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 4x4 and 9x9 mock-up fuel bundles with air-water co-current flow. The spacer had two different effects o n the liquid film flow in the downstream region : (a) liquid droplet e ntrainment and deposition induced by turbulence enhancement in gas cor e flow; and (b) liquid droplet entrainment and deposition induced by a direct interaction of liquid film flow and spacer elements. The forme r effect was taken into account for development of a new spacer model to estimate dependence of critical power on spacer pitch. The liquid f ilm model, including the spacer model, was confirmed to predict the cr itical power with high accuracy.