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Fuel failure simulation test in the experimental fast reactor JOYO

石田 公一; 伊藤 主税; 青山 卓史

Ishida, Koichi; Ito, Chikara; Aoyama, Takafumi

原子炉内で燃料破損が発生した場合、燃料破損の発生とその規模を早期に検知して原子炉を停止し、破損の拡大を防止すること及び破損した燃料集合体を同定することが、プラント安全性の向上、被ばくや環境への影響を低減する観点から重要である。高速実験炉「常陽」MK-III炉心において、燃料被覆管に人工欠陥を設けた試験用燃料要素を炉心中心に装荷して照射した。燃料破損を検知してから原子炉を停止して燃料を取り出すまでの一連のプラント操作を行う「燃料破損模擬試験」を実施した。本試験の結果、FFD-CG法の計数率を監視することにより原子炉起動時の初期欠陥による燃料破損を検知でき、早期に原子炉を停止できることを確認した。また、燃料破損模擬試験用集合体からカバーガス中に放出したFPをCGCS運転及び炉内カバーガスパージによりバックグラウンドレベルまで低減でき、運転初期の破損燃料にあってもナトリウムシッピング法FFDLによる破損燃料集合体同定手法が有効であることを確認した。

When a fuel failure occurs in a nuclear reactor, it is important to detect and identify the failed fuel subassembly to improve reactor safety and reduce radiation exposure and impact on the environment. A Fuel Failure Simulation Test in the experimental fast reactor JOYO was performed to confirm the JOYO plant operation procedures for potential future fuel failure events and help prepare for future planned run-to-cladding-breach tests in JOYO. The test fuel elements with an artificial slit in the fuel cladding was loaded in the core center position of the MK-III core. The test fuel elements were irradiated, the release of fission products was detected, and the location of the leaking test subassembly was identified. The performance of the fuel failure detection system and the failed fuel detection and location systems, and the plant operation procedure of fuel failure were confirmed. To decrease the radioactive concentration in the cover gas, the Cover Gas Clean-up System which collects fission product nuclides by the cryogenic distillation method and furnace-cover-gas-purge were operated, and their operation procedure and performance ware confirmed. This test results will improve the JOYO operation procedures and potential upgrades to the facilities, and be contributed to the safety enhancement of fast breeder reactors.

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