"Safety will be secured because strict screenings have been conducted based on conditions of much larger eruptions than the recent one at Mount Ontakesan," Prime Minister Shinzo Abe said at an Oct. 2 Upper House plenary session, offering assurances of the Sendai plant's safety.
Kyushu Electric Power Co., which operates the plant, is working to resume operations there sometime early next year.
Active volcanoes are located near the Sendai plant, with Mount Sakurajima about 50 kilometers away and Mount Kirishima about 60 km away.
In the past, huge volcanic eruptions have led to the formation of large calderas and pyroclastic flows in the area.
Major eruptions occur in Japan about once every 10,000 years. However, nuclear plants have to implement measures depending on the risk even if the frequency of an event is low.
Nuclear plants are not designed to withstand pyroclastic flows at high temperatures. For that reason, if such flows should reach a facility, the consequences could be disastrous.
Kyushu Electric insists that the possibility of an eruption while the Sendai plant is operating is sufficiently low. The utility also says it would be possible to capture signs of a major eruption and remove nuclear fuel beforehand.
The Nuclear Regulation Authority has supported Kyushu Electric's position.
The removal of the nuclear fuel from the reactors is expected to take several years.
However, the Mount Ontakesan eruption once again demonstrated the limits to accurately predicting volcanic eruptions.
Concerns about the safety measures at the Sendai plant were raised at a Sept. 30 session of a special committee of the Kagoshima prefectural assembly. Some members raised doubts about whether early signs of an eruption could be detected. Others criticized Kyushu Electric's position.
Such concerns have not changed the policy set by Kyushu Electric and the NRA.
"The event expected to occur is totally different," said NRA Chairman Shunichi Tanaka. "It is unscientific to discuss the different events at the same time."
The recent eruption at Mount Ontakesan was of a phreatic type in which groundwater was heated by magma. That is different from a massive volcanic eruption that would spew large amounts of molten lava.
At the same time, there is no difference in the difficulty of predicting the two types of eruptions.
There is the possibility of observing a rise in magma levels by monitoring earthquakes and changes in the mountain size. However, it is difficult to predict what level of unusual activity will lead to a certain scale of eruption in the absence of accumulated observational data.
A study group of volcanologists set up by the NRA said that it would be difficult to make a judgement about a possible eruption based on observations alone.
"Current predictions are based on experience," said Toshitsugu Fujii, chairman of the central government's coordinating committee for prediction of volcanic eruptions. "Because we have never observed a huge eruption, there are many more factors that we do not understand in comparison to the phreatic eruption that occurred at Mount Ontakesan."
Volcanic eruptions were not even considered a major issue in nuclear plant safety until 2013, when new standards called for taking into account eruptions at mountains lying within a 160-km radius of a nuclear plant.
Rather than huge eruptions or pyroclastic flows, one possibility that nuclear plants have addressed is falling volcanic ash.
If ash should accumulate on transmission lines, the weight could cut the lines and the ash could also cause corrosion. Such developments could cut off external power sources to a nuclear plant.
There are also concerns that volcanic ash could lead to clogging of filters attached to ventilation equipment and cause malfunctioning of emergency power generators.
If accumulated ash severs transportation routes, nuclear plants face the possibility of isolation.
At the Sendai plant, assumptions have been made that as much as 15 centimeters of volcanic ash could collect should a major eruption occur at Sakurajima. To deal with such a contingency, plans call for using heavy equipment to remove the ash, and replacement filters will also be prepared.
Because even 2 centimeters of ash could cut off overland routes for supplies, the plant is also considering possible sea routes for access. In addition, a week's supply of food and fuel will be stockpiled at the plant.
"Daily life will be seriously affected with even an accumulation of 5 centimeters of volcanic ash," said Setsuya Nakada, a professor of volcanology at the University of Tokyo. "In addition to measures within a nuclear plant, there will also be a need to think about the issue on a nationwide scale, including the possible conditions facing residents living near such plants."
(This article was written by Takeshi Nakashima and Chikako Kawahara.)
Source: Asahi Shimbun