Experimental observation of adverse and beneficial effects of nitrogen on reactor core cooling

Four experiments with an accumulator injection to a cold leg were performed with the PWR PACTEL facility. In two of the experiments, the break was in the other cold leg and in the other two in the hot leg. The cold leg injection experiments confirmed that nitrogen injection had a small positive impact on the core cooling, but not by shortly redistributing water masses in the vessel as expected. The presence of nitrogen reduced the break water flow rate, delaying the core level depletion and thus postponing core heatup by few minutes. In contrast, with the hot leg break, the injection of the accumulator nitrogen into the primary side had a negative impact on the core cooling. The nitrogen accumulated in the steam generator tubes, blocking the primary side depressurization and causing a core heatup.