Photo/IllutrationKyushu Electric Power Co.’s Sendai nuclear plant in Kagoshima Prefecture (Asahi Shimbun file photo)

A study group at the Nuclear Regulation Authority (NRA) has worked out a draft plan to revise a calculation formula crucial for efforts to improve the earthquake resistance of nuclear plants.

This step is naturally required to ensure compliance with the fundamental principle that safety measures for nuclear plants should be based on the latest scientific knowledge.

The plan will be formalized shortly and instructions about the change will be issued to electric utilities operating nuclear plants.

The revision to the calculation formula will also be applied to nuclear reactors that have been officially certified to fulfill the new safety standards including ones that have already been restarted. This is a system known as “backfitting,” which means the required modification resulting from a new or amended provision in the NRA’s regulations based on the latest scientific knowledge.

This system is based on lessons learned from the Fukushima nuclear disaster. The electric power companies should take appropriate steps to follow the instructions.

Quake-resistance measures for nuclear power plants across the nation are based on the assumed maximum ground shaking due to an earthquake, which is known among experts as “design-basis earthquake ground motion.”

The values of design-basis ground motion for individual nuclear plants are calculated for two categories of earthquakes: trench-type earthquakes and quakes caused by a nearby active fault; and earthquakes that are not caused by any identifiable source and can occur anywhere in the nation.

The NRA’s task force has revised the calculation formula for the second type of earthquake. Since the current method is based on data about just one past earthquake, experts have long pointed out the need to increase the number of cases used to improve the accuracy.

The new formula has been developed by analyzing measurement data of 89 earthquakes that occurred during the period from 2000 through 2017.

The operators of nuclear plants will be required to use the new formula to recalculate the values of design-basis ground motion for their plants while incorporating data concerning specific ground conditions at the sites.

If the re-evaluation process reveals insufficient quake resistance, additional measures will be needed to rectify the problems.

Among the five reactors currently online, the units at Kyushu Electric Power Co.’s Sendai nuclear plant in Kagoshima Prefecture and Genkai plant in Saga Prefecture and Shikoku Electric Power Co.’s Ikata plant in Ehime Prefecture may be affected.

Revisions to the nuclear safety regulations based on the latest seismic knowledge have been applied to all nuclear reactors under the backfitting system. There have been more than 10 such cases concerning fires, volcanic eruptions and tsunami that occur without warning.

The NRA has the authority to order the suspension of reactor operations until the necessary measures are taken. But the nuclear watchdog has never taken this action.

Instead, the NRA has allowed reactor operations to continue in such cases by setting grace periods based on estimates of the scale of the effects and the time required to install additional safety measures.

For nuclear plant operators that do not want to suspend reactor operations, the longer the grace period the better.

In April, electric utilities asked for an extension of the five-year grace period for taking additional measures to protect their nuclear plants from terror attacks involving aircraft as it became clear that they will be unable to meet the deadline.

But the NRA rejected the request, ensuring that the reactors will be taken offline if the required work is not finished by next spring.

The NRA made the right decision by putting a higher priority on safety than on keeping reactors on stream. It should continue strictly implementing the rule according to the safety-first principle while limiting the grace period to a minimum.

Natural disasters and terrorist attacks do not wait for safety measures to be taken.

--The Asahi Shimbun, July 18