When nuclear fuel is used in a reactor, nuclear waste is the material that results. Appearance wise, nuclear waste very closely resembles the fuel that was initially used in the reactor. Visually, nuclear waste appears to be comprised of stacks of ceramic pellets that are encased by assemblies of metal rods.  Once placed in a nuclear reactor, although the contents do not appear to change physically, they do, thus distinguishing them from what first went into the reactor. As a result of the nuclear reactions that take place inside the reactor, the contents transform into radioactive waste. In general, before this transition, the contents of the fuel are comprised of primarily uranium, thorium, steel, and oxygen. Through the transition in the reactor, several of the Uranium atoms split into various isotopes that encompass essentially all of the extremely reactive transition metals from the center of the periodic table of elements. The nature of these transition metals create a dangerous waste that is extremely radioactive and continues to be so for thousands of years. This waste is so toxic when it comes out of the reactor that if someone were to stand within a few yards from it for a few seconds with no protection from it, they would have received a lethal dose of it and would proceed to die from acute radiation sickness a few days later.  In order to prevent this, the used fuel is always submerged in water to keep it shielded for years until the radiation decays enough for concrete to be able to shield people from the waste. The waste can then be put in large concrete crates. The final step to disposing nuclear waste is one of great controversy and it includes deep geologic storage and recycling.  The only other option would be for the sun to consume it but since rockets are not reliable, getting the waste into space is too risky given the possibility of atmospheric.
Due to its composition, the disposal of nuclear waste is extremely difficult, and the management of such dangerous waste is one of the main drawbacks to the use of nuclear energy. There are several factors that contribute to the composition of nuclear waste. The first is what has put into the reactor. Different materials will produce different forms of waste after going through the reactor. The second is how long the reactor is operated for.  If the waste is passed through the reactor for a longer amount of time, it is more likely to form different isotopes and come out with a higher level of radioactivity. The third and final major factor the effects the composition of nuclear waste is how long the waste has been sitting out of the reactor. The longer the waste sits out of the reactor, the more radioactive decay it experiences. Despite a significant decline in their enrichment when they leave the reactor, elements like Uranium remain in the fuel and are used as fuel and a valuable energy source. The minor actinides, which include Neptunium, Americium, and Curium, are extremely durable nuclides that become problematic in that they need to be stored for hundreds of thousands of years.  Fortunately, in fast reactors, these elements are fissionable so they can be used in fuel, leaving behind the fission products.
© Léa Koob. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 R. C. Ewing, "Nuclear Waste Forms for Actinides," Proc Nat. Acad. Sci. (USA) 96, 3432 (1999).
 "Radioactive Waste," U.S. Nuclear Regulatory Commission, April 2015.