アジア・太平洋ジャーナルに掲載されたアージュン・マキャニ博士の再臨界に関する論説

What Caused the High Cl-38 Radioactivity in the Fukushima Daiichi Reactor #1?
という記事では、昨日引用した小出裕章氏とおなじく、東電の公開したデータが正しければ、福島第一原発で「再臨界」が生じている可能性が高いことに言及している。

The presence of highly radioactive water in three turbine buildings at the Fukushima Daiichi nuclear plant is widely understood to be from the damaged fuel rods in the reactors. This has rightly raised concerns because it indicates several problems including extensive fuel damage and leaks in the piping system. Less attention has been paid to the presence of a very short-lived radionuclide, chlorine-38, in the water in the turbine building of Unit 1. The following paper evaluates whether its presence provides evidence of a serious problem – one or more unintended chain reactions (technically: unintended criticalities) – in the reactor. Such chain reactions create bursts of fission products and energy, both of which could cause further damage and aggravate working conditions that are already very difficult.
Chlorine-38, which has a half-life of only 37 minutes, is created when stable chlorine-37, which is about one-fourth of the chlorine in salt, absorbs a neutron. Since seawater has been used to cool, there is now a large amount of salt – thousands of kilograms – in all three reactors. Now, if a reactor is truly shut down, there is only one significant source of neutrons, namely, the spontaneous fission of some heavy metals which are created when the reactor is working and remain present in the reactor fuel. The most important ones are two isotopes of plutonium and two of curium. But if accidental chain reactions are occurring, it means that the efforts to completely shut down the reactor by mixing boron with the seawater have not completely succeeded. Periodic criticalities, or even a single accidental one, would mean that highly radioactive fission and activation products are being (or have been) created at least in Unit 1 since it was shut down. It would also mean that one or more intense bursts of neutrons, which cause heavy radiation damage to people, have occurred and possibly could occur again, unless the mechanism is understood and measures taken to prevent it. Measures would also need to be taken to protect workers and to measure potential neutron and gamma radiation exposure.
This paper examines whether spontaneous fission alone could be responsible for the chlorine-38 found in the water of the turbine building of Unit 1. If that could be the only explanation, there would be less to be concerned about. However, the analysis indicates that it is quite unlikely that spontaneous fission is the sole or even the main explanation for the measured concentration of chlorine-38. Presuming the reported measurements are correct, this leaves only one other explanation – one or more unintended chain reactions. This paper is presented in the spirit of encouraging discussion of whether further safety measures might be needed, and whether supplementary measures to bring the reactors under control should be considered. It is also presented as a preliminary analysis for scientific discussion of a terrible and technically challenging nuclear crisis at the Fukushima Daiichi plant.
Arjun Makhijani March 30, 2011

この論説の著者も小出氏と同じく、東電が公開した資料だけに基づいて「再臨界」の可能性を考慮すべき理由を挙げている。この論説も又、我々が読んで然るべきものであり、最悪の事態を考慮せずに、危険性に直面している現実から目をそむけて眠り込んでしまってはなるまい。東電が公開した資料は、当然、一部の資料だけであろうが、それだけにもとづいても、東電や保安院、安全委員会の説明では言及されていなかった深刻な事態を予告するものとして解読できるのである。保安院の発表は、パニックを恐れるあまり、本当のことを率直に言わないということは、レベル７の事態であったことを一ヶ月後にようやく発表したこと、原子炉建屋の爆発時点での深刻な状況を正確に伝えなかったことなどに明瞭に現れている。
小出氏やアージュン・マキャニ氏の論説およびそれを紹介したジャーナルが信頼できるのは、政府や当事者が言いたくない困難な事実があることを、あえてあるがままに公開し、それにもとづいて適切な対応を呼びかける科学者の良心を尊重しているからである。厳しい状況を直視しつつも、恐怖を煽るのではなく平常心を忘れぬ人間的な勇気を私はそこに見出した。