核分裂の疑い―炉内の混沌を忘れまい

--The Asahi Shimbun, Nov. 3
EDITORIAL: Fears of fission dispel unwarranted optimism about Fukushima
核分裂の疑い―炉内の混沌を忘れまい

Tokyo Electric Power Co. (TEPCO) announced Nov. 2 that nuclear fission may have occurred in the No. 2 reactor at the Fukushima No. 1 nuclear power plant.
　核分裂の疑いがある。もしかしたら、一時的に小さな臨界が起こった可能性もある。

The operator of the disaster-stricken facility also said that a temporary, small-scale state of criticality might have occurred.
　東京電力は福島第一原発２号機についてこんな発表をした。

The news is deeply disquieting.
とても気になる話である。

The remaining nuclear materials are still breaking down gradually in a process known as radioactive decay.
　核分裂とは何か。
　福島第一の事故炉では今も、残った核物質が緩やかに壊れているが、これは核崩壊という。

Nuclear fission occurs inside nuclear fuel in an operating reactor.
　一方、運転中の原子炉の核燃料で起こる現象が核分裂だ。

The nucleus of an atom in nuclear fuel splits into smaller parts after being struck by a stray neutron.
原子核が、飛んできた中性子によって割れるように壊れる。

When the nucleus of an atom is split apart in this way, it throws off free neutrons, thereby causing the fission of the nucleus of another atom.
　このとき、また中性子が出るので、別の原子核の分裂を引き起こすことがある。

When this process continues to occur in a chain reaction, this state is called criticality.
これが続くのが「臨界」だ。

A nuclear power plant generates electricity through the huge amount of energy released when nuclear fission occurs.
　この連鎖反応によって、原子核の膨大なエネルギーをとりだすのが原発である。

In short, criticality must not be allowed to occur in a disabled reactor in which nuclear fuel and control rods are not arranged in an orderly manner but are in a chaotic state.
　裏を返せば、燃料や制御棒が秩序だって並ぶことなく、混沌（こんとん）の極みにある事故炉で臨界はあってはならない。

Nuclear fission in the No. 2 reactor, depending on its scale, could result in a dangerous state of criticality and possibly trigger an unpredictable situation that is impossible to control.
　核分裂の規模によっては、そんな臨界を引き起こし、場合によっては制御できない不測の事態を招きかねない。

Soon after TEPCO workers began operating a gas control system at the No. 2 reactor to purify gases within the containment vessel, traces of what appeared to be radioactive xenon 133 and xenon 135 were found.
　今回は、原子炉格納容器の気体を浄化するシステムを動かしてまもなく、その気体から、核分裂の痕跡といえるキセノン１３３、キセノン１３５らしい放射性物質が見つかった。

The presence of xenon isotopes indicates that nuclear fission has taken place.
The detected elements that appeared to be telltale signs of nuclear fission were later confirmed by the Nuclear and Industrial Safety Agency to indeed be radioactive xenon isotopes.
違うものをみたおそれもあったが、経済産業省原子力安全・保安院が本物と判断した。

Since these isotopes have a half-life of several days, they illustrate what is happening right now inside the No. 2 reactor.
They were not generated in the immediate aftermath of the crisis that flared in March.
　これらの半減期は数日以内なので事故発生時ではなく、今の２号機内の様子を反映していることになる。

The elements were detected soon after the gas purification system was activated. They may point to an ongoing situation in the reactor.
浄化システム立ち上げ直後の検出なので、こうした状況は続いているとみるべきかもしれない。

On the other hand, no significant changes in temperature or pressure inside the reactor have been observed.
　事故炉の温度や圧力に大きな変動はないようだが、

But just the possibility of nuclear fission occurring requires immediate actions to prevent criticality from happening.
核分裂の可能性がある以上、臨界を食いとめる手を急いで打たなくてはならない。

TEPCO's quick move to pump water mixed with boric acid into the reactor to prevent criticality shows that the company takes the discovery of radioactive xenon very seriously.
臨界を抑える働きがあるホウ酸水を、東電が直ちに注入したのも、そんな危機感の表れだろう。

At the end of September, the government lifted its designation of areas in a radius of between 20 kilometers and 30 km of the plant as an emergency evacuation preparation zone.
　政府は９月末に、炉の冷却が安定して進み、緊急事態が起こる可能性が極めて低くなったとみて、半径２０～３０キロ圏の緊急時避難準備区域を解除した。

The decision was made in the belief that stable cooling of the crippled reators had been making progress, thus almost eliminating the possibility of a fresh nuclear emergency.
It seems that the Fukushima plant is on track to reach a stable state known as cold shutdown by the end of the year in line with the government's timetable for its efforts to bring the nuclear disaster under control.
いま福島第一原発は、政府が年内をめざす冷温停止状態に近づいているように見える。

It must not be forgotten, however, that there is melted nuclear fuel, which has no trace of its original form, inside the wrecked reactors and their containment vessels.
　だが事故炉やその格納容器のなかのことを思い浮かべれば、そこには、溶け落ちて燃料の原形をとどめない核物質がある。

The government is responsible for cleaning up the mess so that residents in areas around the plant can pick up the pieces of their lives.
　政府は事故処理を進め、周辺住民の生活を元に戻してゆく責任があるが、

That said, the government must not become oblivious to the remaining risks simply because its priority is on carrying out the cleanup work according to the timetable.
工程表の期限を優先するあまり、こうした現実を見過ごしてはならない。

The discovery that nuclear fission might have occurred should be taken as a serious warning.
　核分裂の痕跡を重い警鐘と受けとめたい。