Government Researchers: Hydrogen Gas Generation by Radiolysis In Reactor SFP May Have Led to Explosion

and as it was revealed at the fall conference of the Atomic Energy Society of Japan on September 20, the water had to be boiling for that to happen.

The article by Mainichi Shinbun on September 13 (linked below) summarizes the findings well enough. According to the article, the government researchers at Japan Atomic Energy Research and Development Agency and at Tokyo University announced the result of their analysis that the explosion of Reactor 4 reactor building at Fukushima I Nuclear Power Plant on March 15 may have been caused by a large amount of hydrogen gas produced by water radiolysis - dissociation of molecules by nuclear radiation in the Reactor 4 Spent Fuel Pool.

Here's from Mainichi Shinbun Japanese (9/13/2011):

東京電力福島第１原発４号機で起きた原子炉建屋の爆発について、沸騰した使用済み核燃料プール内で、水の放射線分解が進んで、水素が大量発生したことが一因との分析を、東京大や日本原子力研究開発機構のチームがまとめた。放射線は、水を水素などに分解する。１９日から北九州市で始まる日本原子力学会で発表する。

An analysis by a team of researchers from Tokyo University and Japan Atomic Energy Agency shows that part of the reason for the explosion of Reactor 4 reactor building at Fukushima I Nuclear Power Plant was the large amount of hydrogen gas generated in the Spent Fuel Pool of Reactor 4. Radiation splits water and creates hydrogen. The analysis will be presented in the Atomic Energy Society of Japan's fall conference that will start in Kitakyushu City from September 19.

　４号機のプールには、事故を起こした１～４号機の中で最も多い１５３５本の燃料棒が入っていた。東日本大震災発生当時、定期検査で運転停止していたが、津波で電源を喪失。冷却機能が失われ、地震発生から４日後の３月１５日に爆発した。

The Reactor 4 Spent Fuel Pool contained 1,535 fuel rods, the most in the reactors at Fukushima that broke down. At the time of the March 11 earthquake/tsunami, the reactor had been stopped for the regular maintenance. But the tunami knocked out the power supply, which caused the loss of cooling, and the reactor building exploded on March 15, 4 days after the quake.

　水素爆発を起こした１、３号機では原子炉内にあった燃料棒が損傷し水素が発生したとされるが、４号機の燃料棒に目立った損傷はなかった。東電は排気筒を共有する３号機から水素が流入して、４号機の水素爆発にいたったと推定している。

In Reactors 1 and 3 which blew up in hydrogen explosions, it is considered that the fuel rods inside the reactors were damaged, which generated hydrogen. However, the fuel rods from Reactor 4 did not have noticeable damages. TEPCO suspects that hydrogen gas from Reactor 3, which shares the exhaust stack with the Reactor 4, flew through to Reactor 4, causing the explosion in Reactor 4.

　しかし、チームは３号機と４号機の爆発に約２０時間の差があることに注目し、他の要因があると推測。フラスコ内の水を室温、９７度、沸騰状態の３段階にして、放射線を照射。発生した水素の濃度を調べたところ、９７度で室温の１．５倍、沸騰状態で１００倍となることが分かった。

However, the team of researchers from Tokyo University and JAEA looked at the 20-hour gap between the explosions of Reactor 3 and 4, and suspected there are some other causes for the explosion [of Reactor 4]. The team irradiated the water in a flask at room temperature, at 97 degrees Celsius, and at boiling temperature and measured the density of hydrogen generated. At 97 degrees Celsius, the density was 1.5 times as much as at room temperature, and at boiling temperature the density was 100 times as much as at room temperature.

　水素は空気中の濃度が４％を超えると爆発の危険性が出てくる。建物上部にたまった水蒸気は壁で冷やされて水に戻るが、水素は気体のままで空気中に占める割合が高まったとみられる。

If the density of hydrogen in the air exceed 4%, there's a danger of explosion. Vapor that went to the upper floors of the reactor building would have been cooled by the walls and condensed into water again, but hydrogen may have remained gaseous, increasing the ratio of hydrogen in the air.

　チームの勝村庸介・東大教授（放射線化学）は「３号機からの流入に加え、放射線分解が重なったのではないか。今後、実際の原子炉建屋やプールの規模で起こるのかを検証したい」と話す。

Professor Yosuke Katsumura of Tokyo University (radiation chemistry), one of the team members, says "Water radiolysis may have happened in addition to the hydrogen flow from Reactor 3. We would like to test whether it can happen in an actual reactor building with a real-size spent fuel pool".

　東電は「理屈上はありうるが、爆発させるほど水素が大量発生するかどうかは不明」としている。

TEPCO says, "It is possible in theory, but we do not know whether a large enough amount of hydrogen is generated that way to cause an explosion".

There seem to be many interesting papers presented at the conference, highly scientific and technical, detailed insights into what may have happened and how at Fukushima I Nuke Plant. A nice departure from muddling through by TEPCO and NISA for the past 6 months. I only wish I had studied physics and chemistry more...

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