Leak rates through cracks and slits in PHT pipes for LBB Original Research Article

The Leak Before Break (LBB) concept is widely used in the nuclear industry to eliminate the consideration of postulated double ended guillotine ruptures. 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 computer codes to estimate the leakage critical flow through a postulated crack. Flow through a crack may not always attain thermodynamic equilibrium; therefore Henryʹs Homogeneous Non-equilibrium Model (HHNM) and a Homogeneous Frozen Model (HFM) have been adopted for the present study. Here HFM has been modified taking into account the contribution of change in liquid phase kinetic energy. According to these models, critical mass flow is expressed in terms of inlet quality and exit pressure and quality. The exit pressure is related to the inlet pressure through the cumulative effect of various pressure losses, e.g. entrance loss, friction loss and loss due to flashing. A hybrid correlation for frictional pressure drop due to surface roughness has been incorporated in the present method. Computer codes have been developed based on these models. The results obtained have been verified against published data and are found to be in good agreement. Parametric studies have been carried out to examine the effects of different physical and thermal–hydraulic parameters and to assess the extent of leakage.