Numerical investigation of heat transfer enhancement phenomenon during thereflood phase of PWR-LOCA

The heat transfer in higher power bundles was enhanced in large-scale reflo od tests at Japan Atomic Energy Research Institute. The heat transfer enhan cement in the core under a radial power distribution is very important to q uantify the safety margin in PWR-LOCA. In this study, we analyzed the physi cal mechanism by numerical simulations with a multi-dimensional two-fluid m odel code, REFLA/TRAC; using data from the large scale reflood test. The he at transfer enhancement is caused by the increase of local upward liquid ve locity resulting from the formation of flow circulation in the core. The fl ow circulation is generated by a radial difference of waterhead below quenc h front under a radial power distribution. The upward liquid velocity depen ds on the bundle power and the cross flow resistance. The higher power and the smaller cross flow resistance give the higher upward liquid velocity, w hich increases the magnitude of the heat transfer enhancement. Through the present study, some guidelines were obtained for the multi-dimensional anal yses to predict the heat transfer enhancement phenomenon with high accuracy .