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冷却材流路を通じた溶融炉心物質の流出挙動に関する基礎試験

Fundamental study on discharging of molten core material through the in-core coolant channel

神山 健司  ; 磯崎 三喜男; 今堀 真司; 小西 賢介 ; 松場 賢一  ; 佐藤 一憲 

Kamiyama, Kenji; Isozaki, Mikio; Imahori, Shinji; Konishi, Kensuke; Matsuba, Kenichi; Sato, Ikken

ナトリウム冷却高速増殖炉での炉心崩壊事故では、溶融炉心物質の一部が冷却材流路等を通じて炉心外へ流出する。溶融炉心物質との混合によって冷却材が急速に蒸発し、流路内から液相冷却材が排除されると、その後に流路内へ流入する溶融炉心物質の固化閉塞が生じ難くなる。このような特性は、溶融炉心物質の炉心外への早期流出を促進し、厳しい再臨界に至る可能性を低減する。本研究では、模擬物質として低融点合金と水を用い、実機相当以上の長さを有する冷却材流路を通じた融体流出試験を実施した。その結果、融体流入初期に冷却材の一部が蒸発し、下部プレナムとの接続部付近まで蒸気が拡大することで流出経路全体がボイド化することが示された。さらに、冷却材ボイド領域の拡大開始条件は融体熱量と冷却材の顕熱比並びに冷却材の加熱領域高さにて整理できること、並びに、冷却材ボイド領域の拡大過程での熱収支評価に際しては、流路壁面への膜状凝縮熱伝達を考慮する必要があることが明らかになった。これらの知見を酸化物融体とナトリウムを用いた試験結果に適用したうえで、実機条件における流出経路のボイド領域拡大挙動について考察した。

In CDA of LMFBR, molten core materials would discharge from the core region through the coolant paths. Rapid vaporization of the coolant by mixing of the molten core materials provides effective evacuation of the liquid coolant from the path and reduces significantly possibility of core-material freezing and blockage formation inside the paths. This characteristic enhances early discharge of molten-core materials and reduces possibility of severe re-criticality events. In this study, melt discharge experiments were conducted with a coolant channel simulating the discharge path with an enhanced length of the path compared with that of the realistic design structure. An alloy and water were used as simulant of the molten fuel and sodium respectively. This series of experiments showed that the discharge path can be entirely voided by vaporization of a part of the coolant at the initial melt discharge phase, followed by vapor expansion toward the end of the coolant channel. Furthermore, it was revealed that the condition where coolant void expansion started can be defined by melt-coolant sensible heats ratio and the heated height of the coolant. The heat balance evaluation during the coolant void expansion phase shows that the film condensation heat transfer should be considered. The coolant-void-expansion behavior in the discharge path of the realistic design condition was estimated based on an application of this knowledge to existing experiments with molten oxide and sodium.

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