The Nuclear and Industrial Safety Agency, overriding expressions of concern from experts, concluded July 27 there is no problem with the strength of the pressure vessel of the Genkai nuclear power plant's aging No. 1 reactor.
The Genkai plant in Saga Prefecture is operated by Kyushu Electric Power Co. and currently offline for regular inspections. Its No. 1 reactor began operating in 1975 and is the oldest in Kyushu.
During a meeting in Tokyo that heard experts' opinions, NISA dismissed all concerns raised over the safety of the No. 1 reactor pressure vessel and virtually slammed the door shut to further discussions on the matter.
NISA has no plans to hold more expert meetings on the issue before it compiles a formal report by the end of August. NISA will be replaced by a new nuclear regulatory commission and cease to exist in September.
The pressure vessel is the heart of a nuclear reactor. Because it is constantly bombarded by neutron irradiation from nuclear fuel, it eventually loses strength.
When an operator starts up a nuclear reactor, it leaves samples made of the same metal material inside the pressure vessel to be retrieved for tests at a later date.
One of the test items is known as ductile-brittle transition temperature, which serves as an indicator of the extent of strength loss.
For example, a metal sample taken out in 1993 showed 56 degrees. But a sample tested in 2009 registered 98 degrees, or 14 degrees above the projected value. The finding triggered immediate concern among experts.
NISA has convened an expert panel on 18 occasions since November to determine if rapid cooling of the nuclear reactor in the event of an accident would destroy the pressure vessel.
NISA ended up endorsing Kyushu Electric's argument that the pressure vessel would not likely crack even if the reactor remained in operation for 60 years.
NISA said an analysis of the metal sample retrieved in 2009 and an inspection of welds in the pressure vessel revealed no anomalies. It concluded the pressure vessel would be able to withstand the impact of rapid cooling.
But neither Kyushu Electric nor NISA was able to offer a convincing explanation for the abnormal high transition temperature.
"You can't call (the pressure vessel) safe when the cause (of the sharp rise in the transition temperature) remains unknown," said one expert during the meeting.
Hiromitsu Ino, professor emeritus of metal materials science at the University of Tokyo who serves on the expert panel, openly expressed objections to NISA's conclusion after the July 27 meeting.
"I just don't understand yet why the transition temperature is so high," Ino said. "I have doubts about the (pressure vessel's) safety. The No. 1 reactor should not be put back in operation."
Although Kyushu Electric took metal test samples out of the pressure vessel on four previous occasions, it has since discarded the samples extracted on the first and second tests.
The samples were inspected only by institutions with close ties to the power industry, including the Central Research Institute of Electric Power Industry.
A member of the expert panel proposed that test samples be provided to university-based researchers for more in-depth analysis.
But Kyushu Electric and NISA dismissed the suggestion, calling CRIEPI a "neutral, academic research institution."
Another member called for additional tests of the samples that still lie in the nuclear reactor, but the proposal was rejected outright.
"We will extract the next sample in 2025 or so," a Kyushu Electric representative said.
(Ikko Ishida contributed to this article.)
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