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Restart of reactors not based on "scientific" jugement

October 6, 2014

Japan Political Pulse: Scientific uncertainty



Distinguishing between a "scientific judgment" and an "unscientific judgment" can prove difficult at times.

The recent eruption of Mount Ontake in central Japan has drawn attention to Kyushu Electric Power Co.'s Sendai Nuclear Power Plant in Satsumasendai, Kagoshima Prefecture. The reason is that the plant is located just 50 kilometers northwest of Mount Sakurajima, a prominent active volcano in Japan.

Nuclear Regulation Authority (NRA) Chairman Shunichi Tanaka, 69, told a news conference on Oct. 1 that it is "unscientific" to discuss Mount Ontake and Mount Sakurajima together. Volcanologists, meanwhile, say the logic behind countermeasures to protect the Sendai Nuclear Power Plant from volcanic eruptions is "unscientific." This gap is at the heart of discussions surrounding volcanic activity and reactivation of the Sendai plant.

Why does the government -- including the NRA -- view it as unscientific to link Mount Sakurajima with Mount Ontake? Confronting experts, we find the following explanation: While the eruption of Mount Ontake resulted in many deaths, it was a phreatic eruption, involving the explosion of steam and rock. No magma was ejected. Meanwhile, the Sendai Nuclear Power Plant is made to withstand an eruption 10,000 times the size of the Mount Ontake eruption.

What's the basis for "10,000 times"? It's actually nothing complicated. The size of an eruption is measured by the amount of ejected matter, and the amount of ejected matter can be roughly determined based on the accumulation of ash and other ejecta.

In the Ontake eruption, the amount of ejected matter was estimated at 1 million tons, an amount comparable to that produced by Mount Sakurajima in a year. At the same time, the 1914 reaction of Sakurajima a century ago produced 100 times the ejecta normally seen in a year. Kyushu Electric Power Co., the operator of the Sendai Nuclear Power Plant, bases its guideline for safety countermeasures on the Sakurajima-Satsuma eruption held to have occurred in the Jomon period roughly 12,800 years ago. The scale of that eruption is believed to be about 100 times bigger than the 1914 eruption.

Multiplying 100 by 100, we get 10,000. If an eruption 10,000 times the size of the Ontake eruption occurred, the pyroclastic flow would not reach the nuclear plant. The amount of ash that would fall on the Sendai plant would reach an estimated 12.5 centimeters. Since it can withstand up to 15 centimeters, there is no problem.

That's what Kyushu Electric Power Co. has to say, and it's also the reason behind the government's judgment that the Sendai plant's countermeasures for a volcanic eruption are adequate.

We see, then, this is rational and scientific. But there is another point from the volcanologists who maintain this is "unscientific" that gives us cause for concern -- namely, the method of determining the possibility of a catastrophic eruption exceeding the scale of the Sakurajima-Satsuma eruption.

Government data based on Kyushu Electric Power Co.'s explanation says, "The space between periods of activity from catastrophic eruptions is about 90,000 years. The latest such eruption occurred about 30,000 years ago, so the possibility of one occurring while the Sendai Nuclear Power Plant is in operation is sufficiently low."

But doubts remain over this theory that a cycle lasts 90,000 years. The space between major eruptions should be evaluated on each caldera. However, Kyushu Electric Power Co.'s evaluation is based on a combination of the Aira caldera including Sakurajima and four other calderas in the area.

Ryusuke Imura, 50, an associate professor at Kagoshima University and an expert on volcanic geology, comments, "A hotchpotch of estimates cannot be called scientific. In terms of volcanology, it is a very rough discussion, on a level that couldn't pass the peer reviews of academic journals or specialist magazines abroad."

Of course, if there were ever to be a disaster on the scale that destroyed all of southern Kyushu, then it would not be a case of simply having to worry about a nuclear power plant. In the name of science, both the government and Kyushu Electric Power Co. share that clear stance.

But hold on -- we have witnessed one catastrophic eruption not just within the past few tens of thousands of years, but in the 19th century, in Indonesia.

From my various questions to people on the issue, it does not seem that a scientific judgment will determine whether the nuclear power plant goes back into operation or not. In the end, it's a choice of values. From Oct. 9 the NRA will start holding meetings to explain its screening results to residents living within 30 kilometers of the Sendai Nuclear Power Plant. (By Takao Yamada, Expert Senior Writer)

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