Temperature coefficient of reactivity in light-water moderated and reflected cores loaded with highly-enriched-uranium fuel.

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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, which was constructed in the Kyoto University Critical Assembly (KUCA). The temperature effect on reactivity was measured for the range of 2070°C to investigate separately the effects of (1) the fuel pitch (H/235U atomic ratio) and (2) the core shape on this physical quantity. The experimental data were analyzed with use of the SRAC code system. The calculated eigenvalue keff agreed with the measured one within 0.5% in the C/E ratio for both the 2- and 3-dimensional diffusion calculations. The experimental data were approximately reproduced by both the eigenvalue and perturbation calculations. It was found that the contribution of the core region was negative to the temperature coefficient of reactivity, whereas that of the reflector region was positive. The synthesis of these contributions made the temperature coefficient negative in total. The degradation of moderation was the main contributor in the core region, whereas the decrease in the neutron absorption in the reflector region. The positive contribution of the reflector region became larger as the H/235U atomic ratio became smaller and the core shape became more slender.
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