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

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  • Photo/Illustraion
  • Photo/Illustraion

The nightmare scenario of a volcanic crater erupting and spewing a pyroclastic flow that engulfs a nuclear plant, causing catastrophic levels of radiation to leak into the atmosphere, doesn't appear on the horizon ... just yet.

But the nation's nuclear watchdog is taking no chances. It plans to install seabed sensors to monitor potential crustal deformations on the Aira Caldera, located just 40 kilometers from the Sendai nuclear power plant in Kagoshima Prefecture.

Little is known of processes that lead to giant eruptions of calderas, or ground depressions formed by volcanic activity, due to a lack of observation data. Such eruptions are extremely rare and occurs every 10,000 years in Japan.

The Aira Caldera, in Kagoshima Bay, was the site of a giant eruption around 30,000 years ago.

The Nuclear Regulation Authority has so far relied on land-based seismic and other sensors to indirectly monitor magma activity and other changes beneath the seabed.

Starting in the new fiscal year from April, the NRA will set up seismic sensors and water-pressure gauges on the seafloor for additional monitoring.

The NRA said it will study the correlation between both sets of data to develop more reliable observation methods.

Current regulation standards, introduced in the aftermath of the Fukushima nuclear disaster of 2011, made it obligatory for electric utilities to take into account the impact of volcanoes that lie within 160 km of a given nuclear facility.

During the screening process for the Sendai plant, whose reactors were reactivated in 2015, plant operator Kyushu Electric Power Co. explained that a giant eruption is considered extremely unlikely during the operating life of the reactors.

The utility also argued it would be possible to remove the nuclear fuel and implement other measures ahead of an eruption, as such an event would be preceded by an expansion of a magma reservoir accompanied by ground surface changes.

The NRA accepted the argument and approved the reactor restarts on condition that the utility continues with land-based monitoring. That, however, drew criticism from volcanologists, who argue it is difficult to predict a massive eruption with certainty even at the last moment, and is almost impossible to forecast one several decades in advance.

NRA officials noted that the Italian government raised the warning level in 2016 after sensors on the seabed detected rising magma pressure and temperatures beneath Campi Flegrei, a caldera lying outside Naples. There is no nuclear plant in the area.

“There is no guarantee that changes will be observed in Japan in the same way that they were in Italy, but we hope to find out if the application will be possible,” said an NRA official.

Huge calderas exist within a 160-km radius of other nuclear plants in Japan, including Kyushu Electric’s Genkai plant in Saga Prefecture, Shikoku Electric Power Co.’s Ikata plant in Ehime Prefecture and Hokkaido Electric Power Co.’s Tomari plant. The NRA intends to conduct studies in those areas, too.

The screening process for the Sendai reactor restarts also included discussions of the Kikai Caldera, located off Kyushu’s southern tip and about 120 km from the plant, which was the site of a giant eruption some 7,300 years ago.

Yoshiyuki Tatsumi, a Kobe University professor of magmatology who has been conducting a vessel survey since 2016 in the waters above the Kikai Caldera, said there is no easy way to detect possible signs of a massive eruption.

“You could make a rough guess that there is a magma reservoir at such and such a location,” the volcanologist said. “But monitoring of detailed changes is a difficult affair.”