NITROGEN IN A STEAM-GENERATOR OF A PWR UNDER SBLOCA CONDITIONS - EXPERIMENTAL INVESTIGATIONS IN THE PKL TEST FACILITY AND COMPARISON WITH ANALYTICAL STUDIES

Since 1976, extensive investigations of the thermal hydraulic behavior of a pressurized water reactor (PWR) during various accident scenario s have been carried out by Siemens/KWU at the integral test facility P KL, which simulates a typical four-loop 1300 MWe PWR of KWU design. On e major topic of the current investigations includes tests under small break loss of coolant accident (SBLOCA) conditions, postulating addit ional system failures. During plant cool down in the course of SBLOCAs , besides other measures, the subsystems of the emergency core cooling system are initiated. Below a primary pressure of 26 bar, the accumul ators (ACCs) automatically inject water into the primary system. With the postulation of additional system failures in the presented PKL exp eriment, nitrogen (N-2) can enter the primary system at a primary pres sure > 10 bar before the ACCs are shut off. Under these very hypotheti cal SBLOCA conditions, nitrogen can accumulate in the steam generator U-tubes, whereby the heat removal mechanism changes from natural circu lation to reflux-condenser mode. This phenomenon is analyzed in detail ; interesting two-phase flow patterns were observed with heat removal under water-steam countercurrent flow with nitrogen present. This PKL experiment shows that a sudden stagnation of natural circulation in al l loops caused by nitrogen at considerably reduced inventory does not impede heat removal from the core; it results in the very effective re flux-condenser mode. The system stabilizes itself at a specific pressu re and temperature level, and consequently the removal of the core pow er is ensured for a given period, until further measures are initiated . (C) Elsevier Science Inc., 1997.