PIV Investigation of natural convective flow inside 8×8 partial fuel assembly

Abstract

Natural convective flow of gas inside a fuel bundle can be found in many applications including spent fuel dry storage system. To fully understand heat transfer inside a fuel bundle, it is important to evaluate how the natural convective flow field is formed inside its sub-channels. However, previous researches focused on experiments and numerical simulations in system scale. Thus, in this study, natural convective flow inside an 8×8 partial fuel assembly with low heat generation rate was investigated in sub-channel scale. An experimental apparatus was designed and manufactured to visualize the natural convective flow and to mimic the nature of decay of spent fuels using electric cartridge heaters. Also, to evaluate the effect of supporting structure such as spacer grid on natural convective flow, special spacer grids providing 42% blockage were designed based on that of prototype. Experiments were conducted with a power level of 4.3 W/rod. For flow field measurement, non-intrusive particle image velocimetry (PIV) technique was utilized. Flow field was investigated at sub-channels inside the half of fuel assembly assuming symmetry at the most upper part of the test section to investigate fully developed flow. Flow inside the sub-channels was found to be laminar in the range of tested power, in which maximum Reynolds number was less than 250. Significant flow acceleration was observed at the 7 mm upstream of spacer grid and decayed exponentially until 30 to 40 mm downstream. Local increase in heat transfer coefficient can be expected from the increased flow.