ICONE19-43676 Assessment of Choking Flow Models for Subcooled Flashing Flow through Steam Generator Tube Cracks

The state-of-the-art modeling of leak rates through individual pipe stress corrosion cracks is based on regression analysis of experimentally determined leak rates and defects size. This has relatively large uncertainties. Accurate leak rate predictions require two key parameters, crack area and choking flow rate. The thermalhydraulics models employed in the leak rate calculations are generally based on the Henry-Fauske model of two-phase flow through long channels. For small length to diameter ratio (L/D) channels such as in steam generator tube cracks both mechanical and thermal nonequilibrium must be accounted. Another very important problem in verifying the model of the critical flow through cracks is the limited data on the crack geometry. Recently there is some database available on choking flow through cracks relevant to steam generator (SG) tubes to model the critical flow. These data are used in assessing the key choking flow models. Based on this assessment a mechanistic choking model is developed. The model is used to predict the choking flow rates for various experimental conditions for subcooled flashing flow through narrow slits with L/D varying from small values (〜5) to large values (100). Results are presented on the effects of thermal and mechanical non-equilibrium on the choking flow for small L/D channels.