[NukeNet] Radiation Redux: Forest fires remobilize fallout from bomb tests

Mon Jul 17 09:37:09 CDT 2006

Excerpt:

"The concentrations of cesium measured by the Yellowknife sensor

during a given month strongly correlate with the sizes of boreal

forest fires then burning upwind, the team reports in the June 28

Geophysical Research Letters."

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Science News

Week of July 15, 2006; Vol. 170, No. 3

Radiation Redux: Forest fires remobilize fallout from bomb tests

Sid Perkins

A sensitive instrument installed in the Canadian Arctic to monitor

fallout from modern nuclear tests has detected small amounts of

radioactive cesium produced by bomb tests decades ago. The material,

which during the Cold War was spread across northern latitudes by

high-altitude winds, is still being redistributed far and wide by

forest fires, researchers say.

Scientists use a worldwide network of sensors to ensure compliance

with the 1996 Comprehensive Nuclear-Test-Ban Treaty. While some

devices are on the lookout for the telltale seismic vibrations

generated by nuclear tests, others sniff the air for radioactive

fallout (SN: 7/14/01, p. 25: Available to subscribers at

http://www.sciencenews.org/articles/20010714/bob11.asp

Beginning in May 2003, a sniffer in Yellowknife, Northwest

Territories - a device that had been switched on for the first time

in January of that year - collected radioactive particles that

included cesium-137, says Gerhard Wotawa, a meteorologist with the

Comprehensive Nuclear-Test-Ban Treaty Organization in Vienna. That

particular isotope of cesium, which has a half-life of about 30

years, is generated when atoms of uranium-235 and plutonium-239

undergo fission within bombs or nuclear reactors.

The Yellowknife sensor regularly detected cesium-137 until

mid-September 2003. In 2004, the radioactive particles showed up

sporadically between late June and mid-September. Detectors at two

other high-latitude sites-one in Iceland, the other on the remote

Norwegian island of Spitsbergen-have detected cesium far less often.

Using computer models and weather reports, Wotawa and his colleagues

pinned down the source of the cesium: the fires that typically rage

unchecked through the boreal forests of Siberia, Alaska, and northern

Canada. The concentrations of cesium measured by the Yellowknife

sensor during a given month strongly correlate with the sizes of

boreal forest fires then burning upwind, the team reports in the June

28 Geophysical Research Letters.

Air samples taken in previous studies near forest fires have

contained cesium-137, says Wotawa, but this is the first time that

scientists have detected long-range redistribution of the radioactive

isotope.

The researchers aren't sure how the radioactive element makes its way

from fallout-tainted soil into the atmosphere. Cesium, a chemical

relative of potassium, is readily taken up by plants, so ash derived

from wood and leaves could contain traces of the element. Another

possibility is that because cesium has a boiling point of 670ï¿½C, some

of the radioactive atoms may be vaporized from the ground by fires

and then condense on airborne ash and soot, says Wotawa.

The cesium-137 lofted during a forest fire is diffusely distributed.

"This isn't a health risk, but it's interesting," Wotawa notes.

Scientists will have to account for the presence of wildfires when

they're interpreting the readings from radiation sniffers, he says.

"[This finding] isn't too surprising, but I hadn't thought of it

before," says Mark Fuhrmann, a geochemist at Brookhaven National

Laboratory in Upton, N.Y. Scientists might use the cesium-137,

strontium-90, and other radioactive isotopes in fallout to track

nutrient cycles in forests, he notes.

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References:

Wotawa, G., et al. 2006. Inter- and intra-continental transport of

radioactive cesium released by boreal forest fires. Geophysical

Research Letters 33(June 28):L12806. Abstract available at

http://dx.doi.org/10.1029/2006GL026206