照射の複合作用を考慮した新しい材料損傷評価法の開発

New evaluation method of material degradation considering synergistic effects of radiation damage

三輪 幸夫; 加治 芳行  ; 大久保 成彰   ; 近藤 啓悦  ; 塚田 隆 

Miwa, Yukio; Kaji, Yoshiyuki; Okubo, Nariaki; Kondo, Keietsu; Tsukada, Takashi

次世代炉の炉内構造材は軽水炉より高温で高照射量まで使用されるため、設計段階で材料の劣化挙動を高精度に予測する必要がある。従来の設計では照射脆化等の劣化要因を個別に検討し構造物の寿命を決めてきた。しかし、照射誘起応力腐食割れ(IASCC)等は複数の劣化要因が複合的に作用し発生する。照射硬化,照射誘起応力緩和等の事象は、異なる照射量依存性を有すため、IASCC等の挙動予測には、照射による材料劣化事象の複合作用を考慮できる損傷評価法が必要である。本論文では、新しい材料損傷評価法の概念を紹介し、イオン照射試験により得られた材料劣化挙動に及ぼす残留応力の影響を評価し、照射温度,照射量及び残留応力依存性の影響をモデル化し、これらモデルの統合によりIASCCの発生挙動を予測するモデルの検討結果を述べる。さらに、そのモデルの統合により炉内構造物のIASCCき裂発生シミュレーションを行うためのプログラミングコードの計算結果の例を紹介する。

In core structural materials of next generation reactors, materials' degradation behavior by neutron irradiation damage and thermal (cyclic) stress should be considered with fair accuracy in design process, because the materials are used under higher temperature gradients and higher neutron flux fields than those in the present light water reactors. In the current experiential design rules, service lives of core structural components were determined by the materials degradation such as the increase of ductile-to-brittle transition temperature after post irradiation examination data. However, other materials degradations such as irradiation-assisted stress corrosion cracking (IASCC), which occurs by the degradation synergistically interacting with radiation hardening, local chemical composition change, swelling and radiation creep, should be considered reasonably in the design process of the next generation reactors, because of the anticipation of the beneficial effects by synergy of radiation damage. To predict material failure by IASCC with reasonable accuracy, in this study, each material degradation phenomenon with different dose dependence was modeled with consideration of radiation induced stress relaxation. In this paper, the models obtained by ion-irradiation experiments and compared by data from neutron irradiation experiments were presented, and the concept of our new evaluation method and the programming code for the failure simulation were outlined.