TOKYO, Feb. 7 (Kyodo) — Workers at the crippled Fukushima Daiichi power plant on Tuesday raised the amount of water injected into the No. 2 reactor to the highest level since the plant achieved a stable state of cold shutdown in December, as concerns grew over the rising temperature recently detected at the bottom of the reactor's pressure vessel.
Following the move, the temperature measured at the same spot on the vessel dropped to 69.0 C at 10 a.m. from 72.2 C logged at 5 a.m., Junichi Matsumoto, spokesman for plant operator Tokyo Electric Power Co. told a press conference, but added that the company needs more time to assess the effect of the latest step.
"It is difficult to judge whether the temperature is rising or dropping unless we monitor the development for about a day," Matsumoto said.
TEPCO said it increased the amount of injected water at 4:24 a.m. Tuesday. The No. 2 reactor is now being cooled with the injection of 13.5 tons of water per hour, up from 10.5 tons.
Nuclear disaster minister Goshi Hosono told a press conference that TEPCO is making utmost efforts to lower the temperature.
Touching on last month's change in the amount of coolant water at the No. 2 reactor for pipe replacement, which is believed to have affected the temperature, Hosono said, "This was a process to enhance stability, but it has become clear that there is a possibility of (replacement work) creating an unstable situation temporarily."
"We have to consider in an even more careful way," he said.
TEPCO's Matsumoto said he believes the No. 2 reactor is maintaining a state of cold shutdown, because the temperature is not rising continuously. Readings on two other thermometers checking the temperature of the bottom of the pressure vessel were around 40 C as of 10 a.m.
A cold shutdown is defined by the Japanese government as a situation in which the bottom part of a reactor pressure vessel is kept below around 100 C and radiation exposure from the release of radioactive substances is significantly held down.
At the Fukushima Daiichi plant in northeastern Japan, the Nos. 1 to 3 reactors have suffered meltdowns as a result of the loss of their key cooling functions in the wake of the devastating earthquake and tsunami on March 11 last year.
TEPCO is now injecting water into the three crippled reactors through a new water circulation system installed after the accident.