Abstract
Since 1976, extensive investigations of the thermal hydraulic behavior of a pressurized water reactor (PWR) during various accident scenarios have been carried out by Siemens/KWU at the integral test facility PKL, which simulates a typical four-loop 1300 MWe PWR of KWU design. One major topic of the current investigations includes tests under small break loss of coolant accident (SBLOCA) conditions, postulating additional 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 accumulators (ACCs) automatically inject water into the primary system. With the postulation of additional system failures in the presented PKL experiment, nitrogen (N 2) can enter the primary system at a primary pressure > 10 bar before the ACCs are shut off. Under these very hypothetical SBLOCA conditions, nitrogen can accumulate in the steam generator U-tubes, whereby the heat removal mechanism changes from natural circulation 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 all loops caused by nitrogen at considerably reduced inventory does not impede heat removal from the core; it results in the very effective reflux-condenser mode. The system stabilizes itself at a specific pressure and temperature level, and consequently the removal of the core power is ensured for a given period, until further measures are initiated.
Published Version
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