EFFECTS OF A SPACER ON THREE-DIMENSIONAL VOID FRACTION DISTRIBUTIONS IN GAS-LIQUID TWO-PHASE FLOW IN A BUNDLE

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Fuel rods in water cooled reactors are supported by a spacer. The behaviors of the coolant inside and near the spacer are comlex and they are important from safety considerations of nuclear reactor to understand the two-phase flow around the spacer.The purpose of this research is to clarify the effects of the spacer on the steady two-phase flow in the rod bundle by measuring the three-dimensional void fraction distribution by a neutron dadiography CT method.Air-water two-phase flow experiments in a 4x4 rod bundle with circular ferrule type spacers were carried out and CT reconstructions with 0.18 mm in the spatial resolution were made. By the CT reconstructions, two handreds horizontal slices with 321x321 points along the conduit were carried out. The horizontal distributions were expressed by the brightness and the surface plot of the void fraction and animations were made to show the all slices for the three-dimensional distributions to develop the methods to understand well the three-dimensional distributions.The effects of the spacer on the void fraction distributions could be clarified by the obtained three-dimensional distributions. It was shown that the void fraction distribution was affected much by the spacer in annular flow region and the water gathered at four cornners in the spacer the effects of which remained downstream of the spacer.