|A uranium fuel pellet|
|Nuclear fuel rod assemblies|
|Fuel assemblies in a nuclear reactor opened for servicing|
The typical commercial reactor will operate for about 12 to 24 months before the fuel must be replaced. By this time, much of the uranium-235 within the assemblies has been changed into fission products (some of the most common being iodine-131, cesium-137, and strontium-90). Isotopes of plutonium will have also been produced when neutrons were absorbed by atoms of uranium-238. The reactor is shut down by fully inserting the control rods. At this point the reactor is "subcritical", meaning that, on average, each fission produces less than one additional fission. The fuel assemblies are then removed from the reactor and are placed in a water-filled pit or pool. Although the nuclear chain reaction has been stopped, the water is required to cool the assemblies since radioactive decay of the fission products produces a significant amount of heat. Eventually the rate of decay will fall sufficiently that the fuel assemblies can be removed from the pool and be placed in a dry cask.
|Spent fuel assemblies in a cooling pool|
I'll be explaining what the above means for Fukushima in a later post.