They contaminate air, water and soil, as well as plants and animals. Some of them become attached to dust particles and water droplets, and come down as rain or snow. If the bomb explodes at ground level, huge quantities of earth are scooped up into the fireball. Many of these materials, originally non-radioactive, become radioactive by absorbing stray neutrons from the fission process. They can contribute significantly to the fallout from an atomic explosion.
Nuclear reactors produce large quantities of fission products.
These are not normally dispersed in the environment except in the case of an accident like the one at Three Mile Island in 1979 or the much more catastrophic accident at Chernobyl in 1986. Four years after the accident, in 1990, reindeer in Scandinavia and sheep in Wales were still judged unfit for human consumption because of radioactive contamination by cesium-137 from Chernobyl. If there are no accidents or leaks, the fission products will remain contained within the spent uranium fuel. Even so, the gamma radiation that they give off is so intense that a person would receive a fatal dose of radiation in less than a minute if he or she stood just a meter or so away from an unshielded spent fuel bundle fresh out of the reactor. It is shipped in special flasks weighing over 50 tonnes, chained to flat-bed trucks or rail cars.