Analysis of thermal stratification in the upper plenum of the “Monju” reactor

• Thermal stratification in the upper plenum of the “Monju” reactor is calculated using the FLUENT code.
• The dominant influence of modeling of inner barrel flow-holes is pointed out.
• A full sector, 1/3 and 1/6 sector models are calculated.
• The turbulence models and mesh dependency affecting on thermal stratification interface are discussed.

The present paper describes a thermal stratification created in the upper plenum after the reactor scram of a liquid metal cooled fast reactor, and discusses the boundary conditions of the calculation and the effect of turbulence models on the axial movement of the thermal stratification interface. The boundary conditions for flow into the upper plenum are re-calculated using a 1D plant system code, and the results of this calculation were used as the new boundary conditions for a CFD calculation. We hypothesize that the inner barrel flow-holes have a rounded edge. A flow-hole with roundness exhibits a much lower local loss coefficient for sodium flow than a straight-edge flow-hole, resulting in an increased outflow of sodium through the flow-holes. Turbulence models are also discussed in the present study under the rounded edge configuration, and the realizable k–ɛ (RKE) turbulence model is selected as a promising model. Better agreement of the temperature distribution in the upper plenum during the transient is obtained when a rounded flow-hole edge is assumed together with the revised boundary conditions.