Effects of residual stress by weld overlay cladding and PWHT on the structural integrity of RPV during PTS

加圧熱衝撃事象時における原子炉圧力容器の構造健全性に及ぼす肉盛溶接と溶接後熱処理による残留応力の影響

宇田川 誠; 勝山 仁哉  ; 鬼沢 邦雄 

Udagawa, Makoto; Katsuyama, Jinya; Onizawa, Kunio

原子炉圧力容器の健全性を評価する際、容器内表面の肉盛溶接部を貫通する表面き裂を想定して破壊力学解析を行う。この解析のためには、原子炉圧力容器内面の肉盛溶接部の残留応力を考慮して、想定き裂の応力拡大係数を精度よく評価することが重要である。そこで、肉盛溶接,溶接後熱処理,耐圧試験及び運転状態を対象として、肉盛溶接部近傍の残留応力を有限要素法に基づく熱弾塑性クリープ解析により求めた。溶接後熱処理の後では、肉盛溶接部には室温で400MPa程度の高い引張応力が生じている。溶接残留応力解析結果に基づいて、加圧熱衝撃事象時における原子炉圧力容器の構造健全性に関して、表面き裂の深さや加圧熱衝撃(PTS)の種類に対する溶接残留応力の影響を評価した。

In order to assess the structural integrity of a reactor pressure vessel (RPV), it is assumed that a surface crack resides through the cladding at the inner surface of the vessel. It is, therefore, important to precisely evaluate stress intensity factor (SIF) taking the residual stress due to weld overlay cladding and post-weld heat treatment (PWHT) into account. In this study, we performed a numerical simulation based on thermal-elastic-plastic-creep analysis using finite element method to evaluate residual stress distribution near the cladding layer produced by weld overlay cladding and PWHT. The tensile residual stress of about 400 MPa occurs in the cladding at room temperature after the PWHT. The residual stress distributions under the normal operating conditions (pressure and temperature) of RPV were also evaluated. The effect of residual stress and evaluation methods on SIF behavior for various crack size were studied under several PTS conditions such as SBLOCA, MSLB and LBLOCA.