Study on the structural integrity of fuel claddings against beam trips for accelerator-driven system

加速器駆動核変換システムにおけるビームトリップに対する燃料被覆管の構造健全性に関する研究

武井 早憲   ; 西原 健司  ; 倉田 有司; 辻本 和文 ; 大井川 宏之 

Takei, Hayanori; Nishihara, Kenji; Kurata, Yuji; Tsujimoto, Kazufumi; Oigawa, Hiroyuki

大強度陽子ビームなどを加速する加速器では、経験的にビームトリップ事象が頻繁に発生することが知られており、加速器駆動未臨界システム(ADS)の燃料被覆管に熱疲労による強度的な影響を与えるおそれがある。この影響を評価するため、定常状態とビームトリップによる過渡状態における燃料被覆管に生じる熱応力を評価した。まず定常状態における評価では、発生する熱応力を一次応力と二次応力に分け、改良9Cr-1Mo鋼の制限値と比較した。その結果、すべての熱応力は制限値よりも小さくなり、燃料被覆管は構造的な強度を有することがわかった。次に、ビームトリップによる過渡状態における評価では、一次応力と二次応力から算出される歪み振幅から許容ビームトリップ頻度を求めた。その結果、許容ビームトリップ頻度は年以上となり、燃料被覆管はADS用加速器で予想されるビームトリップ頻度より20倍以上のビームトリップ事象を許容できることがわかった。

Japan Atomic Energy Agency (JAEA) has promoted the design study of the Accelerator-Driven System (ADS) cooled by lead-bismuth eutectic (LBE) to transmute minor actinide. The structural integrity of the fuel claddings is one of key issues in designing the ADS. For ADS, frequent beam trips as experienced in existing high power proton accelerators may cause thermal fatigue problems which may lead to degradation of the structural integrity and reduction of the lifetime of the fuel claddings. To estimate the structural integrity of the fuel claddings, the thermal stress of the fuel cladding was evaluated for following two states: the steady state and the transient state. For the steady-state analysis, the thermal stress was divided into two types: the primary stress and the secondary stress, which were caused by the inner pressure and the thermal load, respectively. These stresses were compared with the design stress limits. As a result, it was confirmed that their structural strength was sufficient under the conditions of design temperature and corrosion environment without irradiation. For the transient-state analysis, the number of acceptable beam trips was evaluated based on the total strain range due to the primary and secondary stress of the fuel cladding. As a result, the number of acceptable beam trips was evaluated to be over times. If the fuel cladding was replaced once every two years, the acceptable beam-trip frequency was estimated to be greater than times per year. This acceptable frequency was 20 times greater than the expected beam-trip frequency caused by the accelerator for the JAEA's reference ADS.