The Yomiuri Shimbun
One-year-old infants might be at risk of radiation exposure exceeding the level that requires precautionary medication, even if they live outside the 30-kilometer-radius evacuation zone from the troubled Fukushima No. 1 nuclear power station, according to a government panel.
The Nuclear Safety Commission on Wednesday night released its first estimates on infants' internal exposure to radioactive iodine in the atmosphere. The estimates were based on a retroactive simulation of the release of radioactive material from the plant since March 12, one day after the massive earthquake.
The NSC's calculations were based on the assumption that 1-year-old infants were outside the entire time from 6 a.m. on March 12 until Wednesday midnight, a period of nearly 12 days.
They estimate the amount of internal radiation 1-year-old infants would have received from radioactive iodine in the air under those conditions. Infants are more susceptible than older children and adults to the effects of radiation.
According to the estimates, infants could have been exposed to radiation of more than 100 millisieverts, even in some areas outside a 30-kilometer radius of the nuclear power plant. People within that radius were instructed by the government to evacuate or stay indoors, depending on their proximity to the plant.
Potassium iodine should be administered to infants who receive exposure of 100 millisieverts as a precautionary measure.
However, a panel official said: "Even if the exposure level tops 100 millisieverts, it won't pose health risks [to infants]. Furthermore, if infants stay inside, the exposure level drops to between one-fourth and one-tenth [of levels outdoors]."
The simulation on which the estimates were based was conducted using a computer system of the Nuclear Safety Technology Center called the System for Prediction of Environmental Emergency Dose Information (SPEEDI).
The system was originally designed to quickly formulate projections of which areas should be evacuated after an accident at a nuclear power plant. However, power blackouts and hardware malfunctions caused by the March 11 earthquake made it impossible for the commission to measure the amounts of radioactive substances discharged into the air from the plant, and therefore unable to provide useful projections about the need for evacuations.
The panel retrospectively calculated the amounts of radioactive iodine discharged, based on surveys of the atmosphere around the power station from Sunday to Tuesday.
Based on the results of the surveys, the panel calculated how far the radioactive material had spread.
The NSC announced the estimates at 9 p.m. Wednesday, after the U.S. Energy Department announced its own estimates at 9 a.m. that day. The data is available at www.energy.gov/japan2011.
The panel has been criticized by experts for not announcing other estimates it calculated earlier, without specifying the specific amount of radioactive material discharged from the plant.
SPEEDI was designed to make quick, accurate projections of the proliferation of radioactive substances from a nuclear power plant in the case of an accident.
In developing its projections, the system factors in data such as the climate and geographical surroundings.
The then Science and Technology Agency launched development of the system in the wake of the nuclear accident on Three Mile Island in the United States in 1979.
The SPEEDI system has been revised regularly, with 780 million yen allocated to the project in the 2010 budget.
(Mar. 25, 2011)
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