Development of computational method for predicting vortex cavitation in the reactor vessel of JSFR

The conceptual design of Japan sodium-cooled fast reactor (JSFR) is being conducted in a Fast Reactor Cycle Technology Development (FaCT) project. One of specific design in JSFR with economic competitiveness is to compact the reactor vessel. The compact reactor vessel with significantly increased primary flow velocity might cause a vortex cavitation, which should be avoided from the viewpoint of the reactor structural integrity, near the inlet of the hot-leg piping. With the increased speed, cavitation is likely to occur due to the pressure reduction at the vortex center in the accelerating flow. The purposes of the present study are to develop a computational method for predicting vortex cavitation of JSFR, based on the theory of vortex stretching, and to evaluate the influence of the major parameters related to pressure reduction. First, we developed a method to calculate static pressure reduction at vortex centers, utilizing the computational results obtained by a commercial computational fluid dynamics (CFD) code, Star-CD (CDAJ Co., Ltd.), in a combination witha commercial visualization code, FIELDVIEW (VINAS Co., Ltd.). In this study, we calculated static pressure reduction accompanied by the vortex cavitation, focusing on vortices near the inlet of the hot-leg piping. The calculation procedure is as follows. Pressure reduction ΔP is proportional to velocity gradient α and circulation Γ squared. As major parameters for calculating the velocity gradient α, we selected the degree of a polynomial equation approximating the vortex center line position. As major parameters for calculating circulation Γ, we selected the angular division, the second invariant of velocity gradient tensor and the ratio of vorticity to vortex center vorticity. The values of these parameters were qualitatively obtained through examining their effects. In addition, we evaluated the influences of two methods to identify the vortex center, which utilize standard functions in FIELDVIEW and two integration methods for calculating circulation. Next, we analyzed the influence of the major parameters related to the pressure reduction on the major physical quantities of vortex cavitation for a 1/10 scale water flow test for the upper plenum of the JSFR reactor vessel, which was slightly different from the current design. The vortex center lines with static pressure contour obtained by the developed method are shown in Fig.1. These vortices are created by the three dimensional velocity field in complex structures. The predicted cavitation occurrence under the hot-leg piping was in good agreement with the test result. Based on this examination, we tried to apply this method to predicting vortex cavitation at the inlet of the piping of the 1/11 scale water flow test, which recently reflected the current structural arrangement in the reactor design