Thermal hydraulics of PWRS with respect to boron dilution phenomena. Experimental results from the test facilities PKL and UPTF Original Research Article

Within the PWR safety analyses, attention has increasingly focused in recent years on boron dilution events which could potentially lead to reactivity transients. In Germany, the current discussion concerning boron dilution events is focused on small break loss of coolant accident (SB-LOCA) scenarios with reflux-condenser mode and restart of natural circulation (NC). The investigation of this topic was the subject of experiments performed in the Primärkreislauf (PKL) test facility and in the upper plenum test facility (UPTF). The two test facilities represent a typical western-type pressurized water reactor (PWR) and are/were operated by Siemens/KWU in Germany. While the restart of NC was investigated in the PKL system test facility (volume 1:145, height 1:1), the UPTF experiments dealt with the mixing of water flows with different boron concentration in the cold legs, reactor pressure vessel (RPV) downcomer and the lower plenum (all these components were full-scale models). The results from the PKL test facility demonstrate that in case of a postulated SB-LOCA (30 cm2-break in the cold leg; two out of four high pressure safety injection pumps (HP-SIPs) available) NC does not start up simultaneously in all loops. This means, that plugs of non-borated condensate, that might have accumulated in the pump seal during reflux condenser mode of operation, would reach the RPV at different points in time. The UPTF tests showed an almost ideal mixing of water flows with different boron concentration in the RPV downcomer. The most important results from PKL and UPTF tests which concern boron dilution events following the restart of NC are presented in this paper.