Choking Flow of Subcooled Liquid in Steam Generator Tube Wall Cracks

Abstract

Steam generator tubes have a history of small cracks and even ruptures, which lead to a loss of coolant from the primary side to the secondary side. Currently, steam generators operate under a leak-before-break approach. A rupture then signifies the loss of the integrity of the tube itself. Therefore, choking flow plays an integral part not only in the engineered safeguards of a nuclear power plant, but also to everyday operation. Choked flow of subcooled water through small cracks such as in steam generator tube wall cracks is studied both with experiments and analytical models. The knowledge of this maximum flow rate through a crack in the steam generator tubes of a pressurized water nuclear reactor will allow designers to calculate leak rates and design inventory levels accordingly while limiting losses during loss of coolant accidents. Slits of very small channel length to hydraulics diameter ratio (L/D) were manufactured and tested upto 6.89 MPa pressure and range of subcoolings 10–40 °C. Small flow channel length was used (1.3mm) equivalent to steam generator tube thickness with differences in surface roughness. The effect of L/D on the choking flow rates was examined and was contrasted with other data in literature. Analytical models were applied highlighting the importance of non-equilibrium effects and the effects of L/D ranging from 1.3 to 400 on the chocked flow were investigated.