Tokyo Electric Power Co. has belatedly admitted that most nuclear fuel likely melted and dropped to the bottom of the pressure vessels of two other reactors at the Fukushima No. 1 nuclear power plant.
Tokyo Electric Power Co. has belatedly admitted that most nuclear fuel likely melted and dropped to the bottom of the pressure vessels of two other reactors at the Fukushima No. 1 nuclear power plant.
The utility also acknowledged that some fuel possibly escaped to the outer containment vessels.
After analyzing data it released May 16, TEPCO said May 24 it believed that if, as suspected, the fuel rods were exposed after cooling systems failed and water was lost, meltdowns were likely at the No. 2 and 3 reactors.
The estimates have been reported to the Nuclear and Industrial Safety Agency.
TEPCO said the finding will not affect its plan to bring the reactors under control. Officials said they believe that the pressure vessels were not seriously damaged and will not release large amounts of radioactive materials.
Experts pointed out the possibility of a meltdown immediately after the data retrieved from the central control room was released. But TEPCO was reluctant to agree, saying it needed more time for analysis.
According to TEPCO's latest estimates, if water in the reactor was lost, most fuel melted and dropped to the bottom of the pressure vessel at the No. 3 reactor around 3 a.m. March 14, about 60 hours after the March 11 quake and tsunami crippled the Fukushima plant.
Part of the melted fuel escaped through cracks in the pressure vessel and ended up on the floor of the containment vessel, TEPCO said.
If water was lost, most of the fuel melted and dropped to the bottom of the pressure vessel at the No. 2 reactor around 8 p.m. March 15, about 101 hours after the Great East Japan Earthquake, the utility said.
TEPCO presented a second analysis based on a scenario in which some water was retained and part of the nuclear fuel remained submerged.
In this case, the fuel was damaged but did not fall to the bottom of the pressure vessels. The estimates are based on the assumption that water gauges are functioning properly.
TEPCO has already acknowledged that a meltdown occurred at the No. 1 reactor after a broken water gauge was adjusted.
The company said May 15 that melted fuel fell to the bottom of the pressure vessel around 6:50 a.m. March 12.
Cooling systems for the No. 1 reactor stopped three hours after the earthquake. TEPCO estimates that those for the No. 2 and 3 reactors were working for 40 to 75 hours after the quake.
After cooling systems were suspended, water levels fell, temperatures in the pressure vessels shot up, and nuclear fuel was exposed and damaged.
At the No. 3 reactor, water levels began to fall around 7 a.m. March 13, and damage to fuel started around 9 a.m. the same day, according to TEPCO's latest estimates.
At the No. 2 reactor, water levels began to fall around 6 p.m. March 14, and damage to fuel started around 8 p.m. the same day.
TEPCO estimates that the pressure vessel was damaged around 9 a.m. March 14 at the No. 3 reactor and around 4 a.m. March 16 at the No. 2 reactor if water in the reactors was lost.
Still, officials said they believe damage to the pressure vessels is limited, citing temperature and other data.
TEPCO also said the reactors No. 1 to 3 automatically stopped without incident after the earthquake.
Officials said main piping systems and other important safety-related equipment maintained their integrity.
(This article was written by Eisuke Sasaki and Ryoma Komiyama.)