Experimental study on the void reactivity coefficient in the KUCA

The purpose of this study is to measure the reactivity coefficient of voids in the channels between the fuel plates and to investigate the impact of core conversion from the use of high-enriched-uranium (HEU) to the use of medium-enriched-uranium (MEU) in the cylindrical core of the Kyoto University Critical Assembly (KUCA). The voids were produced in the flow channels by bubbling N2 gas through a small needle-like nozzle and the reactivity measured. The void fraction was measured in the out-of-pile experiments. The results indicate that the void effect on reactivity is slightly larger (more negative) in the MEU core than in the HEU core and strongly depends on the location where the voids are generated. (author)