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.