Workers at the Fukushima No. 1 nuclear plant increased the amount of water injected into reactor 2 on Tuesday to the highest level since the plant achieved cold shutdown in December as concerns grew over rising temperatures at the bottom of the pressure vessel.
Following the move, the temperature in the vessel eased to 68.5 degrees by 5 p.m. from 73.3 degrees logged at 7 a.m. Monday, Junichi Matsumoto, a spokesman for Tokyo Electric Power Co., told a news conference.
“The temperature has apparently hit its peak and believed to be on the decline. We will continue to monitor the situation,” he said.
Tepco said it increased the amount of injected water, some of which contained boric acid, at 4:24 a.m. Tuesday. Reactor 2 is now being cooled with 13.5 tons of water per hour, up from 10.5 tons. The boric acid is being used to prevent a sustained nuclear chain reaction, or recriticality.
Nuclear disaster minister Goshi Hosono told reporters that Tepco is making every effort to lower the temperature.
Touching on last month’s change in the amount of coolant water for reactor 2 after pipes were replaced, a move that apparently affected the temperature, Hosono said, “This was a process to enhance stability, but it has become clear that there is a possibility (the replacement work created) an unstable situation temporarily. We have to consider matters in an even more careful way.”
Tepco changed pipes and the amount of coolant water in reactor 2 last month, and suspects the water did not sufficiently cover some parts of the pressure vessel, as only a small amount was injected through a pipe designed to take in large amounts.
Tepco’s Matsumoto said he believes reactor 2 remains in cold shutdown because the temperature is not rising continuously. Readings on two other thermometers at the bottom of the pressure vessel were around 40 degrees as of 10 a.m.
Tepco said it also believes recriticality probably did not occur because it did not detect radioactive xenon, which is generated as a result of nuclear fission.
The utility said it injected water containing boric acid, which works to suppress criticality, and will increase the amount of coolant water further.