Experimental Investigation of Accidental Thermohydraulic Processes under Circuit Depressurization at ISB-VVER Safety Integral Test Facility

The paper presents the results of small-break loss-of-coolant tests carried out at the test facility ISB-VVER located in Elektrogorsk, Russia. Presently, it is the only operating integral model of the Sovjet type VVER-1000 reactor suitable for thermalhydraulic investigations. A small leak in the upper plenum of the reactor vessel was chosen for the test scenario. Different emergency core cooling (ECC) injection modes were applied and compared. It was show that the best results of ECC injection are obtained by a combined injection into both hot and cold leg. It is possible to maintain a reliable core cooling even without an injection by the safety injection tanks, which were assumed to be not available. The tests provided data suitable for code verification. Applied needle shaped void fraction probes delivered detailed information about the two-phase flow in the primary circuit.