急速加熱による円筒の内圧破断に関する構造健全性評価手法の研究

Study on tube rupture strength evaluation method for rapid overheating

小峯 龍司; 和田 雄作

Komine, Ryuji; Wada, Yusaku

蒸気発生器伝熱管破損に起因する隣接伝熱管への破損伝播メカニズムとして、ナトリウム-水反応により急速に伝熱管壁が加熱され破断に至るいわゆる高温ラプチャ現象が考えられる。本研究では、この現象を急速加熱による円筒の内圧破断としての構造強度問題に置き換え、その健全性評価方法を実験と解析に基づき検討した。得られた成果は以下の通り。(1)応力算定法については、内圧破断評価に対して従来の経験式が適用できることが、実験だけでなく詳細FEM弾塑性大変形解析の結果からも示された。(2)高温ラプチャ対象温度領域では非常に短時間の現象であってもクリープ効果が影響し、JIS高温引張試験に従ったひずみ速度では遅過ぎて、時間依存効果を正確には評価できないことが明らかとなった。(3)もんじゅ蒸発器を対象に、2と1/4Cr-1Mo鋼に関して、数分以内の非常に短時間のクリープ破断試験とJIS引張試験よりも2桁近くひずみ速度が速い10%/secの高速引張試験を実施し、構造健全性評価に適用できる強度基準値を定めた。(4)別途実施した伝熱管の一連の内圧破断試験(TRUST-2ガス圧試験)結果を引用することで、上記の応力算定法と強度基準値による構造健全性評価手法の妥当性が示された。(5)もんじゅ過熱器の評価に必要なSUS321の強度基準値を、2と1/4Cr-1Mo鋼に適用した方法に準じて定めた。

A sodium-water reaction drived from the single tube break in steam generator might overheat nabor tubes rapidly under internal pressure loadings. If the temperature of tube wall becomes too high, it has to be evaluated that the stress of tube does not exceed the material strength limit to prevent the propagation of tube rupture. In the present study this phenomenon was recognized as the fracture of cylindrical tube with the large deformation due to overheating, and the evaluation method was investigated based on both of experimental and analytical approaches. The results obtained are as follows. (1)As for the nominal stress estimation, it was clarified through the experimental data and the detailed FEM elasto-plastic large deformation analysis that the formula used in conventional designs can be applied. (2)Within the overheating temperature limits of tubes, the creep effect is dominant, even if the loading time is too short. So the strain rate on the basis of JIS elevated temperature tensile test method for steels and heat-resisting alloys is too late and almost of total strain is composed by creep one. As a result the time dependent effect cannot be evaluated under JIS strain rate condition. (3)Creep tests in shorter time condition than a few minutes and tensile tests in higher strain rate condition than 10%/min of JIS are carried out for 2.25Cr-1Mo(NT) steel, and the standard values for tube rupture strength evaluation are formulated. (4)The above evaluation method based on both of the stress estimation and the strength standard values application is justified by using the tube burst test data under internal pressure. (5)The strength standard values on Type 321 ss is formulated in accordance with the procedure applied for 2.25Cr-1Mo(NT) steel.