The Fukushima prefectural government has received many inquiries because air radiation levels across the prefecture following the crisis at the Fukushima No. 1 nuclear power plant declined considerably late January and have since remained constant, perhaps due to fallen snow blocking radiation above the ground.
According to monitoring by the Education, Culture, Sports, Science and Technology Ministry and others, the rate of decline was particularly large in the Akougi district in Namie and the Nagadoro district in Iitate, located in the expanded evacuation zone around the nuclear plant.
Radiation measuring found that the Akougi district had a reading of 19.7 microsieverts per hour in the morning of Jan. 25, down from 30 microsieverts per hour recorded in the morning of Jan. 18.
Air radiation levels also decreased to 5.9 microsieverts per hour from 10 microsieverts per hour over the same period in the Nagadoro district.
It is believed there were no major changes in air radiation levels before Jan. 18 and after Jan. 25.
According to the ministry's Nuclear Emergency Response Headquarters, the decline can be attributed only to snowfall since decontamination operations were not conducted in the areas at the time.
The Fukushima Meteorological Observatory said snow accumulation is not monitored in Namie and Iitate, but temperatures and other factors suggest the town and the village had snow from Jan. 20 to 22.
The prefectural emergency response headquarters said radiation levels also declined in the city of Fukushima. While such levels measured 0.84 microsievert per hour at 6 p.m. on Jan. 21 when snow began to fall, at 9 p.m. on Jan. 22, after snowfall, radiation levels in the air measured 0.62 microsievert per hour.
Farmer Masuo Kaneko, 63, who evacuated to the city from Nagadoro district, said after reading the newspapers he thought the radiation levels were dropping rapidly. But he was disappointed to hear the decline was due to snowfall.
"I expected radiation levels to halve in about two years time," he said.
Tokyo Institute of Technology Associate Professor Keiji Saneyoshi said air radiation levels may halve if about 20 centimeters of snow falls in certain areas. "Yet decontamination work needs to continue since the levels will rise again once the snow melts," Saneyoshi said.
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.
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