May 12, 2014
EDITORIAL: Threat posed by volcanic eruptions to nuclear plants must be carefully examined

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.

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.

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.

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.

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.

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.

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.

The influences from these mid-scale eruptions must be fully examined as realistic threats.

It is more difficult to assess risks from catastrophic eruptions whose frequency of occurrence is low.

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.

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.”

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.

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.

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.

--The Asahi Shimbun, May 11

0 件のコメント: