Tokyo Electric Power Co. started to inject seawater into the No. 1 reactor at the Fukushima No. 1 nuclear plant a day after the March 11 earthquake and tsunami but suspended the operation for 55 minutes at the direction of Prime Minister Naoto Kan, according to government sources.
With criticism growing that a delay in the seawater injection may have caused a meltdown in the reactor and aggravated the disaster, analysts have said Tepco should have continued pumping the water into the reactor vessel to cool it down.
The government had previously said Tepco started to inject seawater into the No. 1 reactor at 8:20 p.m. March 12, but a document released by the utility Monday revealed it began the work at 7:04 p.m. that day, stopped it at 7:25 p.m. and resumed it at 8:20 p.m.
According to the sources, the Nuclear Safety Commission advised Kan that seawater injection into the reactor vessel could rekindle a chain reaction in a state called “recriticality,” prompting him to require the utility to suspend the operation.
But the commission later confirmed that the injection wouldn’t cause any problem, and Tepco, which initially began pumping seawater at the discretion of on-site workers, restarted the work by also putting in boric acid, which works to suppress criticality, the sources said.
On Friday, workers at the plant entered the No. 1 reactor building to measure the level of water that has leaked into its basement, the utility said.
The water, which may be leaking from the reactor and thus could be highly radioactive, was visually confirmed to be around 4.2 meters deep when it was found last Saturday, the utility said.
Four other workers entered the reactor building the same day to check radiation levels using a special device called a “gamma camera,” which can image points of high-level radiation, Tepco said.
It is believed that a fuel meltdown occurred March 12 in the No. 1 reactor, creating holes in its pressure vessel.
Meanwhile, a huge steel floating structure arrived Saturday at a nearby pier. It will be used to store contaminated water accumulated in the plant, according to Tepco officials.
The 136-meter-long, 46-meter-wide “megafloat” can store some 10,000 tons of water, which will go through a decontamination process before it is pumped aboard, the officials said.