Koichi Masaki

Effect of Partial Welding on the Residual Stress and Structural Integrity of Piping Welds

When weld defects are observed during an inspection after welding, repair welding is performed after removing the defects. However, partial repair welding can potentially complicate the weld residual stress distribution. In this study, we performed thermal-elastic-plastic analyses to evaluate the weld residual stress produced by repair welding after pipe butt-welding. The analysis results were validated through comparison with actual measurements. In addition, based on the analysis results for varying repair-welding conditions, we also performed structural integrity assessments related to stress corrosion cracking using the probabilistic fracture mechanics analysis code pascal-sp. It was clearly observed that the tensile stress in the repair-welded region increased and that compressive stresses occurred outside the repair-welded region. A deeper mechanical cutting depth caused larger increases in the tensile residual stress of the repair-welded region. It was also concluded that partial repair welding may favorably affect the break probability of piping welds.

Verification of Probabilistic Fracture Mechanics Analysis Code PASCAL

1. 緒言 国内において確率論的破壊力学 (PFM) の適用性向上を図るためには、破損頻度の算出に用いら れる PFM 解析コードの検証が不可欠である。著者らは、原子力機構が整備を進めている原子炉圧力容器 (RPV) に対する PFM 解析コード PASCAL に対する検証の一環として、PASCAL に導入した機能の検証を 行うとともに、米国の PFM 解析コード FAVORとのベンチマーク解析を実施した。 2. PASCALの機能整備 PASCALの適用性向上を目的として、条件付亀裂進展確率 (CPI) 及び条件付亀裂 貫通確率 (CPF) に関する低損傷確率評価機能、認識論的不確実さを考慮した信頼度評価機能等を PASCAL に導入した。例えば、信頼度評価機能の整備では、これまで不確実さの分類を考慮せずにモンテカルロ法 により CPI 及び CPFを算出していたのに対し、二重ループ化する改良を行った。具体的には、内側のルー プで偶然的不確実さのみを考慮して求めた CPIと CPFを、外側のループで認識論的不確実さに基づき統計 処理することにより、前報で示したとおり破損頻度の信頼度を評価できるようにした。また、整備した全 ての機能について検証を実施し、適切に動作していることを確認した。 3.ベンチマーク解析 PASCAL の信頼性向上を図る ため、FAVOR との比較計算を実施した。PASCALと FAVOR の機能の相違を確認した上で、単一亀裂に対 する CPI及び CPF の比較、RPV 炉心領域に対する亀 裂進展頻度 (FCI) 及び亀裂貫通頻度 (TWCF) の比 較等を実施した。PASCAL と FAVOR とでは開発機 関が異なるため、個々の機能には違いがあるが、本 比較計算を通じて、RPV 寸法や化学成分、応力拡大 係数等の条件を合わせた場合にはほぼ同等の結果が 得られることを確認した (図1)。 4. 結論 PASCAL に整備した個々の機能検証、及び PASCAL と FAVOR のベンチマーク解析を通じて、 PASCALの信頼性を検証し、実用性の向上を図った。 5. 謝辞 本報告は、原子力規制庁からの受託事業「平成28年度高経年化技術評価高度化事業(原子炉一 次系機器の健全性評価手法の高度化)」で得られた成果である。関係各位に謝意を表する。

Study on Application of PFM Analysis Method to Japanese Code for RPV Integrity Assessment Under PTS Events

A probabilistic fracture mechanics (PFM) analysis method for pressure boundary components is useful to evaluate the structural integrity in a quantitative way. This is because the uncertainties related to influence parameters can be rationally incorporated in PFM analysis. From this viewpoint, the probabilistic approach evaluating through-wall cracking frequencies (TWCFs) of reactor pressure vessels (RPVs) has already been adopted as the regulation on fracture toughness requirements against PTS events in the U.S. As a study of applying PFM analysis to the integrity assessment of domestic RPVs, JAEA has been preparing input data and analysis models to calculate TWCFs using PFM analysis code PASCAL3. In this paper, activities have been introduced such as preparing input data and models for domestic RPVs, verification of PASCAL3, and formulating guideline on general procedures of PFM analysis for the purpose of utilizing PASCAL3. In addition, TWCFs for a model RPV evaluated by PASCAL3 are presented.Copyright

Estimation of Through-Wall Cracking Frequency of RPV Under PTS Events Using PFM Analysis Method for Identifying Conservatism Included in Current Japanese Code

To assess the structural integrity of reactor pressure vessels (RPVs) during pressurized thermal shock (PTS) events, the deterministic fracture mechanics approach prescribed in Japanese code JEAC 4206-2007 [1] has been used in Japan. The structural integrity is judged to be maintained if the stress intensity factor (SIF) at the crack tip during PTS events is smaller than fracture toughness KIc. On the other hand, the application of a probabilistic fracture mechanics (PFM) analysis method for the structural reliability assessment of pressure components has become attractive recently because uncertainties related to influence parameters can be incorporated rationally. A probabilistic approach has already been adopted as the regulation on fracture toughness requirements against PTS events in the U.S. According to the PFM analysis method in the U.S., through-wall cracking frequencies (TWCFs) are estimated taking frequencies of event occurrence and crack arrest after crack initiation into consideration.In this study, in order to identify the conservatism in the current RPV integrity assessment procedure in the code, probabilistic analyses on TWCF have been performed for certain model of RPVs. The result shows that the current assumption in JEAC 4206-2007, that a semi-elliptic axial crack is postulated on the inside surface of RPV wall, is conservative as compared with realistic conditions. Effects of variation of PTS transients on crack initiation frequency and TWCF have been also discussed.Copyright