Experimental studies on nitrogen’s effect on reactor core cooling during a hot leg SBLOCA in a scaled EPR model

Abstract

The presence of non-condensable gases in the reactor cooling system can significantly influence the operation of several safety systems within a nuclear power plant. The pressurized nitrogen volume at the top of the accumulator tank is the driving force for injecting accumulator water. In certain nuclear power plants, the release of gaseous nitrogen to the primary side is inhibited by an automatic closure of the accumulator injection line at the end of the discharge. However, if this automatic closure system fails, nitrogen will inadvertently flow into the reactor cooling system once the accumulators have been depleted. Furthermore, it is important also to note that the water within the accumulator is saturated with dissolved nitrogen, resulting in the injection of some nitrogen into the primary system alongside every accumulator discharge.

The impact of nitrogen on core cooling during loss of coolant accidents (LOCAs) has been investigated experimentally using the PWR PACTEL facility. The main observations were that when a break occurs in the hot leg, the injection of nitrogen from an accumulator can effectively prevent depressurization of the primary side. Consequently, the core can experience a heat-up at primary pressure somewhat above the typical low-pressure safety injection (LPSI) shut-off head. The decoupling of primary and secondary side pressures depends on the amount of nitrogen released from the accumulator, how much of it accumulates into the U-shaped steam generator heat exchange tubes, decreasing the condensation heat transfer, and the number of steam generators participating in the secondary side depressurization. Furthermore, the size of the break significantly affects the volume of nitrogen escaping through the break, which in turn influences the nitrogen levels within the system. Specifically, larger breaks permit a greater flow of nitrogen, thereby reducing the likelihood of disrupting heat transfer between the primary and secondary sides while also mitigating the depressurization of the primary side.