Controlling doses at Fukushima plant
Radiation levels across the Fukushima Daiichi power plant site have dropped significantly as clean-up efforts continue. Most workers receive less than 1 milliSievert of radiation per month – well within normal limits.
Tokyo Electric Power Company (Tepco) has employed some 25,837 workers at the power plant since March 2011, keeping records of their radiation doses during the accident as well as the clean-up operation. One important task has been to protect workers and make the site workable by decontaminating the site and clearing away the radioactive rubble that was spread by the hydrogen explosions. Dust inhibiting sprays were used and heavy rubble was handled by remote control earth moving equipment. An airtight cover has been installed over unit 1 to prevent airborne release and the majority of rubble has been cleared from all the reactor building roofs.
A radiation survey map of the site made last week revealed substantial progress: the highest dose rate anywhere on the site was 0.15 milliSieverts per hour (mSv/h) near units 3 and 4. Two years ago a similar survey put the highest dose rate at 300 mSv/h near rubble lying alongside unit 3.
Elsewhere on site, the latest survey showed three areas with rates of 0.13 and one with 0.10 mSv/h. Below that, the higher levels found were spots at 0.06 mSv/h and 0.04 mSv/h. The majority of the power plant area is at less than 0.01 mSv/h, although of course many areas inside the power plant buildings remain too hazardous for workers to enter.
This reduction in on-site radiation is one factor in low doses for workers, also shown by recent data: During January, the 5702 workers at the site received an average of 0.86 mSv, with 75% of workers recorded as receiving less than 1 mSv. In total, only about 2% of workers received over 5 mSv and the highest dose in January was 12.65 mSv for one worker.
Occupational health and safety rules place a limit of 50 mSv/y per year on professionals working with radiation in Japan. Exposure limits of 100 mSv and 250 mSv were in place during the most difficult days of the accident but these have since been taken away in favour of more normal radiation protection standards.
Tepco managers assign tasks on the basis of each worker’s exposure to date, their specific expertise and the predicted exposure from a planned task. This allows managers to make progress on site while keeping doses as low as they can. In total from March 2011 to the end of January 2013 some 25,837 workers had spent time on site. Of these, over 95% received less than 50 mSv during the 25 month period; 4% recieved 50-100 mSv and fewer than 1% received over 100 mSv.
Source: World Nuclear News