Approach to the lifetime assessment of the bayonet back plate for IFMIF target

IFMIFターゲットのバイオネット型背面壁の寿命評価への取り組み

Agostini, P.*; 井田 瑞穂; Miccich, G.*; 中村 博雄; Turroni, P.*

Agostini, P.*; Ida, Mizuho; Miccich, G.*; Nakamura, Hiroo; Turroni, P.*

国際核融合材料照射施設(IFMIF)のリチウム(Li)ターゲットの背面壁は施設中の機器のうちで最も強く中性子照射に曝されるが、その交換方式の一つとしてバイオネット方式の背面壁の開発を進めている。この背面壁の交換頻度を左右する背面壁寿命を主なダメージ原因について評価し、以下の点を明らかにした。リチウム中の窒素濃度が10wppm以下に制御されるIFMIFでは低放射化フェライト鋼製の背面壁の腐食損耗は年間0.003mm以下である。重陽子ビームのBraggピークが1mm背面壁側にずれた場合でも背面壁温度上昇は12度以下であり、疲労荷重は十分低い。中性子照射による背面壁中心部の熱応力も93MPa以下であり、弾性限界より十分低い。背面壁の寿命に対して最も影響が大きいのは照射による延性脆性遷移温度(DBTT)の上昇であり、年間60dpaの照射損傷条件下では3か月以下の寿命となるが、背面壁に脱着型ヒーターを取り付けてアニールすることによってより長寿命にできる。

In the International Fusion Materials Irradiation Facility (IFMIF), a back-plate of liquid lithium (Li) target is most severely irradiated by high flux of neutrons. The conceptual configuration of the IFMIF target, based on the bayonet back plate, has been developed. The life time analysis of the back-plate has to be made carefully, in order to credibly estimate its expected replacement frequency. The various interconnections between the main damaging causes were discussed in order to evidence the most plausible reasons of the back-plate malfunctioning. The results were as follows. Under IFMIF Li condition with nitrogen content of 10 wppm or less, the erosion/corrosion rate of the back-plate made of the reduced activation ferritic steel is 0.003 mm/year or less. Even in case that the Bragg peak of heat deposition by deuterons approaches to the back-plate by 1 mm, temperature of the back-plate increases by only 12 degrees or less, and induced fatigue load is well low. Thermal stress due to the neutron irradiation is 93 MPa at the back-plate center, which is well below the plastic limit. The positive shift in the ductile brittle transition temperature (DBTT), the most effective factor, reduces the back-plate lifetime to three months or shorter under irradiation damage rate of 60 dpa/year. However, the lifetime can be recovered by the post irradiation annealing heat treatment.