Analysis of FLECHT-SEASET 163-rod blocked bundle data using COBRA-TF

Abstract

Flow blockage and spacer grid heat transfer models for rod bundle arrays have been developed for a two-phase flow situation characteristic of a PWR reflood. These models have been incorporated into COBRA-TF, which is a three-dimensional, three-field, two-fluid mechanistic two-phase flow subchannel computer code. Comparison of the predicted flow blockage heat transfer in large rod bundle arrays with test data indicates that the blockage and grid heat transfer models used with the COBRA-TF code agree quite well with the measured data. Bias plots of the predicted and measured temperature rises from different tests indicate that, in general, the computer code calculatons tend to underpredict the heat transfer improvement observed to have been caused by grids and blockage in the experiments. The principal reason for heat transfer improvement due to blockage and grids is the breakup of the entrained liquid droplets in the superheated steam flow above the quench front. The breakup of these entrained drops results in a population of much smaller drops, which are more easily evaporated in the superheated vapor. The enhanced heat transfer observed in and downstream of blockages and grids is also attribute to increased turbulence caused by the droplets in the steam flow. The resultingmore » computer models and methods of modeling both grids and blockages, which are described in this report, are believed to be applicable to PWR safety analysis. Application of such models is expected to significantly reduce or eliminate the calculted peak clad temperature penalty due to flow blockage for a hypothetical PWR LOCA, using the Appendix K criteria.« less