Conceptual design for Japan Sodium-cooled Fast Reactor, 2; Thermal-hydraulic design for reactor upper sodium plenum in JSFR

Oyama, Kazuhiro*; Watanabe, Osamu*; Yamano, Hidemasa

In the present study, three-dimensional thermal-hydraulic analyses of the reactor upper plenum in JSFR were applied to evaluating the following countermeasures including the geometrical structure. The basic ideas of countermeasures are as follows. (1) In order to mitigate the thermal striping phenomenon, locations where hot sodium discharged from fuel assemblies meets with cold sodium from control rods and/or radial blanket subassemblies are kept away from un-replaceable structures above the core. (2) In order to prevent the vortex cavitations, asymmetric flow in the upper plenum due to the radial slit with UIS is mitigated by installing a cylindrical structure named as dummy plug instead of the fuel handling machine only used for refueling period. Furthermore, flow holes on perforated plates with UIS are extended to mitigate radial sodium jet from lower part of UIS. (3) In order to prevent the cover-gas entrainment, Dipped Plate (DP) is installed below the sodium free surface. Original design of DP was a double wall type with many labyrinth seals, so the manufacturing regarded as difficult. Then a single wall type DP has been newly designed and examined in this analyses.