原発と火山―噴火の脅威を直視せよ

May 12, 2014
EDITORIAL: Threat posed by volcanic eruptions to nuclear plants must be carefully examined
原発と火山―噴火の脅威を直視せよ

Now is the time to rethink the risk of operating nuclear power plants in Japan, which is one of the most volcanically active countries in the world.
　日本は世界有数の火山国である。そこに原発を抱えるリスクを、改めて熟考すべきときだ。

Kyushu Electric Power Co. is currently aiming to restart the operations of idled reactors in its Sendai nuclear power plant in Kagoshima Prefecture. However, in the Nuclear Regulation Authority's inspection process on whether to permit the restarts, the possible consequences of volcanic eruptions in surrounding areas is attracting attention.
　九州電力が再稼働をめざす川内（せんだい）原発（鹿児島県）の審査で、周辺火山の噴火の影響がクローズアップされている。

Based on the new safety standards worked out in 2013, the NRA is examining the threat posed by eruptions and the effectiveness of measures to deal with them. To tell the truth, it is the first time that Japan has seriously evaluated the safety of nuclear power plants from the standpoint of the danger posed by volcanoes.
　昨年できた新規制基準に基づき、原子力規制委員会が噴火の影響や対策の有効性を検討している。原発の安全性を火山との関係で本格的にチェックするのは、実はこれが初めてだ。

In the March 2011 accident at the Fukushima No. 1 nuclear power plant, people in charge had to reflect on the insufficient measures to deal with tsunami. However, disasters at nuclear power plants could be caused not only by tsunami but also by volcanic eruptions and even terrorists. Given the seriousness of disasters caused by these factors, it is a matter of course to think seriously about the risks posed by them, which have been made light of so far.
　福島第一原発の事故で、津波対策の遅れが大きな反省材料になった。津波に限ったことではない。事故の深刻さを踏まえれば、噴火やテロなど従来軽視されてきたリスクも真剣に考えるのは当然である。

The new safety standards require electric power companies to consider possible influences from volcanoes located within a radius of 160 kilometers from nuclear power plants. Therefore, Kyushu Electric examined the effects of eruptions from 39 volcanoes. As a result, it concluded that it is sufficient to take measures based on the assumption that ash from Sakurajima volcano in Kagoshima Prefecture would accumulate in the compound of the Sendai nuclear power plant to a height of up to 15 centimeters.
　新基準は１６０キロ以内の火山の検討を求めており、九電は３９の火山の影響を評価した。その結果、桜島の火山灰が敷地に最大１５センチ積もる想定で対策をとれば十分とした。

As one of the measures, the utility will stockpile fuel for emergency generators in preparation for a situation in which power transmission lines were severed due to the weight of volcanic ash. Another measure is that it will clean filters for air ventilation equipment or emergency generators or replace the filters with new ones if they become clogged.
火山灰の重みで送電線が切れた時に備えて非常用発電機の燃料を備蓄したり、換気設備や非常用発電機のフィルターの目詰まりに掃除や交換で対処したりするという。

However, will those measures really work given the possibility that accumulation of volcanic ash to a height of only several millimeters will seriously impede workers and vehicles? If the intake of water to cool nuclear reactors is also impeded, the reactors will be immediately plunged into dangerous situations.
　しかし、数ミリの降灰で人や物の移動が難しくなる恐れが指摘される。対策は実際に機能するだろうか。原子炉を冷やす水の取り込みに支障が出れば、たちまち原発は危険な状態に陥る。

The influences from these mid-scale eruptions must be fully examined as realistic threats.
　こうした中規模の噴火の影響は、現実的な脅威として十分に検討されなければならない。

It is more difficult to assess risks from catastrophic eruptions whose frequency of occurrence is low.
　頻度が低い破局的噴火のリスクは、判断がさらに難しい。

In those eruptions, the pyroclastic flow, which consists of hot gas and rock, travels more than 100 kilometers, causing devastating damage in surrounding areas. In the areas around the Sendai nuclear power plant, there are several calderas, or bowl-shaped depressions, that were formed by the collapse of land caused by catastrophic eruptions.
　高温の火山噴出物が火山ガスと一体となって広がる火砕流が半径１００キロ以上も流れ、破滅的な被害を出す。川内原発の周辺には破局的噴火で山が陥没したカルデラ地形が複数ある。

Kyushu Electric assessed that, given those calderas, catastrophic eruptions have occurred at an interval of about 60,000 to 90,000 years. Based on the assessment, it says, “Not much time has passed since the latest catastrophic eruption occurred. Therefore, the possibility is extremely low that the next catastrophic eruption will take place within the coming several decades when the nuclear power plant is operating. There will be no problems if we continuously monitor the signs of eruptions.”
　九電は、カルデラをまとめて破局的噴火が約６万～９万年間隔で起こっていると評価。「最新の破局的噴火からあまり時間がたっていないので、原発が運用される数十年のうちに次の破局的噴火に見舞われる可能性は十分低い。予兆を継続的に監視すれば大丈夫」と主張する。

However, some experts offer contrasting views, saying that forecasts of the intervals of eruptions are not reliable and that it is uncertain whether the signs of an eruption can really be foretold.
　これに対し火山学者らは「こうした噴火間隔の推測には疑問がある」「予兆が観測できるか分からない」と反論する。

Nuclear power plants are not the only facilities that would suffer devastating damage from catastrophic eruptions. If those nuclear plants are destroyed, however, radioactive materials will continue to be scattered throughout the world. It is a challenge not only for the Sendai nuclear power plant but also for many other nuclear power plants in Japan.
　破局的噴火で壊滅的打撃を受けるのは原発だけではない。だが、原発が破壊されれば放射性物質は世界にまき散らされ続ける。川内に限らず、日本の多くの原発に共通する難題だ。

Methods to assess the possible impact of eruptions have yet to be established in the world. The NRA bears responsibility for conveying the potential consequences, including the limits of human knowledge as to forecasting eruptions, to the public in an easy-to-understand manner.
　噴火の影響評価手法は世界的にも確立していない。規制委には火山噴火をめぐる人知の限界も含めて、国民にわかりやすく説明する責任がある。

--The Asahi Shimbun, May 11