Failure probability analyses for PWSCC and NiSCC in Ni-based alloy welds

Ni基合金異材溶接部のPWSCC及びNiSCCに関する破損確率解析

宇田川 誠; 勝山 仁哉  ; 鬼沢 邦雄 

Udagawa, Makoto; Katsuyama, Jinya; Onizawa, Kunio

Ni基合金異材溶接部に対する確率論的破壊力学解析コードPASCAL-NPは、加圧水型原子炉におけるPWR一次水質環境中応力腐食割れ(PWSCC)及び沸騰水型原子炉におけるNi基合金のBWR水質環境中応力腐食割れ(NiSCC)を対象とし、構造健全性に影響を及ぼすパラメータのばらつきを適切に考慮して機器の漏えいや破断確率を評価することができる。本報では、PWSCC及びNiSCC発生までの時間を算出する機能を本解析コードに導入し、大飯3号機原子炉圧力容器上蓋貫通部におけるPWSCC損傷事例及び浜岡1号機制御棒駆動機構ハウジングにおけるNiSCC損傷事例の破損確率解析を実施した。その結果、本解析コードによる結果は実機における検査結果と概ね一致した。

A probabilistic fracture mechanics (PFM) analysis code PASCAL-NP has been improved continuously with reference to the latest knowledge. This code treats not only primary water stress corrosion cracking (PWSCC) in PWR but also Ni-based alloy stress corrosion cracking (NiSCC) in BWR. This code can also evaluate both leakage and break probabilities due to these types of SCC, considering the scatters of parameters which affect the structural integrity of Ni-based alloy welds. In this paper, failure probability analyses have been performed focusing on vessel head penetrations affected by PWSCC detected at Ohi unit 3 nuclear power plant as well as control rod drive housings affected by NiSCC at Hamaoka unit 1. In order to perform case studies for these actual plants, analytical models of predicting the incubation time to PWSCC and NiSCC initiation were introduced to PASCAL-NP. Failure probabilities calculated by PASCAL-NP reasonably agreed with inspection results in these plants.