Analyses of Reactor Physics Experiments in the Kyoto University Critical Assembly

Both experimental and analytical studies have been performed on the temperature coefficient of reactivity in a light water moderated and reflected core loaded with highly enriched uranium fuel at the Kyoto University Critical Assembly. The temperature effect on reactivity was measured for the 20 to 70°C range to investigate separately the effects of the H/²³⁵U atomic ratio and the core shape on this quantity. The results of both the eigenvalue and perturbation calculations by the SRAC code system approximately reproduced the experimental data. It was found that the contribution of the core region to the temperature coefficient was negative due to the degradation of moderation, whereas that of the reflector region was positive due to the decrease in neutron absorption. The positive contribution of the reflector region became larger as the H/²³⁵U atomic ratio became smaller and the core shape became more slender.