A metastable jet model for leaks in steam generator tubes

[Display omitted] •Phenomena of subcooled flow through leaks in thin walls are studied.•Laboratory tests on flow through thin-walled apertures with mass flow and optical measurements are performed.•A computational model assuming a jet of metastable liquid water is proposed and applied.•Comparative analysis of leak rates with different models on selected tests. The modeling and prediction of flow rates through leaks in steam generator tubes requires an understanding of the rapid decompression of a subcooled fluid flowing through a small and short flow path. The thin walls of steam generator tubes may lead to the phenomenon that the coolant water cannot evaporate within the leak flow path, which is a key difference compared to leak flow in thick-walled components. Experimental investigations in the literature were investigated and additional laboratory tests on dedicated specimens were performed, including mass flow measurements and optical measurements, which study the flow pattern at the outlet of the leak. These experiments cover a ratio of flow length (wall thickness) to hydraulic diameter L/dh range from 0.4 to 12. Based on the findings, a metastable jet model is proposed, which is based on the formation of a jet of metastable liquid water. The model parameters are fitted with the experiments. The model is compared with alternative approaches, which demonstrates that it is suitable to approximate the mass flow of leaks in steam generator tubes with a sufficiently large leak width or cross section. Due to the small absolute wall thickness of steam generator tubes only very narrow flaws are found to be successfully modeled with ordinary two-phase flow models.