Flow blockage model for LOCA analyses

A flow blockage model applicable in PWR's during loss-of-coolant analyses is presented. During a postulated loss-of-coolant accident the high temperature may weaken the clad sufficiently to cause it to bulge in response to the internal gas pressure being higher than the surrounding pressure. Mechanical property models are used in BULGEX (ENC clad deformation model) to predict the onset of rod cladding instability or bulging; however, these models cannot predict the extent of bulging. Experimental data are available for determining the probability and extent of bulging. Statistical models are developed for predicting the magnitude of flow blockages in reactor fuel assemblies. Parameters in these models are evaluated with experimental rod bulging data from single and multi-rod tests that simulated LOCA conditions. The statistical models utilize bulge strain data and may be used to evaluate the following effects on maximum planar flow area blockage fraction: Presence of guide tubes; deformation of rods which have not bulged; movement of rod centers; different bulging probabilities between rods; and axial bulging distribution. The experimental data has shown the clad stress at bulging to be the primary variable for flow blockage. (U.S.)