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information about Fukushima published in English in Japanese media info publiée en anglais dans la presse japonaise

What is an active fault?

October 25, 2012
Editorial: Nuclear regulator must prioritize safety in active fault assessments

http://mainichi.jp/english/english/perspectives/news/20121025p2a00m0na007000c.html

 

The government's Nuclear Regulation Authority (NRA) will launch on-the-spot surveys to confirm whether there are active faults under nuclear power stations across the country, beginning at the Oi Nuclear Power Plant in Fukui Prefecture in November. If active faults just below a nuclear plant trigger a powerful quake, it could cause catastrophic damage to the reactors.


NRA Commissioner Kunihiko Shimazaki, the nuclear regulatory body's second-in-command, said the panel will review what constitutes an active fault in its nationwide surveys. The government's current guidelines for quake-resistance of nuclear plants recognize faults that have moved in the past 120,000-130,000 years as active faults. The NRA reportedly intends to broaden the scope to cover those that were active hundreds of thousands of years ago. We laud this move to prioritize safety.


Active faults are generally defined as faults that show traces of repeated past movement and may trigger earthquakes in the future. Countermeasures against earthquakes triggered by active faults are regarded as an important pillar of nuclear plant safety inspections, and the government does not allow nuclear reactors and other key facilities in nuclear power stations to be built above such faults.


However, suspicions have arisen that the government's assessments of faults around nuclear plants conducted in the wake of the March 2011 Great East Japan Earthquake may have overlooked active faults below some of the power stations. In response, the NRA is set to conduct on-the-spot surveys on five other facilities, including Japan Atomic Power Co.'s Tsuruga plant in Fukui Prefecture and Hokuriku Electric Power Co.'s Shika power station in Ishikawa Prefecture, following the Oi plant run by Kansai Electric Power Co.


It has been pointed out that electric power companies have underestimated the impact of earthquakes that could hit their nuclear stations. For example, utilities' forecast of the scale of earthquakes that could occur near their power plants tends to be smaller than predictions by the government's Headquarters for Earthquake Research Promotion. In a report released in 2010, the headquarters defined faults that triggered temblors 400,000 years ago on as active faults. In other words, such a wide gap in the definition of active faults was unreasonable. The NRA should investigate whether power companies and the NRA's predecessor had deliberately covered up active faults near nuclear plants.


A serious problem is that there is a possibility that even experts will be split on whether the faults are active. However, once a serious accident breaks out at a nuclear plant, its impact will be immeasurable. Regarding a survey on faults below the Oi plant, NRA Chairman Shunichi Tanaka said, "We must make a serious judgment even if faults are only suspected of being active, let alone cases in which the faults are confirmed to be active."


The NRA should closely examine faults near nuclear plants across the country, and if the panel deems them active, it should demand that the power stations near them be suspended or decommissioned. It goes without saying that priority must be placed on the safety of members of the public over power suppliers' profits.


The NRA should incorporate criteria for assessing active faults from the standpoint of safety and procedures for decommissioning nuclear plants based on assessment results into new safety standards for nuclear power stations it will draw up by July next year. The NRA must keep in mind that there are active faults all over this earthquake-prone nation.

 

 

 

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