Tensile mechanical properties of a stainless steel irradiated up to 19 dpa in the Swiss spallation neutron source

スイス核破砕中性子源で19dpaまで照射されたステンレス鋼の引張り特性

斎藤 滋   ; 菊地 賢司*; 濱口 大; 宇佐美 浩二; 遠藤 慎也; 小野 勝人; 松井 寛樹; 川合 將義*; Dai, Y.*

Saito, Shigeru; Kikuchi, Kenji*; Hamaguchi, Dai; Usami, Koji; Endo, Shinya; Ono, Katsuto; Matsui, Hiroki; Kawai, Masayoshi*; Dai, Y.*

核破砕中性子源やADSのビーム入射窓及び構造材料は、高エネルギー陽子及び核破砕中性子の照射により、損傷を受ける。核破砕条件における材料の照射損傷特性を明らかにするために、スイスのPSIを中心として核破砕ターゲット材料照射プログラム(STIP: SINQ Target Irradiation Program)が進行中である。本プログラムは1996年に始まり、PSIの加速器で各種材料を580MeVの陽子で照射し、参加国がPIEを分担して行っている。原子力機構も照射試料の一部を輸送し、照射後試験を行った。本論文ではSTIP-II試料の中からJPCA鋼の引張り試験の結果を報告する。引張り試験の結果、JPCA鋼は照射後大きく硬化するが、耐力の増加は11dpa付近で飽和した。伸びも大きく低下したが、全伸びは19.5dpa照射後も約15%保っていた。試験後の破面観察の結果、粒界破面や割れなどは見られなかった。このJPCA鋼には約1600appmのHeが生成していると見積もられ、表面からの反跳分を除いた多くが材料中に残留していると推定される。TEM観察でも、Heバブルが組織中にほぼ一様に分布しており、特に粒界析出が見られなかったことと一致する結果と考えられる。

To evaluate the lifetime of the beam window of an accelerator-driven transmutation system (ADS), post irradiation examination (PIE) of the STIP (SINQ target irradiation program, SINQ; Swiss spallation neutron source) specimens was carried out. The specimens tested in this study were made from the austenitic steel JPCA (Japan primary candidate alloy). The specimens were irradiated at SINQ Target 4 (STIP-II) with high-energy protons and spallation neutrons. The irradiation conditions were as follows: the proton energy was 580 MeV, irradiation temperatures ranged from 100 to 430C, and displacement damage levels ranged from 7.1 to 19.5 dpa. Tensile tests were performed in air at room temperature (R.T.), 250C and 350C. Fracture surface observation after the tests was done by SEM (Scanning electron microscope). Results of the tensile tests performed at R.T. showed the extra hardening of JPCA at higher dose compared to the fission neutron irradiated data. At the higher temperatures, 250C and 350C, the extra hardening was not observed. Degradation of ductility bottomed around 10 dpa, and specimens kept their ductility until 19.5 dpa. All specimens fractured in ductile manner. The result from a microstructure observation on a specimen irradiated to 19.3 dpa at 420C indicates that some agglomeration of bubbles on grain boundaries was observed in the specimen irradiated to 19.3 dpa at 420C. However the tensile specimen irradiated up to 18.4 dpa at 425C still exhibited little loss of ductility. Since He/dpa was very high on SINQ target irradiations, the formation of highly dense small bubbles in the matrix consequently avoided the accumulation of He on grain boundaries, which might have resulted in avoiding grain boundary embrittlement.