Background: Commercial nuclear power plants are water-cooled. They are fuelled by ceramic uranium fuel pellets stacked inside long narrow rods made of zirconium metal. A number of these rods are bound together into a fuel assembly — in Canada such an assembly is called a fuel bundle.
Heat is produced by splitting uranium atoms. That heat is transported by the liquid water coolant which flows past the zirconium tubes containing the fuel. The heat is used to produce steam that will turn the blades of a steam turbine to generate electricity.
As the uranium fuel undergoes nuclear fission (splitting uranium atoms), hundreds of varieties of intensely radioactive byproducts build up inside the fuel. These are (1) broken fragments of uranium atoms, called “fission products”; (2) heavier-than-uranium elements, including plutonium, called “transuranic actinides”. These byproducts are millions of times more radioactive than the original fuel.