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 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.