中性子照射ステンレス鋼のIASCC感受性に及ぼす照射開始時の温度履歴の影響

Influence of temperature histories during reactor startup periods on IASCC susceptibility of austenitic stainless steel irradiated with neutrons

笠原 茂樹; 知見 康弘; 橘内 裕寿*; 越石 正人*

Kasahara, Shigeki; Chimi, Yasuhiro; Kitsunai, Yuji*; Koshiishi, Masato*

BWR一次系模擬高温水中での低ひずみ速度引張(SSRT)試験の既往知見によると、BWR炉心で3dpa程度まで中性子照射された低炭素ステンレス鋼は照射誘起応力腐食割れ(IASCC)感受性を有するが、JMTR照射ステンレス鋼では照射量によらず粒界破面は生じないとされている。この相違の検討に資するため、BWRの起動時の温度履歴を模擬した条件で約3dpaまでJMTRで照射したSUS316LのSSRT試験を実施した。SSRT試験後の破面SEM観察の結果、粒界破面が確認された。既往のJMTR材を使ったIASCC感受性評価研究では、JMTRの起動時に予め昇温した後に照射を開始した供試材を用いており、照射開始時の温度履歴が粒界破面の発現に影響したことが示唆される。この結果を受け、照射開始時の昇温条件が異なる照射材の引張特性評価結果を踏まえた上で、IASCC感受性と引張特性パラメータの関連について検討した。

According to existing data of slow strain rate tensile (SSRT) test under high temperature water conditions, which simulated BWR primary coolant environment, low carbon stainless steel, which was irradiated with neutrons up to about 3 dpa in BWR core, shows susceptibility of Irradiation associated stress corrosion cracking (IASCC). On the other hand, the stainless steel irradiated by using the JMTR did not show IASCC susceptibility, regardless of neutron fluence. To investigate this different result about IASCC susceptibility, the JMTR operated to simulate temperature history at start-up of BWR, and a tensile specimen of SUS316L was irradiated up to about 3 dpa under the condition. After that, the specimen was examined by SSRT test to evaluate IASCC susceptibility. The result of fracture surface observation after the SSRT test indicated that the specimen fractured by Inter-granular mode and was evaluated to be susceptible to IASCC. In the comparison of the data of IASCC sensitivity by the JMTR irradiated materials, which did not show IASCC susceptibility, the difference of them was suggested to attribute to different temperature histories at the start of irradiation. The relationship between IASCC susceptibility and the parameters obtained from tensile tests was discussed, in consideration of the difference of the tensile parameters which are suffered from the irradiation condition under the different temperature history during the start period of the irradiation.