Radionuclide storage in forests, Chernobyl . Scientific American

http://www.scientificamerican.com/article.cfm?id=at-chernobyl-radioactive-danger-lurks-in-the-trees

At Chernobyl, Radioactive Danger Lurks in the Trees

For 26 years forests around Chernobyl have been absorbing radioactive elements, but a fire would send them skyward again–a concern as summers grow longer, hotter and drier

By Jane Braxton Little and The Daily Climate

CHERNOBYL, Ukraine – Most days Nikolay Ossienko patrols the forests surrounding the Chernobyl nuclear power plant, clearing brush and dead trees from the grid of fuel breaks that crisscross the 1,000-square-mile area. But on hot July afternoons, when black thunderheads loom on the horizon, he climbs a rusty ladder 75 feet up a rickety fire tower. When he spots smoke, he radios the six other towers to pinpoint the location, then trucks off to the blaze.

“Our number one job is to save the forest from fire,” said Ossienko, a burly, blue-eyed Ukrainian whose warm smile winks with a missing tooth.

It’s a job with international consequences. For almost three decades the forests around the shuttered nuclear power plant have been absorbing contamination left from the 1986 reactor explosion. Now climate change and lack of management present a troubling predicament: If these forests burn, strontium 90, cesium 137, plutonium 238 and other radioactive elements would be released, according to an analysis of the human health impacts of wildfire in Chernobyl’s exclusion zone conducted by scientists in Germany, Scotland, Ukraine and the United States.

This contamination would be carried aloft in the smoke as inhalable aerosols, that 2011 study concluded.

And instead of being emitted by a single reactor, the radioactive contamination would come from trees that cover some 660 square miles around the plant, said Sergiy Zibtsev, a Ukrainian forestry professor who has been studying these irradiated forests for 20 years.

“There’s really no question,” he added. “If Chernobyl forests burn, contaminants would migrate outside the immediate area. We know that.”

Overcrowded pines
Combined with changes in climate, these overcrowded pines are a prescription for wildfire. In their assessment of the potential risks of a worst-case fire, Zibtsev and the team of international scientists concluded that much of the Chernobyl forest is “in high danger of burning.”

Zibtsev has been worrying about catastrophic wildfire in Chernobyl since witnessing runaway wildland fires in the western United States while on a Fulbright Scholarship in 2005. He has watched the threat get worse each passing year. Rainfall in the region is decreasing and seasonal droughts are lasting longer, changes Zibtsev attributes to climate change. Scientists say these patterns of drier and longer summers are contributing to forest drying and increased insect attacks.

The predominantly pine forests themselves are part of the problem. After the explosion – the worst nuclear accident in human history – the area surrounding the power plant was evacuated, the fields and forests abandoned. To keep the contamination from moving beyond the area known as the “zone of alienation,” the Ukraine government forbade all commercial activity. For forests, this meant a halt to logging, thinning and removing dead trees. While most of Ukraine boasts woodlands that are carefully manicured, the Chernobyl forests have grown into unmanaged thickets with dense brush below and lifeless canopies above.

The risk of fire in these forests has concerned scientists since 1992, a drought year when more than 65 square miles of forests burned. They know that these ecosystems are trapping radionuclides and slowly redistributing them in soil and vegetation, a process called “self-repair.” In some places the contamination level is the same as it was in 1986, most of it in the top 10 centimeters of the soil. Absorbing cesium, plutonium and strontium helps contain radionuclides within the exclusion zone, but it dramatically heightens the alarm over wildfire.

Two-acre test fire
A 2002 test fire offers insight on the scope of the radioactive risk. Set to assess plume and radionuclide behavior, the two-acre ground fire near the failed power plant released up to five percent of the cesium and strontium in the biomass. A high-intensity crown fire would release much higher amounts than burning needles and leaf litter, said Vasyl Yoschenko, who set the fire and heads the radioecological monitoring laboratory at the Ukrainian Institute of Agricultural Radiology. Other studies predict that the fine particles emitted from a forest fire could be transported hundreds of miles away.


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