Expert opinion is divided on the basis for the Hiroshima High Court's Sept. 25 decision to green-light the restart of a reactor at the Ikata nuclear plant in southwestern Japan's Ehime Prefecture.
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The Hiroshima High Court on Sept. 25 overturned an injunction issued by the same court in December last year ordering the Ikata plant's No. 3 reactor be suspended. The injunction was based on the potential risks posed to the nuclear station by a future major eruption of Mount Aso, about 130 kilometers to the southwest in Kumamoto Prefecture in Kyushu. The latest decision accepted plant operator Shikoku Electric Power Co.'s assertion that no major eruption of Mount Aso -- which last occurred about 90,000 years ago -- could be expected within the operational lifetime of the reactor.
One expert sided with the latest court decision, agreeing that the possibility of a catastrophic eruption is extremely slim. Others said the decision lacked scientific grounds as it is extremely difficult to predict volcanic events.
Tadashi Narabayashi, specially appointed professor of nuclear reactor engineering at Tokyo Institute of Technology, told the Mainichi Shimbun, "The judgment is in line with social norms and is appropriate. If the plant were located in a dangerous area, then all residents of Kyushu would have to evacuate." He added, "It is a common view that the chance of catastrophic eruptions is extremely low."
Yukio Yamaguchi, co-leader of the Citizens' Nuclear Information Center, said the court "abandoned its responsibilities with its talk of 'social norms' and 'majority public opinion'" to justify green-lighting the Ikata reactor restart.
"The court reasons that even if the operator does not assume catastrophic eruptions can occur, it cannot be said the plant is unsafe unless reasonable grounds to expect such eruptions is presented. But were there any 'reasonable grounds' for predicting the tsunami that caused the Fukushima nuclear crisis?" Yamaguchi continued.
Professor emeritus of volcano physics at Kyoto University Kazuhiro Ishihara commented, "That the past frequency of volcanic eruptions is low doesn't necessarily mean that the frequency of eruptions while the plant is in operations will also be low. In the latest case, the court never discussed the chance Mount Aso would erupt, and its decision was unscientific."
He added that he believed the court "ignored the statistical probability (of an eruption) because they don't understand it."
The secretariat to the Nuclear Regulation Authority (NRA) took the court decision calmly.
"We're not a party to the matter so are in no position to comment on the decision," an official said. "Our position to enforce strict regulations in safety inspections on nuclear plants is unchanged."
An official in charge of safety inspections said he was "not surprised" at the court decision, adding that the court apparently recognized the NRA's own evaluation of the Ikata plant was appropriate.
The NRA safety checklist states that a nuclear plant's location can be deemed appropriate only if there is a "sufficiently small chance" of volcanic pyroclastic and lava flows reaching the grounds.
Following the December 2017 Hiroshima High Court ruling, the NRA clarified its position. The authority stated that a power station's risk of being hit by the volcanic flows is considered "sufficiently small" if there are no grounds for predicting a nearby catastrophic eruption within the 40-year operational life of the plant.
Moreover, the authority state that it is permissible not to consider safety measures against volcanic eruptions in light of social norms, unless the possibility of a catastrophic eruption is specified.
(Japanese original by Riki Iwama and Koki Matsumoto, Science & Environment News Department, and Koji Endo, City News Department)