Modeling critical flow through steam generator tube cracks

Modeling of leakage rates through geometries representative of steam generator tube cracks is being investigated. These cracks are characterised by very small flow areas and low length to diameter ratios. Two sets of experiments were conducted by researchers at Purdue University measuring flow rates through several slits in 3.175 mm and 1.3 mm thick samples, with L/D ratios as low as 1.2. A pressure differential of 6.8 MPa was applied across the samples with varying degrees of subcooling. Flow rates through these samples were modeled using the thermal-hydraulic system codes RELAP and TRACE, using different nodalization techniques and both the Henry Fauske and Ransom Trapp critical flow models available in RELAP. Model results are compared to experimental values and modeling techniques are discussed. TRACE and RELAP were found to have similar accuracy in predicting flow rate trends, with higher accuracy at larger L/D. In general best results were achieved by modeling the crack as a junction component.