The Leak Before Break(LBB) concept is widely used in the nuclear industry to eliminate consideration of postulated double ended guillotine breaks. This is based on the premise that a detectable leak will develop before a catastrophic break occurs. A 'throughwall flaw' is postulated at critical sections so that any leakage from the postulated flaw is sure to be detected. The main purpose of the present study is to develop a computer code to estimate the leakage critical flow through a postulated crack area. Flow through crack may not always attain thermodynamic equilibrium, therefore Henry's non-equilibrium model and Homogeneous Frozen Model have been adopted for the present study. In these models, critical mass flow is expressed in terms of inlet quality and exit pressure and quality. The exit pressure is related to inlet pressure through cumulative effect of various pressure losses e.g. entrance loss, viscous loss, losses due to flashing, loss due to surface roughness along the crack, etc.
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