[in Japanese]

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[in Japanese]

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According to the Long-term Program for Research, Development and Utilization of Nuclear Energy (June, 1994) in Japan, the Rokkasho Reprocessing Plant will be operated shortly after the year 2000, and the planning of the construction of the second commercial plant will be decided around 2010. Also, it is described that spent fuel storage has a positive meaning as an energy resource for the future utilization of Pu.
Considering the balance between the increase of spent fuels and the domestic reprocessing capacity in Japan, it can be expected that the long-term storage of UO₂ spent fuels will be required.
Then, we studied the effect of long-term storage of spent fuels on Pu-thermal fuel cycle. The burnup calculation were performed on the typical Japanese PWR fuel, and the burnup and criticality calculations were carried out on the Pu-thermal cores with MOX fuel. Based on the results, we evaluate the influence of extending the spent fuel storage term on the criticality safety, shielding design of the reprocessing plant and the core life time of the MOX core, etc.
As the result of this work on long-term storage of LWR spent fuels, it becomes clear that there are few demerits regarding the lifetime of a MOX reactor core, and that there are many merits regarding the safety aspects of the fuel cycle facilities. Furthermore, long-term storage is meaningful as energy storage for effective utilization of Pu to be improved by technological innovation in future, and it will allow for sufficient time for the important policymaking of nuclear fuel cycle establishment in Japan.

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In order to develop advanced transparent radiation shielding materials against fast neutrons, three kinds of shielding materials were trially manufactured by using hydrogen-rich acrylic resin and two kinds of cycloolefin copolymers as raw materials. These three shielding materials were evaluated on their optical, mechanical and fast neutrons shielding properties. Especially, the shielding property against fast neutrons was evaluated by the experiments using ²⁵²Cf and the calculation using MOX fuel as a neutron source, respectively. As a result, it was clarified that two shielding materials made of cycloolefin copolymer had better shielding property than that of acrylic resin, which was typical transparent neutron shielding material, and similar optical and mechanical properties to those of acrylic resin. Therefore, these cycloolefin copolymer can be used practically as transparent neutron shielding material to panels of glove boxes in nuclear fuel treatment facilities, and so on.

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Recently, there has been a great demand for high-performance zirconium alloys which are durable even in severe circumstances in light water reactors to be designed for the higher burn-ups of nuclear fuels and also for increasing efficiency of power generation. In order to respond to this strong demand for new alloys, it is firstly necessary to get fundamental information of alloying effects on the various properties of zirconium.
In this study, special attention was directed towards the mechanical properties. Both the Young's modulus and the Vickers hardness were measured systematically with various Zr-2 mol% M binary alloys, where M's were 3d, 4d, 5d transition metals and several non-transition metals. The measured Young's moduli of the alloys changed with M, following the position of elements, M, in the periodic table, except for the 3d transition metals, while showing a close correlation with the values of the Young's moduli for M in the pure metal state. The Vickers hardness also exhibited such a periodical change with M, even though it was modified in some way with test temperatures.
The fives types of characteristics diagrams to show alloying behavior of respective elements in zirconium were newly made, employing the present results together with autoclave corrosion-test data and also thermal-neutron absorption cross sections of elements. These diagrams will be useful for the design and development of zirconum alloys.

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