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Spent nuclear fuel
Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant) to the point where it is no longer useful in sustaining a nuclear reaction.
Nuclear reprocessing can separate spent fuel into various combinations of reprocessed uranium, plutonium, minor actinides, fission products, remnants of zirconium or steel cladding, activation products, and the reagents or solidifiers introduced in the reprocessing itself.
Alternatively, the intact spent fuel can be disposed as radioactive waste. The US is currently planning disposal in deep geological formations, such as Yucca Mountain, where it has to be shielded and packaged to prevent its migration to mankind's immediate environment for thousands if not millions of years.
Additional recommended knowledge
Nature of spent fuel
Large John H Radioactive Decay Characteristics of Irradiated Nuclear Fuels, January 2006 
Spent low enriched uranium nuclear fuel is an example of a nanomaterial which existed before the term nano became fashionable. In the oxide fuel intense temperature gradients exist which cause fission products to migrate. The zirconium tends to move to the centre of the fuel pellet where the temperature is highest while the lower boiling fission products move to the edge of the pellet. The pellet is likely to contain lots of small bubble like pores which form during use, the fission xenon migrates to these voids. Some of this xenon will then decay to form cesium, hence many of these bubbles contain a lot of 137Cs.
In the case of the MOX the xenon tended to diffuse out of the plutonium rich areas of the fuel, and it was then trapped in the surrounding uranium dioxide. The neodymium tended to not be mobile.
Also metallic particles of an alloy of Mo-Tc-Ru-Pd tends to form in the fuel. Other solids form at the boundary between the uranium dioxide grains, but the majority of the fission products remain in the uranium dioxide as solid solutions. A paper describing a method of making a nonradioactive uranium active) simulation of spent oxide fuel exists.
Table of chemical data
For natural uranium fuel: Fissile component starts at 0.71% 235U concentration in natural uranium). At discharge, total fissile component is still 0.50% (0.23% 235U, 0.27% fissile 239Pu, 241Pu) Fuel is discharged not because fissile material is fully used-up, but because the neutron-absorbing fission products have built up and the fuel becomes significantly less able to sustain a nuclear reaction.
Some natural uranium fuels use chemically active cladding, such as Magnox, and need to be reprocessed because long-term storage and disposal is difficult .
For highly enriched fuels used in marine reactors and research reactors the isotope inventory will vary based on in-core fuel management and reactor operating conditions.
Spent fuel corrosion
Uranium dioxide films
Uranium dioxide films can be deposited by reactive spluttering using an argon and oxygen mixture at a low pressure. This has been used to make a layer of the uranium oxide on a gold surface which was then studied with AC impedence spectroscopy.
Noble metal nanoparticles and hydrogen
According to the work of the corrosion electrochemist Shoesmith  the nanoparticles of Mo-Tc-Ru-Pd have a strong effect on the corrosion of uranium dioxide fuel. For instance his work suggests that when the hydrogen (H2) concentration is high (due to the anaerobic corrosion of the steel waste can) the oxidation of hydrogen at the nanoparticles will exert a protective effect on the uranium dioxide. This effect can be thought of as an example of protection by a sacrificial anode where instead of a metal anode reacting and dissolving it is the hydrogen gas which is consumed.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Spent_nuclear_fuel". A list of authors is available in Wikipedia.|