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.