As concern over the depletion of and environmental consequences over the uses of fossil fuels has grown, nuclear power has enjoyed a rebirth in interest. However, a nuclear renaissance is still fraught with concerns over proliferation, potential depletion of currently economically recoverable uranium supplies, short-term environmental contamination from accidents, and long-term environmental contamination from radioactive wastes. Spent nuclear fuel processing provides an option that increases the amount of energy that can be obtained from uranium supplies while also decreasing long-lived nuclear wastes. However, these benefits come at an increased risk of proliferation of nuclear weapons materials, particularly plutonium.
Nuclear fission creates a host of products, including precipitates containing elements such as rubidium, cesium, palladium, and gold; volatiles such as certain isotopes of xenon and bromine; and oxides containing heavy metals such as cerium and promethium that remain dissolved in the fissile fuel.  These products are of concern because they must be separated, transported, and stored as long as they are radioactive to avoid environmental contamination. Separation of these products from the nuclear fuel is also of importance since the once these have been removed from the fuel, the fuel can be reused in a reactor, thus removing more of the energy from the original uranium source. This additional energy recovery can be substantial-up to 25% more of the energy can be recovered from the resource.  Nuclear fuel reprocessing is this separation that allows for better storage and disposal of products in addition to improved energy recovery.
The still usable fissile materials are separated from the waste products through series of transformations. First, the lighter fission products are evaporated off and then deposited in glasses for later sequestration in deep underground repositories.  The remaining products are then treated through solvent extraction. One of the most prevalent such extraction processes is the Purex process, which involves nitrate ions and organic phosphates.  These complex molecules are then treated with water to to produce uranium oxides or mixed oxides (MOX). These oxides can then be placed back in reactors to obtain continue fission to provide more electric power.
This ability to separate plutonium and uranium from fission products raises the prospect that reprocessing could be used to divert nuclear material from peaceful electricity uses to dangerous weapons. In particular, the wide-spread Purex treatment method allows for the separation of pure plutonium from the other fission products. However, the plutonium produced by processing resulting in MOX contains too high of percentages of Pu-240, an isotope that discourages its use in weapons. 
Chemical reprocessing of spent nuclear fission material both reduces radioactive waste and increases the utilization of the energy present in the original resource. Though proliferation is a concern, properly managed cycles produce plutonium containing isotopes that discourage the creation of weapons from the separated fissile materials.
© Thomas Parise. 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.
 H. Kleykamp, "The Chemical State of the Fission Products in Oxide Fuels," J. Nucl. Mat. 131, 221 (1985).
 "Spent Fuel Processing Options," International Atomic Energy Agency, TECDOC 1587, August 2008.
 C. Madic et al., "Separation of Long-Lived Radionuclides From High Active Nuclear Waste," C. R. Physique, 3, 797 (2002).
 J.E. Birkitt et al., "Recent Developments in the Purex Process for Nuclear Fuel Reprocessing: Complexant Based Striping for Uranium/Plutonium Separation," Chimia 59, 898 (2005).
 R.J. Taylor et al., "The Applications of Formo- and Aceto-Hydroxamic Acids in Nuclear Fuel Reprocessing," J. Alloys and Compounds, 271-273, 534 (1998).
 B. Pellaud, "Proliferation Aspects of Plutonium Recycling," C. R. Physique, 3, 1067 (2002).