Void distribution analysis with high-speed x-ray CT scanner for design of PWR grid spacer

A grid spacer of PWR fuel plays a dominant role to increase a thermal margin for safety operation of PWR since cooling effects of fuel rod are significantly promoted by fluid mixing induced by mixing vanes of grid spacers. Recently, CFD has become an available tool for the prediction of DNB performance of designed grid spacers by examining a fluid mixing effect. However, for complicated flow path like PWR fuel assembly, the current CFD is not applicable to the prediction of a two-phase slug flow due to the existence of complicated structure of interfaces between gas and liquid phases. As a result of that, an alternative approach is required to compensate the shortage of CFD applicability to the evaluation of DNB performance in the slug flow condition. In the present study, void distributions of air and water two-phase flows inside a 3x3 test rod bundle were experimentally investigated to examine the effects of grid spacers on the slug flow. A high-speed X-ray CT scanner was employed as a measurement tool that can provide with instantaneous void distributions in a cross section. Grid spacers of different types were used and the measured data were examined to investigate the differences of void distributions. Since it can be assumed that the DNB performance will be influenced by void distribution, our water DNB test data were referred to take into account the relationship between the void distributions and the performance of resistance to DNB. As a result, the detailed examinations of void profiles in flow subchannels provide the corresponding void distributions with the results of the water DNB tests. It was confirmed that the void distributions obtained by the high speed X-ray CT scanner could yield one of the available aspects from the view point of flow fields when we compare the DNB performance under two-phase slug flow condition between designed grid spacers. (author)