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

130,000 or 400,000 years?

February 17, 2013


News Navigator: What is the future of active fault inspections at nuclear plants?



The Nuclear Regulation Authority has revised safety standards that power companies must follow when building and running nuclear power plants. The revisions, to take effect from July, include relaxing the conditions for determining a fault is active. The Mainichi answers common questions readers may have regarding these changes.

Question: What are the current standards for judging active faults?

Answer: Active faults are underground cracks that have evidence of moving recently and are predicted to move again in the future. If the force that caused them to previously move struck again, the faults would move once more. Researchers believe that such energy continues to apply itself to faults for around 400,000 years, and the government's Earthquake Research Committee therefore generally considers faults as active if they have moved within that period.

However, in the case of nuclear power plants, the standards for active faults have been changed to those that have moved in the last 120,000 to 130,000 years. This is because around that time the Earth's climate was warmer and ocean levels were higher, making it easier to identify unusual ground layers from that period.

Q: How will the standards change?

A: When it is clear that a fault moved within the last 120,000 to 130,000 years, the current standards will be applied. However, when it is not clear, faults will be investigated for activity in the last 400,000 years. Even at their slowest rates, active faults move every tens of thousands of years. If a fault moved within the past 400,000 years, it also should have moved within the latest 120,000 to 130,000 years.

Q: What will be the effect on the operation of nuclear plants?

A: For at least the No. 1 and No. 2 reactors at the Kashiwazaki-Kariwa Nuclear Power Plant in Niigata Prefecture and the Tomari Nuclear Power Plant in Hokkaido, the power companies owning the plants have dismissed the danger of faults there, saying they have not had confirmed activity for at least 200,000 years. Under the new standards, the faults would have to be further examined.

Q: What would happen if they were found to be active?

A: The new standards clearly forbid the building of important facilities like reactor-housing buildings directly above active faults. This could force the dismantling of existing reactors if they were found to be above such faults. Even if an active fault is not directly beneath such a structure, if there is one nearby then structures would have to be prepared against the largest predictable shockwaves for a nuclear plant to be allowed to operate. (Answers by Ei Okada, Tokyo Science & Environment News Department)


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