NATURAL CIRCULATION EXPERIMENTS WITH A VVER REACTOR GEOMETRY

Experiments were conducted to study natural circulation behaviour in t he PACTEL facility, a medium-scale integral test loop patterned after Soviet-designed VVER pressurized water reactors. Using stepwise invent ory reduction and small-break experiments, primary loop flow behaviour was studied over a range of coolant inventories. The tests revealed a trend toward decreasing primary side mass flow rate with inventory. I n the small-break experiments, flow stagnation and system repressuriza tion were observed when the water level in the upper plenum fell below the entrances to the hot legs and coolant flow into the hot legs chan ged from single to two-phase flow. The cause of this flow interruption was attributed to the hot leg loop seals, which are a unique feature of the VVER geometry. Finally, an experiment was conducted to demonstr ate how loop seal refilling behaviour at low coolant inventories depen ds upon the steam flow rate through an individual hot leg. It was show n that loop seal refilling results when low steam velocities permit co untercurrent flow in the upflow side of the loop seal.