#Fukushima I Nuke Plant: Unintended "Water Entombment" of Reactor 1

They didn't mean to, but ended up doing what some experts had suggested as a way to cool the melted fuel in the Reactor 1's Pressure Vessel.

Mainichi Shinbun (10:01 PM JST 4/23/2011; emphasis added) reports that TEPCO is "entombing" the Reactor 1 with water by accident:

Fukushima I Nuke Plant: 6 meters of water inside the Reactor 1 Containment Vessel

TEPCO disclosed on April 23 that there is 6-meter deep water inside the Reactor 1 Containment Vessel at Fukushima I Nuclear Power Plant. The "water entombment", whereby the Containment Vessel is to be filled with water above the level of the fuel rods [inside the Pressure Vessel] is one of the goals listed in TEPCO's "roadmap" within the next 3 months. What is happening seems to be the de facto "water entombment", not by design but by accident. It is not clear whether the water continues to rise above the level of the fuel rods.

According to TEPCO, about 70% of the fuel rods are damaged in the Reactor 1, and the total 7,000 tons of water have been poured into the Pressure Vessel in an effort to cool the Vessel. The steam from this cooling effort may have escaped into the Containment Vessel and condensed into water, or the pipes that connect the Pressure Vessel and the Containment Vessels may have been damaged in the earthquake, allowing the water to leak into the Containment Vessel.

The water level is an estimate by TEPCO from the change in pressure within the Containment Vessel as they injected the nitrogen gas to prevent a hydrogen explosion. TEPCO believes the Suppression Chamber near the bottom of the Containment Vessel is now completely filled with water, and the flask-shaped dry well (17.7 meter in diameter) is filled with 6-meter deep water.

Water is also being injected into the Pressure Vessels of Reactors 2 and 3. In Reactor 2, the Suppression Chamber is damaged and the highly radioactive water has been leaking from the Containment Vessel; it is not known whether any water remains inside the Containment Vessel.

The "water entombment" method is not without problems. In the Reactor 1's Containment Vessel, 10,700 cubic meters of nitrogen gas, twice the capacity of 6,000 cubic meters, have been injected but the pressure has stopped rising. It is possible that the Containment Vessel is damaged, and the water may leak from the damaged area if the water keeps rising. Also, the vulnerability of the Containment Vessel to an earthquake due to increased weight from the water is "under the final examination", according to Nuclear and Industrial Safety Agency. In addition, it will be necessary to treat the highly contaminated water [inside the Containment Vessel] at some point.

福島第１原発：１号機格納容器内に水６メートル

　東京電力は２３日、福島第１原発１号機の原子炉格納容器に深さ約６メートルの水がたまっていることを明らかにした。格納容器を燃料棒の上部まで水 で満たして原子炉を冷やす「水棺」作業は、事故収束に向けた工程表で最初の３カ月目標に掲げた対策の一つ。同社が意図しない形で事実上の水棺状態が進行し ているとみられるが、このまま燃料棒上部まで水位が上がるかどうかについては不確定要素もある。

　東電によると、１号機は燃料棒の損傷が推定７０％と最も激しく、圧力容器にこれまで約７０００トンを注水して冷却を続けてきた。ここで発生した蒸 気が格納容器に移って水になっている可能性や、圧力容器と直結する配管などが地震で損傷し、格納容器に水が漏れ出ている可能性が考えられるという。

　水位は、水素爆発を防ぐための窒素注入による格納容器の圧力変化から東電が推計した。その結果、格納容器下部にある圧力抑制プールは既に満水と なっており、「ドライウェル」と呼ばれるフラスコ状の球形部（直径１７・７メートル）も深さ約６メートルの水がたまっていることが分かった。

　２、３号機も同様に圧力容器への注水が続けられているが、２号機では圧力抑制プールが破損し、高濃度の放射性汚染水が外部へ漏れ出ており、格納容器内の水のたまり具合は分かっていない。

　一方、水棺方式には課題もある。格納容器には既に容量（約６０００立方メートル）の２倍近い窒素約１万７００立方メートルを注入しているが、一定 以上に圧力が高まっていない。容器の損傷も考えられ、このまま水位が上がれば、損傷部からの水漏れが懸念される。また、水の重量の負荷に伴う耐震性は「最 終チェックしている段階」（経済産業省原子力安全・保安院）の上、長期的には高濃度に汚染された水の処理も必要となる。【八田浩輔、阿部周一】

Mainichi has a diagram that they created, based on the information from TEPCO. 11.8 meters to go for the water to rise above the level of the fuel rods.

At least TEPCO didn't claim this "entombment" was by design. Some honesty here. I have a feeling that TEPCO has grown so weary and worn out to make up stories any more.

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