AbstractIf a loss-of-vacuum event (LOVA) occurred in a fusion reactor, buoyancy-driven exchange flows would take place at breaches of a vacuum vessel (VV) due to the temperature difference between the inside and outside of the VV. The exchange flows may bring mixtures of activated materials and tritium in the VV to the outside through the breaches, and remove decay heat from the plasma-facing components of the VV. Therefore, the LOVA experiments were carried out under the conditions that one or two breaches were opened and that the VV was heated to a maximum 200°C, using a small-scaled LOVA experimental apparatus. Air and helium gas were provided as working fluids. Fluid and wall temperature distributions in the VV were measured and the flow patterns in the VV were estimated from these temperature distributions. It was found that: (1) the exchange mass in the VV depended on the breach positions; (2) the exchange flow at the single breach case became a counter-current flow when the breach was at the roof of the VV and a stratified flow when it was at the side wall; (3) and that at the double breach case, a one-way flow between two breaches was formed.
核融合炉において真空喪失事象（LOVA）が発生した場合、真空容器（VV）の内外の温度差により、破口部において浮力駆動型の交換流が生じる。この交換流により、真空容器内の活性化材料やトリチウムの混合物が破口を通じて外部へ運ばれる可能性があり、またプラズマ対向機器からの崩壊熱が除去される可能性がある。そこで本研究では、小型スケールのLOVA実験装置を用いて、破口を1箇所または2箇所開口した条件下で、真空容器を最高200℃まで加熱したLOVA実験を実施した。作動流体には空気およびヘリウムガスを用いた。真空容器内の流体温度分布および壁面温度分布を測定し、これらの温度分布から真空容器内の流動パターンを推定した。その結果、以下のことが明らかとなった：（1）真空容器内の交換流量は破口位置に依存すること、（2）破口が1箇所の場合、破口が真空容器の頂部にあるときは対向流が形成され、側壁にあるときは成層流が形成されること、（3）破口が2箇所の場合、2つの破口間で一方向の流れが形成されること。