Development of Spectrum Shifter Region for Neutrons Produced in Nuclear Reactor through Critical Experiments

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In accordance with a recent development in applied neutron science, the needs for so-called controlled neutrons are becoming stronger. To realize a spectrum-controlled irradiation field, it is desirable to establish a spectrum shifter region in research reactors.For this purpose, a basic and systematic study on the development of a spectrum shifter region in nuclear reactors was carried out through critical experiments at the Kyoto University Critical Assembly (KUCA). By using polyethylene moderated and reflected cores loaded with highly enriched uranium fuel, three cores of different moderator-to-fuel volume ratios with spectrum shifter regions, which consists of beryllium, graphite, aluminum or natural uranium, were constructed to measure the neutron flux, reaction rate, spectrum index (i.e.reaction rate ratio) distributions mainly by the activation wire method. The measured results were compared with the calculated ones by a neutronics code currently used in the neutronics design of research reactors.Through the KUCA experiments, it was demonstrated that the spectrum shifter region can be realized even in the reflector region of current research reactors, although the harder neutron spectrum in the core region would be much favorable for the higher performance. This fact indicates that, by selecting the material of reflector, it is possible to controll the slowing down process of fission neutrons to some extent.Finally, a neutronics design of the spectrum shifter region was performed on the basis of the present study in view of upgrading the Kyoto University Research Reactor (KUR).