The 1986 nuclear plant explosion saw the 4800 square kilometres of land surrounding it evacuated and abandoned. The exclusion zone has been taken over by a dense boreal forest over the following decades—and now wildfires are spreading the radiation from the incident further than we'd hope.
The original disaster saw 85 petabecquerels of radioactive caesium released, and best estimates predict that between 2 and 8 petabecquerels still lie in the upper surface of soil around Chernobyl. It was hoped that it would gradually sink into the earth, but the thick, abandoned forest picks that up the radiation, dead leaves return it to the top surface of the soil, and so the cycle continues. Now, forest fires—more severe because of thick vegetation present—can release larger amounts of radiation to the surrounding than expected.
In fact, analysis of satellite images of forest fires in 2002, 2008 and 2010, and measurements of radioactive caesium-137 deposited on the area, by researchers at the Norwegian Institute for Air Research quantify that effect. They reveal that those three fires alone released as much as 0.5 petabecquerels in their smoke. The radiation spread across Euope, even reaching Italy and Scandinavia.
The releases are, arguably, small when averaged over entire populations. It's estimated that the fires likely gave people in nearby Kiev a 10 microsievert dose of radiation, which is only 1 per cent of the permitted yearly dose. But it's also noted by the researchers that the fires also likely dump radioactive strontium, plutonium and americium too, often unevenly. That can directly affect people, or work its way into the food chain.
Perhaps worst of all, models created by the researchers suggest that forest wildfires in the exclusion zone won't peak until between 2023 and 2036, well ahead of the point by which radiation will have decayed away. It's currently not clear what Ukraine officials will do about the threats—indeed, it has plenty else to worry about right now—but currently, it looks like wildfires could resurrect Chernobyl's radiation across the rest of Europe if something isn't done to stop it. [Ecological Monographs via New Scientist]