Spent nuclear fuel is discharged from a reactor because it no longer has enough reactivity to keep the reactor critical. However, spent fuel still contains a lot of U-235 and plutonium that can be removed from the spent fuel and recycled back into reactors. The removal of the valuable components of spent fuel is called spent fuel reprocessing. This is a technique in which the solid spent fuel is dissolved and chemically separated to partition off the reusable components (mainly the U and Pu). Spent fuel reprocessing can also help to decrease waste volumes. This is because the spent fuel is composed of approximately 96% U, 1% Pu, and 3% fission products and minor actinides. The uranium (which is the bulk of the spent fuel) does not add significantly to the radiotoxicity of the spent fuel. Thus, it can be removed and disposed of as low level waste. This will remove 96% of the volume of the spent fuel. In addition, if the plutonium were also removed, a large portion of the long-term radiotoxicity of the waste would also be removed. The radiotoxicity of the remaining activation products and fission products drops below the reactivity of the original uranium ore body in around 300 years.
There are several different types of spent fuel reprocessing techniques but by far the most common is the PUREX process and its variants (specifically the COEX process). The basic PUREX process is as follows:
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