reactors (I); Numerical investigation for the Rationalization of Hydrodynamics in the upper plenuln

not registered; not registered; Yamaguchi, Akira

A large-scale sodium-cooled fast breeder reactor in 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 a gas entrainment behavior from free surfaces, a flow-induced vibration of in-vessel components, a 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 understand the thermohydraulic characteristics in the upper plenum, and to investigate suitable in-vessel structure for the elimination of gas entrainment possiblity. From the analysis, the following results were obtained. (1)It is difficult to rationale in-vessel flow patterns through adjustments of porous ratio and pressure loss for a hold down plate and baffle plates installed in an upper core structure. (2)Dummy plug insertion to a slit of the upper core structure is one of effective measures to stabilize in-vessel flow patterns. (3)Flow guide devices such as a baffle ring and a partial inner barrel are also effective measures to eliminate impinging jet to a dipped plate (D/P) and to reduce horizontal flow velocity components at free surface. (4)Installations of labyrinth structures to a R/V - D/P gap is successful for decreasing of free surface horizontal flows. (5)Gap closing of an in-vessel fuel pot and two cold trap components has the effects of reductions for free surface horizontal flows and for the difference of free surface levels. Following future investigations are important preventive measures against the gas entrainment from the free surface. (1)Flattening of spatial axial velocity distributions at the R/V - D/P gap. (2)Alleviation measures of vortex concentration at free surface. (3)Separation measures of 3-dimensional vortex distributi