In its latest findings, TEPCO also said that most of the nuclear fuel in the No. 3 reactor at the plant in Fukushima Prefecture melted through the pressure vessel and continued down to the bottom of the outer containment vessel. The finding may make it even more difficult to decommission the plant.
According to previous findings, including the final report made in July 2012 by the government’s Nuclear Incident Investigation and Verification Committee, TEPCO manually stopped an emergency high-pressure coolant injection apparatus at the No. 3 reactor and tried to switch to another water injection system as operation of the apparatus became unstable before 3 a.m. on March 13, 2011. The attempted switch failed, as TEPCO was unable to secure power source.
Initially, TEPCO had assumed that from that time on, a “hiatus of coolant injection” had started, leading to a fall in the water level inside the pressure vessel and triggering a core meltdown inside the vessel, sometime after 9 a.m. on the same day.
However, a record made by a worker at the plant that details water levels inside the reactor was found in the autumn of 2012. The record suggests it is possible that the coolant apparatus had already ceased functioning nearly seven hours before TEPCO stopped the coolant-injection device.
A new analysis of conditions inside the reactor, made on the basis of the uncovered record, led to the latest finding that the temperature in the reactor core reached the fuel’s melting point of 2,200 C at around 5:30 a.m. on that day.
TEPCO has come to assume that the core meltdown was highly likely to have started in the early morning of March 13.
As the core meltdown is now believed to have started earlier than was previously thought, the amount of melted nuclear fuel that passed into the containment vessel through the pressure vessel is considered to have been greater, making it technically more difficult to extract the melted fuel and dispose of it.