Research and development of a long-life control rod for fast reactors is being conducted at Joyo. One of the challenges in developing a long-life control rod is the restraint of absorber-cladding mechanical interaction (ACMI). First, a helium-bonding rod was selected as a control rod for the experimental fast reactor Joyo, which is the first liquid metal fast reactor in Japan. Its lifetime was limited by ACMI, which is induced by the swelling and relocation of B 4 C pellets. To restrain ACMI, a shroud tube was inserted into the gap between the B 4 C pellets and the cladding tube. However, once B 4 C pellets cracked and broke into small fragments, relocation occurred. After this, the narrow gap closed immediately as the degree of B 4 C pellet swelling increased. To solve this problem, the gap was widened during design, and sodium was selected as the bonding material instead of helium to restrain the increase in pellet temperature. Irradiation testing of the modified sodium-bonding control rod confirmed that ACMI would be restrained by the shroud tube regardless of the occurrence of B 4 C pellet relocation. As a result of these improvements, the estimated lifetime of the control rod at Joyo was doubled. In this paper, the results of postirradiation examination are reported.
The effects of lower-dose-rate irradiation on the tensile properties of 12% cold-worked type 316 stainless steel hexagonal ducts which were irradiated in the outer rows of EBR-II were investigated. Tension tests were conducted on a total of seventeen irradiation conditions. Fractographic examinations were performed on four irradiation conditions which were selected based on the results of tension tests. The results of the tension tests and fractographic examinations showed that the 316 stainless steel hexagonal ducts exhibited irradiation hardening typical to 316 stainless steels irradiated at similar temperatures and retained respectable ductility after the irradiation. There was no significant effect of dose rate on tensile properties for the range of irradiation conditions of this study.
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