- Spent nuclear fuel
Spent nuclear fuel, occasionally called used nuclear fuel, is
nuclear fuelthat 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 reprocessingcan 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 of years. [http://www.largeassociates.com/3145/3145-a1%20FINAL.pdf] (
Nature of spent fuel
Spent low enriched uranium nuclear fuel is an example of a
nanomaterialwhich existed before the term nanobecame fashionable. In the oxide fuelintense temperature gradients exist which cause fission productsto migrate. The zirconiumtends to move to the centre of the fuel pelletwhere the temperatureis highest while the lower boiling fission products move to the edge of the pellet. The pellet is likely to contain lots of small bubblelike pores which form during use, the fission xenonmigrates to these voids. Some of this xenon will then decay to form caesium, 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
neodymiumtended to not be mobile.
Also metallic particles of an
alloyof 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 dioxideas solid solutions. A paper describing a method of making a non radioactive"uranium active") simulation of spent oxide fuel exists. ["Microstructural features of SIMFUEL - Simulated high-burnup UO2-based nuclear fuel", P.G. Lucuta, R.A. Verrall, Hj. Matzke and B.J. Palmer, Journal of Nuclear Materials, 1991, 178, 48–60.]
*3% of the mass consists of fission products of 235U (also indirect products in the
decay chain), nuclear poisons considered radioactive wasteor separated further for various industrial and medical uses. The fission products include every element from zincthrough to the lanthanides, much of the fission yield is concentrated in two peaks, one in the second transition row (Zr, Mo, Tc, Ru, Rh, Pd, Ag) while the other is later in the periodic table (I, Xe, Cs, Ba, La, Ce, Nd). Many of the fission products are either non radioactive or only shortly lived radioisotopes. But a considerable number are medium to long lived radioisotopes such as 90Sr, 137Cs, 99Tc and 129I. Research has been conducted by several different countries into partitioning the rare isotopes in fission waste including the Fission Platinoids (Ru, Rh, Pd) and Silver (Ag) as a way of offsetting the cost of reprocessing, however this is not currently being done commercially.
: The fission products can modify the thermal properties of the uranium dioxide, the
lanthanideoxides tend to lower the thermal conductivity of the fuel while the metallicnanoparticles slightly increases the thermal conductivity of the fuel. [Dong-Joo Kim, Jae-Ho Yang, Jong-Hun Kim, Young-Woo Rhee, Ki-Won Kang, Keon-Sik Kim and Kun-Woo Song, "Thermochimica Acta", 2007, 455, 123–128. ]
Table of chemical data
*1% of the mass is 239Pu and 240Pu resulting from conversion of 238U, which may either be considered a useful by-product, or as dangerous and inconvenient waste. One of the main concerns regarding
nuclear proliferationis to prevent this plutonium from being used by states other than those already established as Nuclear Weapons States, to produce nuclear weapons. If the reactor has been used normally, the plutonium is reactor-grade, not weapon-grade: it contains much 240Pu and less than 80% 239Pu, which makes it less suitable, but not impossible, to use in a weapon. [ cite web
title= Additional Information Concerning Underground Nuclear Weapon Test of Reactor-Grade Plutonium
date= |year= |month= |work= |publisher=
U.S. Department of Energy
accessdate= 2008-05-18 ] If the irradiation period has been short then the plutonium is weapon-grade (more than 80%, up to 93%).
*96% of the mass is the remaining uranium: most of the original 238U and a little 235U. Usually 235U would be less than 0.83% of the mass along with 0.4% 236U.
Reprocessed uraniumwill contain 236U which is not found in nature; this is one isotope which can be used as a fingerprintfor spent reactor fuel.
*Traces of the
minor actinidesare present in spent reactor fuel. These are actinidesother than uranium and plutonium. These include neptunium, americiumand curium. The amount formed depends greatly upon the nature of the fuel used and the conditions under which it was used. For instance, the use of MOX fuel (239Pu in a 238U matrix) is likely to lead to the production of more 241Am and heavier nuclides than a uranium/thorium based fuel (233U in a 232Th matrix).
natural uraniumfuel: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. [ cite web
title= RWMAC's Advice to Ministers on the Radioactive Waste Implications of Reprocessing
author= |last= |first= |authorlink= |coauthors=
date= 3 November, 2002 |work= |publisher= Radioactive Waste Management Advisory Committee (RWMAC)
accessdate= 2008-05-18 ]
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.
pent fuel corrosion
Uranium dioxide films
Uranium dioxide films can be deposited by reactive
sputteringusing an argonand oxygenmixture at a low pressure. This has been used to make a layer of the uranium oxide on a goldsurface which was then studied with AC impedancespectroscopy. [F. Miserque, T. Gouder, D.H. Wegen and P.D.W. Bottomley, "Journal of Nuclear Materials", 2001, 298, 280–290. ]
Noble metal nanoparticles and hydrogen
According to the work of the
corrosionelectrochemist Shoesmith [ cite web
title= David W. Shoesmith
date= |year= |month= |work= | publisher=
University of Western Ontario
accessdate= 2008-05-18 ] [ cite web
title= Electrochemistry and corrosion studies at Western
date= |year= |month= |work= |publisher= Shoesmith research group, University of Western Ontario
accessdate= 2008-05-18 ] 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 steelwaste 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 anodewhere instead of a metal anodereacting and dissolving it is the hydrogen gas which is consumed.
Spent nuclear fuel shipping cask
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