Amid the chaos on March 11 as the crisis at the Fukushima No. 1 nuclear plant started to spiral out of control, workers manually shut down an "isolation condenser," or a cooling device in the plant's No. 1 reactor, Tokyo Electric Power Co. said on Nov. 22.
Amid the chaos on March 11 as the crisis at the Fukushima No. 1 nuclear plant started to spiral out of control, workers manually shut down an "isolation condenser," or a cooling device in the plant's No. 1 reactor, Tokyo Electric Power Co. said on Nov. 22.
The workers were concerned that continued operation of the isolation condenser could damage the equipment and lead to a discharge of radioactive substances, according to TEPCO, the plant operator.
The circumstances behind shutting down the isolation condensers--the only means to cool the core of the No. 1 reactor where a meltdown first occurred--are a focus of an investigation by the government's Investigation Committee on the Accident at the Fukushima Nuclear Power Stations.
Whether the decision to shut down the condenser was appropriate may emerge as a subject for review.
Only the No. 1 reactor is equipped with isolation condensers, which can cool vapor from the nuclear reactor, turn it into water and recycle that water to cool the reactor core. The equipment is made of two channels. At each condenser, vapor is sent through a pipe that runs inside a tank filled with water. The devices can function even if the power supply is lost.
TEPCO said the loss of power supply following the tsunami spawned by the Great East Japan Earthquake made it impossible to tell whether the isolation condensers were operating or not.
The condensers were later confirmed as having stopped when the power supply was restored temporarily.
At 6:18 p.m., workers opened motorized valves to set one of the condensers in motion, but the generation of vapor, due to the heating of water in the tank, was not confirmed, so the workers closed a valve at 6:25 p.m. to shut down the operation. The workers thought it possible that the tank contained no water, which would have meant that continued operation could damage the piping and lead to a release of vapor containing radioactive substances, TEPCO officials said.
An on-site survey in October found out that the water level remained at about 65 percent of the capacity of the tank.
The workers reopened the valve at 9:30 p.m. TEPCO said that the equipment retained only limited functionality and that the core meltdown would have been inevitable at any rate, even if continued operation could have delayed it to a certain extent.
TEPCO's analysis suggested that a core meltdown would have occurred four hours after the earthquake hit at 2:46 p.m. if the isolation condensers had not operated, and would have taken place in seven hours, or some time before 10 p.m., even if the condensers had begun operating at around 6 p.m.
The decision to shut down the operation was made by workers in the central control room, including the chief worker on duty, but it is not known if the director and other senior officials of the nuclear plant were informed of the decision, TEPCO officials said.