Report of long-range plutonium find tardy

By MIZUHO AOKI
10/4/11
The Japan Times

The science ministry was tardy when it reported last week for the first time that traces of plutonium fallout were found outside the Fukushima No. 1 power plant's compound through tests conducted in June, a nuclear expert said Monday.

The plutonium traces, which are too low to present a hazard to human health, were found at six spots far away from the plant's premises.

A soil contamination map released Friday by the science ministry shows that plutonium drifted as far as 45 km northwest from the crippled plant to the village of Iitate, Fukushima Prefecture.

In late March, plutonium and strontium isotopes were reportedly found in soil at Fukushima No. 1 based on tests by Tokyo Electric Power Co.

It is not clear what caused the plutonium, a heavy element, to drift so far.

"The results came too late. The government should have conducted the tests much earlier," said Michiaki Furukawa of the nonprofit Citizens' Nuclear Information Center, a noted antinuclear group based in Tokyo.

The largest amount of plutonium-238 detected was 4 becquerels per square meter in the town of Namie, 24 km northwest of the plant. As for plutonium-239 and -240, the largest combined amount found was 15 becquerels per square meter, the ministry's report said.

If someone lives for 50 years in an area contaminated with 4 becquerels of plutonium-238, his total dose would be 0.027 millisieverts, the report said. The area with 15 becquerels would provide a cumulative dose of 0.12 millisievert, it said.

A cumulative dose of 100 millisieverts increases one's cancer risk by 0.5 percent, scientists say.

"Plutonium won't do harm unless it gets into people's bodies. And from the amount detected, (that) possibility is very low. People shouldn't be concerned about it," said Furukawa.

Plutonium-239 has a half-life of 24,000 years and plutonium-238 about 88 years. Plutonium-240 has a half-life of about 6,600 years.

Unlike cesium, which emits gamma rays, plutonium emits alpha rays, which can be stopped by skin or clothing but pose a greater risk to cellular material within the body.

Furukawa said when plutonium is inhaled it may stay in the lungs for a very long time. But because the amount found was very low, the chance of inhaling it is very small, he said.

As for strontium, the amount detected was, on average, less than 1 percent of the radioactive cesium, the ministry report said.

The largest strontium amount was 5,700 becquerels per sq. meter in the town of Futaba, about 3 km north of the power plant, Friday's report said. Someone living in the area for 50 years would get a cumulative dose of 0.12 millisieverts, it said.

Strontium-90, which has a half-life of 29 years, is known to accumulate in the bones when ingested or inhaled. But the small amount of strontium-90 that was detected is also not worth worrying about, Furukawa said, emphasizing that the biggest risk is being posed by cesium isotopes.

The ministry said it has no plans to expand its checks beyond the survey's current 100-km radius, which means that Tokyo — whose massive population could magnify the consequences of any results — will be excluded.

Although it would be better if the government conducted more soil tests outside that range, it would be very difficult because conducting an analysis of strontium and plutonium takes much longer than it does for cesium, Furukawa said.

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