When spent fuel rods are removed from a nuclear reactor, they are giving off heat and emitting radiation, primarily from fission products. They are stored in special pools of water or boric acid to allow the heat and radiation to diminish. The cooling fluid absorbs the radiation and is circulated through heat exchangers to get rid of the heat. It can take several years for the heat and radiation to drop to a safer level. The rods may be transferred to dry storage casks after at least a year has elapsed.
Spent fuel is either considered waste and stored indefinitely or is reprocessed into fuel to be used again in reactors. If it is not going to be reprocessed, the spent fuel may be kept at the reactor site indefinitely or transported to a central repository for storage. If it is going to be reprocessed, then it will ultimately be transported to a reprocessing facility. Special spent nuclear fuel casks are required to transport spent nuclear fuel. They are designed to withstand possible accidents such as crashes without releasing their contents.
Nuclear waste is classified into three different categories. Low-level waste is created during all the stages of the fuel cycle. Intermediate level is produced during the burning of the fuel in the reactor and during reprocessing. High-level waste contains fission products from fuel reprocessing and includes the spent fuel itself.
Nuclear waste contains many different radioactive isotopes which have half-lives that vary from a few years to billions of years. It is possible to process nuclear waste in some types of reactors where radioactive isotopes can be converted to other elements which may have lower or no radioactivity.
There is currently no facility where nuclear waste can be disposed of permanently. The technical issues of such disposal have been studied extensively and permanent disposal sites have been propose. Some of the major issues involve how to package the waste in a safe and stable way, geological stability of the area, possibility of leaching into ground water, and other potential problems. There have even been efforts to create such sites but none have been completed to date.
The various radioactive isotopes that are present in the spent fuel can be separated with chemical processing and used to make more fuel. The amount of U-235 remaining in spent fuel is higher than the amount found in natural ores. The uranium in spent fuel can be enriched to the point where it can be sent back to the reactor and burned again. Alternatively, the P-238 produced by fission can be used to make mixed oxide fuel where plutonium oxide is combined with uranium oxide. . This fuel is burned in what are referred to as mixed oxide or MOX reactors. The P-238 takes the place of U-235 found in uranium oxide fuel
Spent fuel is reprocessed in Europe for European and Japanese reactors. The United States has banned reprocessing because of concerns about nuclear weapons proliferation utilizing P-238. There has been work setting up reprocessing plants under international agreement where plutonium can be processed into a type of fuel that could be burned in a reactor but cannot be used to make nuclear weapons.