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

根据应用中子科学的最新发展，对所谓的受控中子的需求越来越强烈。为了实现谱控辐照场，需要在研究堆中建立谱移区，为此，在京都大学临界装置（KUCA）开展了核反应堆谱移区发展的基础性和系统性研究。采用聚乙烯慢化反射堆芯，装载高浓铀燃料，分别由铍、石墨、铝和天然铀组成的3个不同慢化剂与燃料体积比的堆芯，采用活化线法测量了中子注量率、反应率和能谱指数（反应率比）的分布。通过KUCA的实验验证了谱移区在现有研究堆的反射层区也是可以实现的，尽管堆芯区较硬的中子能谱有利于提高性能。这表明，通过选择反射器的材料，可以在一定程度上控制裂变中子的慢化过程。最后，针对京都大学研究堆（KUR）的升级改造，在本研究的基础上，对谱移区进行了中子学设计。

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).