From RT, April 29, 2015

Smoke from burning forests in the Chernobyl exclusion zone is capable of spreading contaminants across great distances, even after the fire has been stopped, ecology experts told RT.
The forest fire near the crippled Chernobyl nuclear power plant started on Tuesday and triggered an emergency alert, with police and National Guard mobilized to bring the flames under control.
By Wednesday, the country’s Emergency Ministry, as well as the prime minister, who went to the affected area, said the spread of the fire had been stopped and firefighters were containing the remaining flames.
Although the sarcophagus remains untouched by the fire, decades-old contaminants could still be released and travel far and wide, borne aloft by the smoke, nuclear safety expert John H. Large told RT:
“Brush fires and forest fires were the greatest concern in terms of the means by which you can disperse a secondary radiological impact from the original dissipation that occurred in 1986,” he said.
John went to Chernobyl in 2006 to assess the situation there and spoke to dozens of scientists working on containing the contamination.
“In the exclusion zone and further away you have an area that has been abandoned for farming, abandoned for man management,” John says. “That means you’ve got lots of brush and young wood growing out of control, and that means there’s a big fuel load to have a fire.”
He says the high temperatures and volumes of smoke produced in a forest fire can take contaminants hundreds of kilometers away from the exclusion zone: “Radiation really doesn’t respect any international boundaries.”
https://soundcloud.com/rttv/chernobyl-fire
Forest fires have happened in the area before, but have never been so serious, Timothy Mousseau, biology professor at the University of South Carolina, told RT:
“Previous forest fires had re-released about eight percent of the radiation from the original catastrophe. The fire that we’re seeing today seems to be on a much larger scale, and so we could see a re-dispersion of a very significant component of the original radiation.”
Another problem is that as the trees that have absorbed contaminants burn up and release smoke, this turns radioactive particles into a much more dangerous form than if they simply lie in the ground.
“Internal radiation from inhalation – in other words, if you inhale something radioactive and it gets inside you – is very much more dangerous than just the background radiation that comes off the ground,” says Christopher Busby, the scientific secretary of the European Committee on Radiation Risks.
http://rt.com/news/254193-chernobyl-fire-radiation-spread/
French nuclear safety research institution IRSN created this simulation video, modelling the spread of caesium-137 from the 1986 Chernobyl disaster.
Christopher Busby commented on how far radioactive particles can potentially spread: “After Chernobyl itself, they ended up in the atmosphere and they went right across the Baltic States and into Finland, and over Sweden, and then to the United Kingdom, where they caused significant increases in cancer.”