7 Août 2014
August 7, 2014
TEPCO: Nearly all nuclear fuel melted at Fukushima No. 3 reactor
THE ASAHI SHIMBUN
Almost all of the nuclear fuel in the No. 3 reactor of the Fukushima No. 1 nuclear power plant melted within days of the March 11, 2011, disaster, according to a new estimate by Tokyo Electric Power Co.
TEPCO originally estimated that about 60 percent of the nuclear fuel melted at the reactor. But the latest estimate released on Aug. 6 revealed that the fuel started to melt about six hours earlier than previously thought.
TEPCO said most of the melted fuel likely dropped to the bottom of the containment unit from the pressure vessel after the disaster set off by the Great East Japan Earthquake and tsunami.
The utility plans to start fuel removal operations at the No. 3 reactor no earlier than in the latter half of fiscal 2021.
“(The new estimate) does not mean we are now facing a higher risk (in the planned removal work),” a TEPCO official said. “It is still impossible for us to evaluate the potential impact (of the findings) on the decommissioning of the reactor.”
TEPCO released its first report on the nuclear crisis in June 2012. But because many details of the disaster remain unknown, TEPCO is still looking into the causes of the disaster.
The previous report was compiled on the assumption that an emergency cooling mechanism for the No. 3 reactor, known as a high pressure core flooder system, continued to properly inject water into the reactor until it was manually shut off in the early morning of March 13, 2011.
Because the system does not work properly unless certain levels of pressure inside the reactor are secured, TEPCO made a new estimate based on the premise that the cooling mechanism stopped functioning at 8 p.m. on March 12, when the internal pressure dropped sharply.
According to the latest estimate, fuel at the No. 3 reactor began melting at 5:30 a.m. on March 13, and almost all the melted fuel had dropped to a broad area on the bottom of the containment vessel soon after 7 a.m. on March 14.
TEPCO officials said they believe part of the melted fuel still remains inside the pressure vessel, citing the fact that the temperature inside the pressure vessel fell after a later water injection.
The estimated start of the fuel melting is roughly consistent with when neutrons were detected near the front gate of the nuclear plant, according to the officials.
Neutrons were also detected when nuclear fuel started to melt at the No. 2 reactor at midnight on March 14. For this reason, TEPCO estimates that radioactive substances released from the No. 3 reactor emitted neutrons near the front gate.
All of nuclear fuel at the No. 1 reactor is estimated to have melted after the disaster, while around 60 percent is believed to have melted at the No. 2 reactor. TEPCO said it will now consider how to remove the melted nuclear fuel from the No. 3 reactor.
August 6, 2014
Tokyo Electric Power Co. said Wednesday that its new estimate shows that all the fuel rods in reactor 3 at the Fukushima No. 1 nuclear power plant apparently melted down and fell onto the bottom of the containment vessel.
In November 2011, the company had said it believed only about 63 percent of reactor 3′s fuel core had melted.
The utility updated its estimate as part of an effort to probe unclear points about the triple meltdown at the Fukushima No. 1 plant caused by a megaquake and monstrous tsunami in March 2011.
The revised estimate is based on the finding that an emergency cooling system, known as HPCI, of reactor 3 stopped working six hour earlier than previously thought, and that the meltdown had also started more than five hours earlier.
Tepco had previously said that the HPCI had shut down at 2:42 a.m. on March 13, 2011. But further investigation over the past year determined that the HPCI appeared to have lost its cooling function about at 8:00 p.m. on March 12.
According to the new estimate, all the melted fuel penetrated the pressure vessel, fell onto the bottom of the containment vessel and melted about 68 cm into the concrete.
The pressure vessel is located inside the massive containment vessel.
The analysis shows that the fuel did not penetrate the containment vessel, according to Tepco.
While the new analysis announced on Wednesday, based on temperature, pressure and other data, shows that all the fuel had melted down to the containment vessel, Tepco has a more optimistic view.
“We think some fuel still remains at the core part based on the actual plant data,” said Shinichi Kawamura, a Tepco spokesman, during a news conference.
According to Kawamura, this is because the temperature of the pressure vessel decreased when the water was injected, meaning some warm fuel was still there.
August 7, 2014
Fukushima No. 3 reactor meltdown worse than previously believed: TEPCO
TOKYO (Kyodo) -- Fuel inside the No.3 reactor at the Fukushima Daiichi nuclear plant began melting earlier than thought, and more fuel than previously estimated fell into the outer primary container after the cooling function failed following the 2011 earthquake and tsunami, a new analysis showed Wednesday.
Tokyo Electric Power Co. initially estimated that around 60 percent of the fuel melted through the base of the pressure vessel and dropped into the outer primary containment vessel.
But a company official told a press conference now most of the fuel "is believed to have dropped," and the utility is still studying ways to remove it.
TEPCO's latest analysis is expected to add to concern that safely removing the melted fuel at the reactor could be even more difficult.
A massive earthquake and tsunami ravaged the northeastern Japan on March 11, 2011, sparking the nuclear crisis in which three reactors including the No. 3 unit at the Fukushima complex suffered meltdowns.
The new analysis showed the fuel meltdown at the No. 3 reactor is likely to have begun at around 5:30 a.m. on March 13, about five hours earlier than originally estimated.
The reactor's cooling system is believed to have failed at around 8 p.m. on March 12, about seven hours earlier than initially thought.
Even after more than three years, details of what happened at the time of the crisis and the situations surrounding the reactors remain unknown, and TEPCO is continuing its investigation.