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

• Published in: Nuclear engineering and design 2018-06, Vol.332, p.111-118
• Main Authors: Riikonen, Vesa, Kouhia, Virpi, Kauppinen, Otso-Pekka, Sjövall, Heikki, Hyvärinen, Juhani
• Format: Article
• Language: English
• Subjects: Accumulators; Boilers; Cooling; Cooling effects; Core loss; Flow rates; Flow velocity; Gases; Hydraulic equipment; Hydraulics; Injection; Leg; Loss of coolant accidents; Loss-of-coolant-accident (LOCA); Nitrogen; Nitrogen release effect; Noncondensable gases; Nuclear accidents & safety; Nuclear reactors; Pressure; Pressure reduction; Pressurized water reactors; PWR PACTEL integral facility; Reactors; Steam; Tubes; Water flow; Water levels
• ISSN: 0029-5493; 1872-759X
• DOI: 10.1016/j.nucengdes.2018.03.027

•The nitrogen gas injection from the accumulators can affect core cooling.•In contrast to some earlier studies, no piston effect driving water into the core.•Nitrogen can postpone core heatup.•Nitrogen can block the primary side depressurization and cause a core heatup. Noncondensable gases, if present in the reactor cooling system, affect the coolability of the nuclear reactor core. In Loss-Of-Coolant Accidents (LOCA), nitrogen from hydroaccumulators will enter the reactor systems, and can temporarily alter the water level in the core by a piston effect. The piston effect on the downcomer side can increase water level in core and improve core cooling (Damerell et al., 1993). The effect of nitrogen on core cooling in LOCA situations was studied experimentally in the PWR PACTEL facility. The main goal of this testing was to independently verify whether the claimed positive effect of nitrogen on the core cooling can be reproduced and to generate data for the development and validation of thermal–hydraulic system codes. 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.