In the world of nuclear reactor science and safety, the ductile-brittle transition temperature, which is used to measure the strength of the inner wall of a reactor pressure vessel, is a critical factor.
The steel walls of a reactor vessel wear out through years of direct exposure to neutron irradiation, and when they are weakened they can become brittle with sudden temperature drops.
A high DBTT means the walls can shatter at a relatively high temperature when the vessel is going through the cooling process, similar to pouring ice-cold water into a hot glass, causing it to shatter.
Even though the government is mulling new stress-test standards and when to give the green light to reboot nuclear power plants across Japan, some experts warn that aged reactors and their high DBTTs should be considered key factors when assessing safety levels.
And when it comes to the brittleness factor, the one causing the most concern is reactor 1 at Kyushu Electric Power Co.’s Genkai nuclear plant in Saga Prefecture.
It has a DBTT of 98 degrees, the experts say.
The reactor “exceeded the ductile-brittle transition temperature of 81 degrees Celsius of the No. 1 nuclear reactor in Mihama (Fukui Prefecture),” Hiromitsu Ino, a University of Tokyo professor in metal physics, wrote in a thesis submitted in February to Citizen’s Nuclear Information Center, a nonprofit organization.
In terms of age, the Mihama nuclear plant in Fukui Prefecture run by Kansai Electric Power Co. is older than unit 1 at Genkai.
The DBTT figure for reactor 1 at Genkai indicates that any attempt to cool it in an emergency would have to be conducted near at temperatures near boiling point.
“It can be considered the most dangerous reactor in Japan,” Ito continued.
Genkai, with four reactors, is near the northwestern tip of Kyushu and is “surrounded by beautiful sea and grandeur of nature,” as its website says. Reactor 1 is the oldest of the four, having begun producing electricity in 1975.
Reactors 1 and 4 are in operation, while 2 was shut down for routine checks in December, as was reactor 3 in January.
The plant drew recent media attention when it looked likely that reactors 2 and 3 would get the OK to restart, the first to do so since the March 11 quake and tsunami caused three reactor meltdowns at Tokyo Electric Power Co.’s Fukushima No. 1 nuclear plant.
But Prime Minister Naoto Kan abruptly stepped in and declared that all reactors nationwide would need to undergo stress tests to ensure their safety before they can be restarted.
The Genkai reboot bid fell through.
According to Kyushu Electric Power, reactor 1’s DBTT has been soaring in recent years — from 35 degrees in 1976 to 56 degrees in 1993 and then to 98 degrees in 2009.
“There are so many things to do even before conducting a stress test at Genkai,” Kobe University professor Katsuhiko Ishibashi said last week regarding the threat.
Ishibashi, who decades ago coined the term “genpatsu-shinsai” (nuclear-quake disaster) has been active in warning of the dangers of operating nuclear plants in earthquake-prone Japan for decades.
In a gathering in Tokyo, he said the level of metal fatigue is alarmingly high.
“No one, not even (industry minister Banri) Kaieda, can truly say the Genkai nuclear power plant is safe and ready for operation,” he warned.
Regarding the extremely high DBTT of reactor 1, Kyushu Electric has acknowledged it is “above the expected level” but claims “safety is secured,” based on test samples deliberately taken from a spot with high exposure to radiation and that the actual reactor vessel has an 80-degree DBTT.
Kyushu Electric also added that even if the unit continued to operate for approximately two more decades, the DBTT would only go as high as 91 degrees, 2 degrees lower than the standard required for any newly built reactors.
“At this point cooling off the vessel does not pose any threat,” the utility said, adding DBTT shouldn’t be taken as a definite temperature that will cause metal to fracture.
Kyushu Electric also stresses that a 4.9-meter-high tsunami induced by an 8.1-magnitude quake — considered the largest possible temblor — in the surrounding sea won’t damage the nuclear plant because it is located well above sea level.
Such a temblor would hit the power plant with a level of acceleration of 120 gal, but that is still far below the 540 gal that reactor 1 is designed to withstand, the utility said.
Kobe University’s Ishibashi said optimistic speculation can end up inducing grave consequences, as was the case in Fukushima.
For one, the seismology expert pointed out that Kyushu Electric’s earthquake and tsunami assumptions are “too low, to the point that it should be considered criminal.”
“The utility needs to calculate the possibility of a large-scale tsunami and losing all electricity power” and then figure out if having a weakened nuclear reactor that can’t sustain sudden cooling is still safe under such circumstances, he insisted.
The tsunami that knocked out the Fukushima nuclear plant is believed to have been at least 14 meters, and well above Tepco’s estimates. Historic records show that the region not far from Genkai was hit with major earthquakes above magnitude 7.0 in 679, 1700 and 2005, Ishibashi added.
“I wouldn’t be surprised if a major earthquake hit the area around the nuclear power plant,” Kobe University’s Ishibashi said, reiterating that combined with the high DBTT, the consequences can exceed the damage caused in Fukushima.