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Radioactive materials emit radiation on a typical timescale known as a half-life. Over time they become less and less radioactive in a process known as exponential decay. Searches for isotopes which may undergo non-exponential decay have, to date, proved fruitless. Most nuclear fuel consists of material with a very long half-life, such as uranium-235 (over seven hundred million years). Other materials which are suitable for nuclear fuel have mostly decayed away and are no longer found in nature.

Material created in a nuclear reactor during normal operation is a complex mix of isotopes which have a wide variety of half lives. It is common when mentioning radioactive waste to focus on those isotopes whose half-life is particularly long. However, these isotopes actually do not emit the bulk of the instantaneous ionizing radiation from spent nuclear fuel. Spent fuel is currently stored in pools after use to allow it time to cool down and emit the bulk of its ionizing radiation from the short-lived isotopes. In the future, advanced fuel recycling technology may allow for the re-use of nuclear fuel with reduced need to mine new fuel and the potential to radically reduce the total amount of nuclear waste by separating out only those isotopes which last longer than 10's of years for long-term storage. The core of the HT3R is designed with a space for the long-term storage of material to be irradiated over a long period of time, a capability which exists at only one other research facility in the United States. This facility will be used to experiment with the creation of scientifically important but rare isotopes whose extremely long half-life makes them difficult to study. It will also be possible to study the transmutation of long-lived isotopes into short-lived isotopes, another frontier in the science of nuclear waste reduction.