"Radioactive particles" refers to tiny pieces of material that give off harmful radiation, which can be dangerous to humans and the environment. Full definition
The mix of radioactive particles released into the atmosphere would vary with the length of time expired since the fuel units were moved from reactors to the pools. (scientificamerican.com)
If left exposed to air, they may catch fire or explode, spreading radioactive particles into the air. (scientificamerican.com)
Being exposed to the toxic environment for so many years, these pups have radioactive particles on their fur and inside their bodies. (petsmagazine.com.sg)