ESTIMATION OF HYDROGEN REDISTRIBUTION IN ZIRCALOY CLADDING OF SPENT FUEL DURING DRY STORAGE AND ITS INFLUENCE ON MECHANICAL PROPERTIES OF CLADDING

用夏比法测量了含氢量为100ppm和400ppm的Ziraloy-2合金(尺寸为5 mm×10 mm×50 mm)在室温至250ﾟC温度范围内的韧性。测试结果表明，氢化物分布不均匀的试样的韧性比氢化物均匀分布的试样的韧性大。从热力学角度分析了干式储存条件下轻水冷却堆燃料锆合金包壳中氢化物的再分布现象，并用计算机模拟计算了氢化物的再分布速率，其中燃料燃耗为40~55Gwd/t，包壳吸氢量为300~400ppm。在计算中，假定干仓的温度条件为150-450ﾟC。得出如下结论：(1)在设计的干仓温度条件下，反应堆内产生的氢化物在锆包覆层径向的不均匀分布不会松弛。(2)在用含氢400ppm的锆包覆层干法储存乏燃料的情况下，假设锆包覆层的温度因某些事故而升高到480ﾟC以上，则氢化物在锆包覆层中的不均匀分布将被瓦解，并在一小时内形成均匀的氢化物分布。氢化物的这种重新分布将导致锆合金覆层韧性的显著下降。这些成果已在国际原子能机构的会议上和日本源代株式会社秋季会议上介绍。

The toughness of Ziraloy-2 specimens (size : 5mm x 10mm x 50mm) containing 100 ppm or 400 ppm hydrogen was measured with the Charpy test in a temperature region from room temperature to 250ﾟC.The hydrogen concentration in the specimens was gradient in one dimension, which was prepared with the thermomigration of hydrogen. The measurement suggested that the toughness of the specimen, in which the hydride is nonuniformly distributed, was larger than that of the specimen in which the hydride was uniformly distributed. The redistribution phenomena of hydride in the Zircaloy cladding of the fuel for the light water cooled reactors under the conditions of the dry storage was analyzed from the thermodynamical aspect and the rate of redistribution of the hydride was calculated with the computer simulation, in which the burn-up of the fuel was assumed as 40-55Gwd/t and the amount of hydrogen absorbed in the cladding was also assumed as 300-400ppm. In the calculation, the temperature conditions of the dry storage was assumed as 150-450ﾟC.Followings were concluded : (1) The nonuniform distribution of the hydride in the radial direction of the Zircaloy claddings, which was brought about in the reactors, would not be relaxd under the designed temperature conditions of the dry storage. (2) In the case of the dry storage of the spent fuel with the Zircaloy cladding containing 400 ppm hydrogen, the nonuniform distribution of the hydride in the Zircaloy cladding would be collapsed and uniform distribution of the hydride would be built up in an hour provided that the temperature of the Zircaloy cladding would be increased over 480ﾟC because of certain accidents. This redistribution of the hydride would bring about the marked decrease of the toughness of the Zircaloy cladding. These results were presented in the meeting of International Atomic Energy Agency and in the Fall Meeting of Nihon Genshiryoku Gakkai.