Report says Fukushima Reactor 2 containment possibly failed shortly after the earthquake

Researchers from the Japan Atomic Energy Agency have conducted simulations that show that Fukushima Daiichi Reactor 2 may have sustained damage immediately after the earthquake.   The simulations showed that a hole possibly 6 or 7 inches in diameter may have occurred in the containment vessel.

This is the first estimate of damage to the concrete containment vessels, and was presented in a forum held in Kitakyushu City.  The report said that the heat from the melting fuel should have caused pressures to exceed the design basis for the containment within 48 hours, but at Reactor 2 it took over 72 hours before the reactor reached the critical pressure levels.

TEPCO has been working to calm fears after a flurry of failed thermometers in the Unit 2 reactor in recent months, 25 of 42 gauges have failed to date.  According to a report to the Nuclear and Industrial Safety Agency, the utility is considering putting a thermometer down to a depth of 15 to 20 meters through an existing pipe to gauge the temperature of water at the bottom of the No. 2 reactor in late July at the earliest.  After the government declared that the plant had been brought to cold shutdown in December, contaminated water leaked from 44 locations at the plant.

“One of the important lessons from this accident is that we have to care very much about the temperature of the containment vessel; that means some cooling of the containment vessel is needed” following a disaster, Akira Omoto, a member of the five-person Atomic Energy Commission (AEC) of Japan said.

東日本大震災直後に、東京電力福島第一原子力発電所2号機の格納容器が損傷、直径約7・6センチ相当の穴が開いた可能性のあることが、日本原子力研究開発機構の柴本泰照研究員の模擬実験で分かった。

格納容器の損傷度を示す具体的な数値が推定されたのは初めて。北九州市で開かれている日本原子力学会で21日、発表された。

柴本さんは、交流電源が喪失した後に、蒸気で注水を継続する非常用冷却装置「原子炉隔離時冷却系」(RCIC)の動作状況、圧力の推移など東電が発表したデータを活用。RCICへの水の供給源は、震災14時間後に、枯渇した復水貯蔵タンクから格納容器の底部の「圧力抑制室」に切り替わった。

この場合、熱が外部に逃げないため、圧力は、震災後、2日程度で設計圧力(約5気圧)の2倍まで急上昇する。しかし、実際は、圧力の上昇は緩やかで、7気圧に達するまで3日以上経過していた。

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