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An explosion Saturday at the Fukushima No. 1 power plant may have exposed 1-year-olds to more than 100 millisieverts of radiation — even outside the 30-km danger zone — a government computer simulation shows.

“There are some cases in which they could have received more than 100 millisieverts of radiation, even if they’re outside the 30-km radius and in the event that they spent every day outdoors since the explosion at the Fukushima nuclear plant,” Chief Cabinet Secretary Yukio Edano told a news conference.

Nuclear Safety Commission Chairman Haruki Madarame told reporters, “The figure represents the level that 1-year-olds would have received and accumulated in their thyroids by midnight Wednesday since the explosion.”

Madarame said the radiation dose accumulated by adults outside the 30-km zone would be lower.

People exposed to doses greater than 100 millisieverts are required to take potassium iodide, Madarame said. An annual dose of 100 millisieverts is believed to be associated with an increased risk of cancer.

“(But) our analysis shows that people outside the 30-km radius would not be in a situation where they would have to evacuate immediately or remain indoors,” Edano added.

“As a precautionary measure, I would like to recommend that if people (outside the zone) are on the leeward side of the nuclear power plant, they close their windows and stay indoors inside sealed buildings as far as possible,” he said.

The simulation was conducted on the Nuclear Safety Technology Center’s networked computer system, called the System for Prediction of Environmental Emergency Dose Information, known as SPEEDI.

The system enables the center to predict the spread of radioactive substances and their amounts over 48 hours by analyzing landscape and wind direction.

The radiation dose for 1-year-olds outside the radius was calculated by using readings of radiation levels and amounts of radioactive substances detected in areas surrounding the nuclear plant.

Meanwhile, the science ministry said Wednesday that radiation levels detected in Tokyo and Chiba Prefecture since Tuesday had more than tripled from three days earlier.

The radiation detected in the capital between 5 p.m. Tuesday and 5 p.m. Wednesday rose to 0.155 microsieverts per hour from 0.142 microsieverts in the previous 24 hours, while that in Chiba rose to 0.125 microsieverts from 0.106 microsieverts.

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