Experimental Approach to Flow Field Evaluation in Upper Plenum of Reactor Vessel for Innovative Sodium Cooled Fast Reactor

In Japan Atomic Energy Agency, an innovative sodium cooled fast reactor of 1500MWe class, JSFR, has been investigated on the Fast Reactor Cycle Technology Development project. A compact reactor vessel (R/V) and a column type upper inner structure with a radial slit for an arm of a fuel-handling machine are adopted. These result in increase of the spatial-averaged velocity on the horizontal cross section of the R/V by factor of 2.5. These high velocities may cause gas entrainment at the free surface in the upper plenum and also the cavitations. Therefore horizontal dipped plates (D/P) are set below the free surface to prevent the gas entrainment. We performed two water experiments using an 1/10th scaled full-sector model of the upper plenum of R/V and a large scaled partial model. The flow optimization in the upper plenum was performed in the full-sector model. It was observed in the large scaled model that the gas entrainment occurred under the extreme velocity conditions which were far from the rated condition in the reactor design of JSFR. Consequently, there is a chance for this design of the compact reactor vessel to suppress the gas entrainment and cavitation.