As stated previously, most of the nuclear energy plants in the world are based on light water reactors and their variants. One disadvantage with the light water reactor design is that it only utilizes a small percentage of the Uranium-235 that is available within a fuel rod. Over ninety percent of the volume of spent fuel is uranium-235 that can be reprocessed to produce more fuel for nuclear energy plants and greatly reduce the volume and radioactivity of material to be disposed of. While the practice of fuel reprocessing remains a politically sensitive issue in many parts of the world, the fears that it will somehow lead to the increased proliferation of nuclear arms are erroneous.
There are different types of reprocessing methods, and some are still in the experimental stage, or only exist on paper. The most common type of nuclear fuel reprocessing is the PUREX (Plutonium and Uranium Recovery by EXtraction) method. The PUREX process was originally invented by Herbert H. Anderson, and Larned B. Asprey during their work on the Manhattan project in 1947. However, the spent material from fuel-grade uranium is too high in plutonium-240 to allow for the production of nuclear warheads.
PUREX and Its Derivative Processes
Nitric acid is used to dissolve the spent fuel in the reprocessing center where insoluable debris is removed in order to prevent contamination of the of the solute. A Tributyl phosphate/kerosene mixture is then used to claim the available plutonium and uranium from the nitric acid solute, leaving the transuranic elements such as curium and americium behind. Ferrous sulphmate is used to separate the reclaimed uranium from the plutonium as it reduces plutonium's oxidation state to +3 allowing the plutonium nitrates to be separated from the uranium nitrates. This liquid extraction process must be repeated several times to get an acceptable amount of plutonium and uranium reclaimed from the spent fuel. Uranium and plutonium are typically converted to their oxide forms for ease of storage and fabrication into MOX (Mixed OXide) fuel.
The UREX (URanium EXtraction) technique is based on the PUREX method to prevent the separation and extraction of plutonium. The plutonium is reduced with acetohydroxamic acid before any metal extraction takes place. This greatly increases the difficulty of recovering neptunium and plutonium isotopes in the solute. UREX was designed to add an extra measure of proliferation resistance during fuel reprocessing, although this is largely unnecessary.
Transuranics are radionuclides that have atomic numbers greater than 92 in the periodic table. They are produced in nuclear reactors or as a result of nuclear chemistry experiments. These elements typically have half-lives that are greater than twenty years, and can produce moderate to high amounts of alpha radiation. The PUREX reprocessing method can be modified to carry out the TRUEX process which allows for the extraction of the transuranics, which reduces the radioactivity of the resulting MOX fuel. TRUEX is nearly identical to PUREX, except that octyl(phenyl)-N, N-dibutyl carbamoylmethyl phosphine oxide and tributylphosphate are added to the solution after the uranium and plutonium have been extracted to allow for the extraction of transuranics. Some of these transuranics such as americium, have industrial uses. The SANEX (Selective Actinide EXtraction) process has been proposed to allow for the extraction of specific radionuclides. Although the SANEX process is still theoretical, researchers are looking into the viability of using bis-triazinyl bipyridines or dithiophosphinic acids as reagents.
Another reprocessing method based on PUREX is the UNEX (UNiversal EXtraction) process. This was developed to facilitate the separation and extraction of the more toxic actinides left behind in spent fuel; such as cesium-137, strontium-90, and other minor actinides. The UNEX process is identical to the TRUEX process, except that the extracted actinides are diluted with polar aromatic compounds.
In the past, spent fuel was often processed by using solvation techniques involving reactions with a chemical reagent that served to increase the concentration of the solute within a solution to serve as a carrier for the desired elements. The bismuth phosphate, hexone, and butex techniques were phased on in favor of the current PUREX system because of the large amount of extra material to be disposed of that was created in these prior techniques.
Pyroprocessing is the name given to reprocessing methods that involve the application of high temperatures and metallurgical properties of radioisotopes rather than water and organic solvents. Pyroprocessing methods for fuel fabrication from spent nuclear material would have several advantages over the current PUREX method. This is because pyroprocessing techniques would be more streamlined and would also allow for on-site reprocessing of spent nuclear material. In addition, some pyroprocessing methods would facilitate the extraction of several different radioisotopes at the same time without having to carry out further extraction steps. In addition, many pyroprocessing methods are more efficient at extracting useful fissile material as the remaining waste that would result would be smaller in volume as well has have a significantly shorter half-life.
The most promising pyroprocessing potential lies with the molten-salt reactor (MSR) designs of the Oak Ridge National Laboratory designs of the 1960s in the US. The liquid core of a molten salt reactor can be chemically separated into its different elements for reprocessing without having to suspend them in solution. In addition, the remaining radionuclides contained within the core would allow for a greater degree of burn-up resulting in less material to be disposed of. Finally, the reactor would not have to be shut down during fuel fabrication because the molten fuel could be continually pumped in and out of the reactor chamber.