Simulation of non-condensable gas flow in two-fluid model of APROS – Description of the model, validation and application

In Loviisa VVER-440 type nuclear power plant the nitrogen used to pressurize hydro-accumulators and other passive safety systems is gradually dissolved to the accumulator water during the long period of normal plant operation. If a primary circuit leakage takes place, the accumulator water is injected into the primary circuit, where lower pressure is prevailing and as a consequence the dissolved nitrogen is released from the liquid phase to gas phase. It is also possible that after the liquid has run out of the accumulator the gaseous nitrogen may flow into the primary system and may thus disturb the circulation in the primary circuit. It is important that the system codes that are used in safety analysis work are capable to simulate flows of non-condensable gases and that they can take into account the release of the dissolved gases. In this paper the non-condensable gas model of the APROS two-fluid safety analysis system code is described. The model has been validated using one experiment carried out in the PACTEL VVER-440 test facility, where the release of the nitrogen dissolved in the accumulator water has been studied. The model has been used to analyze the primary–secondary leakage (PRISE) in the Loviisa nuclear power plant. In this leakage incident the dissolved nitrogen from the accumulator was assumed to flow into the primary circuit of the plant.