Gap Width Effect on Critical Power based on Tight-Lattice 37-Rod Bundle Experiments

Abstract

Since most critical power data have been collected for tube, annulus, or BWR geometries under BWR flow conditions, there is a lack of the critical power data for very tight triangular lattice bundles under low mass velocity flow conditions that is indispensable for the thermal-hydraulic design of the Innovative Water Reactor for FLexible Fuel Cycle (FLWR). Large-scale thermal-hydraulic experiments using two tight-lattice 37-rod bundle test sections with 1.0 and 1.3 mm gaps respectively were therefore carried out within the range of 2–9 MPa in pressure and 150–1200 kg/m2 s in mass velocity. It was confirmed that the fundamental characteristics of the flow parameter impact on critical power are similar between 1.0 and 1.3 mm gaps. Then, the gap width effect was discussed using the relationship between critical quality and mass velocity. No significant differences are recognized under high mass velocity conditions (>700 kg/m2 s), whereas the critical quality in the 1.0 mm gap experiments tends to be lower under low mass velocity conditions (>700 kg/m2 s) and the difference is less than 10%.