2 Décembre 2018
November 27, 2018
EDITORIAL: Nuclear plants must take threat of volcanic ash more seriously
The Nuclear Regulation Authority will reassess the safety risks posed by possible natural disasters to certain nuclear power plants that have been declared to be fit for operation under the new safety standards.
The nuclear watchdog’s unusual decision has been prompted by recent discoveries of new facts concerning possible effects of volcanic eruptions on the Mihama, Oi and Takahama nuclear power plants operated in Fukui Prefecture by Kansai Electric Power Co.
It is a totally reasonable decision based on the principle of putting the top priority on safety in regulating nuclear plants.
Initially, Kansai Electric asserted that volcanic ash posed no threat to the safety of the three nuclear plants. Its claim was based on its own estimate of the amount of volcanic ash that would fall on the plants.
Using research findings and geological surveys as well as simulations of eruptions of Mount Daisen, a volcanic mountain in Tottori Prefecture located about 200 kilometers from the plants, the Osaka-based utility estimated that the nuclear compounds could be coated with up to 10 centimeters of ash from a major volcanic eruption.
The NRA accepted the company’s assessments of volcanic hazards for these plants and allowed the No. 3 and No. 4 reactors at both the Oi and Takahama plants to come back online.
After the NRA’s safety screenings, however, a 30-cm ash layer from an eruption of Mount Daisen that occurred 80,000 years ago was discovered in Kyoto, 190 km from the mountain.
Kansai Electric argued that the thickness of ash from the mountain cannot be estimated accurately because ash from other sources was mixed in.
But the NRA confirmed that the layer of volcanic ash from the mountain is 25 cm thick through its own on-site inspection and other research, concluding that the eruption was greater in scale than the utility’s estimate of a maximum possible incident.
These developments have led to the regulator’s unusual decision to reassess the risks posed by volcanic ash fall to the safety of the plants.
A massive fall of volcanic ash could cause a malfunction of the emergency power generation system at a nuclear power plant and cut off the power supply, which is crucial for preventing a severe nuclear accident during a natural disaster.
The new findings have made it inevitable to re-evaluate the estimate of maximum possible volcanic ash fall for each nuclear plant and consider the necessity of additional safety measures.
One important component of the new tighter nuclear safety standards introduced after the 2011 Fukushima nuclear disaster is the so-called “back-fit” system, which applies the latest safety requirements to existing reactors.
The NRA acted on this new rule when it decided to reassess the threats posed by volcanic eruptions to the safety of the nuclear plants by incorporating the implications of the newly discovered facts.
The body should adopt the same stance toward safety risks posed by other natural disasters such as earthquakes and tsunami.
But the NRA has also decided not to order the utility to suspend the operations of the four reactors, at least for now, because there is a certain safety margin in the measures to deal with volcanic ash fall taken at the three nuclear plants in Fukui Prefecture.
But it should not hesitate to order the shutdowns of these reactors if more new facts are discovered with risk implications for them.
Bodies of scientific knowledge concerning earthquakes, tsunami and volcanic eruptions change constantly due to new findings from research and surveys.
Kansai Electric Power’s response to the new discovery deserves to be criticized as an attempt to escape from an inconvenient new fact.
Electric utilities operating nuclear plants need to make constant efforts to gather the latest information and face new facts concerning the safety of their nuclear plants in a humble and honest manner.
The back-fit system was introduced to ensure the safety of nuclear plants in this nation as a policy response to the lessons learned from the catastrophic accident at the Fukushima No. 1 nuclear power plant. This should never be forgotten.
--The Asahi Shimbun, Nov. 27