This is the concern:
Plants take up nutrients in the soil, including unstable isotopes, and concentrate those in their structure. Burn the plants, and those isotopes become part of the smoke and ashes, further concentrated, and carried on the wind.
Lest we forget, the original plumes from Chernobyl were the result of fires burning the graphite moderating material in the reactor unit core, carrying isotopes vaporized by the fire.