Investigation of two-phase flow phenomena at integral test facilities using needle conductivity probes

The paper deals with the utilization of needle shaped conductivity probes developed in Rossendorf during a small break LOCA experiment at the Budapest PMK test facility. A 1-%-break at the down-comer inlet was simulated, the ECC tanks were switched off. The probes worked properly under primary circuit conditions (12.3 MPa, 300 °C), the data acquision system provided clearly readable data in spite of the high electrical disturbance level caused by the directly heated fuel rod simulators of PMK. The paper presents plots of the signals of the probes. The level drop in the upper plenum of the reactor, the hot leg loop-seal clearing and the emptying of the steam generator cold collector were indicated. Closely after the loop-seal clearing void fraction oscillations with a period of about 22-24 sec were observed at the reactor outlet and the steam generator inlet.