Shielding technology for upper structure of HTTR

HTTR上部構造の遮へい技術

植田 祥平  ; 坂場 成昭 ; 沢 和弘

Ueta, Shohei; Sakaba, Nariaki; Sawa, Kazuhiro

1次冷却材に遮へい能力のある水ではなくヘリウムガスを用いる高温ガス炉の遮へい設計では、特に制御棒用スタンドパイプが貫通する原子炉上方向への中性子ストリーミングに留意する必要がある。第IV世代原子炉システムの超高温ガス炉(VHTR)の遮へい設計技術の確立に資するため、HTTRの遮へい性能を実証した。その結果、スタンドパイプ室内の線量の9割以上が高速中性子によるものであり、中性子及び線量の測定値は検出下限相当であった。また、設計に用いた安全裕度に50倍程度の余裕があることが明らかとなり、遮へい設計の保守性が定量的に示された。また、スタンドパイプ室の線量当量率は、当初の予想通り制御棒の引抜効果により非線形的に変化することが示された。今後の高温ガス炉の遮へい設計の最適化やそれに伴う経済性の向上に役立つと期待される。

In the shielding design for the High Temperature Gas-cooled Reactor (HTGR), special attentions shall be needed to avoid neutron streaming, since helium gas as a coolant does not work for shielding. Japan Atomic Energy Agency has demonstrated the performance of shielding through testing operations of the High Temperature Engineering Test Reactor (HTTR) in order to establish design method for shielding of the Very-High-Temperature Reactor (VHTR) as a Generation-IV nuclear power system. As results of the test, it was confirmed that dose equivalent rates for neutron and -ray at on-operating acceptable areas were less than detection limit and as low as background, respectively. The measured dose at the stand-pipe room corresponded to the detection limit, and it was found that over 90% of dose derived from fast neutron. It was indicated that there was still a margin of factor 50 in addition to the design which excluded the safety factor. The measured dose rates showed good agreement with the predicted considering the control rod withdrawing effect. The knowledge on the design method and the demonstration of shielding by the HTTR will strongly contribute to realizing and optimizing the designs of future VHTRs.