The Japan Nuclear Regulation Authority (NRA) is planning to install seabed sensors to monitor potential crustal deformations on the Aira Caldera, located 40km from the Sendai nuclear plant in Kagoshima Prefecture.
Current regulations, introduced after the Fukushima accident of 2011, require utilities to take into account the impact of volcanoes that lie within 160km of a given nuclear facility.
Calderas are ground depressions formed by volcanic activity and eruptions are extremely rare, occurring every 10,000 years in Japan. The Aira Caldera was the site of a giant eruption around 30,000 years ago.
NRA has so far relied on land-based seismic and other sensors to monitor magma activity and other changes beneath the seabed. However, from April, the NRA will set up seismic sensors and water-pressure gauges on the seafloor for additional monitoring. It will study the correlation between both sets of data to develop more reliable observation methods.
During the screening process for restarting the Sendai reactors, plant operator Kyushu Electric Power Co said an eruption is considered "extremely unlikely" during the operating life of the reactors. Kyushu said it would be possible to remove the nuclear fuel and implement other safety measures ahead of an eruption.
NRA approved the reactor restarts in 2015 on condition that Kyushu continues land-based monitoring. Volcanologists criticised the decision arguing that it is difficult to predict a massive eruption with certainty even at the last moment.
The Sendai screening process also included discussions of the Kikai Caldera, located off Kyushu’s southern tip and about 120km from the plant, which was the site of a giant eruption some 7300 years ago.
Calderas exist within a 160km radius of other reactors 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 also intends to conduct studies in those regions.