Spacer Effect on Liquid Film Flow and Critical Power in BWR Fuel Bundles

Liquid film thickness measurements and flow pattern observations near a spacer have been carried out using 4×4 and 9×9 mock-up fuel bundles with air-water co-current flow. The spacer had two different effects on the liquid film flow in the downstream region: (a) liquid droplet entrainment and deposition induced by turbulence enhancement in gas core flow; and (b) liquid droplet entrainment and deposition induced by a direct interaction of liquid film flow and spacer elements. The former effect was taken into account for development of a new spacer model to estimate dependence of critical power on spacer pitch. The liquid film model, including the spacer model, was confirmed to predict the critical power with high accuracy.