To study gas tightness of float balls of nitrogen gas compressed emergency core cooling system (TH) accumulators of Loviisa VVER-440 NPP in conditions corresponding to primary-to-secondary leakage (PRISE) and loss-of-coolant accident (LOCA), a scaled down test rig was designed and constructed at Lappeenranta University of Technology in 2010. The test rig consists of a ball vessel (height 1.5 m, volume 170 l), full-scale float balls, DN200 piping, a pump, a water separator, closing- and back-pressure valves, rupture disks, an electric heater, measurement instrumentation and a data acquisition system. The maximum operating pressure and temperature of the test rig are 16 bar and 100 °C, correspondingly. To reduce heat losses the ball vessel is insulated with mineral wool.
The test facility is equipped with temperature, pressure, pressure difference and flow rate measurements. Additional instrumentation includes a linear sensor for measuring velocity of the float balls. During the experiments movement of the float balls can be recorded with a digital high-speed video camera through the polycarbonate window on the ball vessel wall.
In 2011, the test facility was equipped with a buffer tank and connecting pipelines between the ball vessel and the buffer tank. With this configuration, the incluence of residual water on the bottom of the TH-accumulator for tightness of TH-balls was studied. In addition, a brand-new float ball design was tested.
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