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.[1]

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Nature of spent fuel

Large John H Radioactive Decay Characteristics of Irradiated Nuclear Fuels, January 2006 [2]

Nanomaterial properties

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.^[1]

Fission products

• 3% of the mass consists of fission products of ²³⁵U (also indirect products in the decay chain), nuclear poisons considered radioactive waste or separated further for various industrial and medical uses. The fission products include every element from zinc through 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 lanthanide oxides tend to lower the thermal conductivity of the fuel while the metallic nanoparticles slightly increases the thermal conductivity of the fuel.^[2]

Table of chemical data

Plutonium

• 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 proliferation is 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 [4]. If the irradiation period has been short then the plutonium is weapon-grade (more than 80%, up to 93%).

Uranium

• 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 uranium will contain ²³⁶U which is not found in nature; this is one isotope which can be used as a fingerprint for spent reactor fuel.

Minor actinides

• Traces of the minor actinides are present in spent reactor fuel. These are actinides other than uranium and plutonium. These include neptunium, americium and 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).

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 [5].

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.^[3]

Noble metal nanoparticles and hydrogen

According to the work of the corrosion electrochemist Shoesmith [6][7] 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.

See also

• Nuclear power
• Spent nuclear fuel shipping cask

References

1. ^ 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.
2. ^ 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
3. ^ F. Miserque, T. Gouder, D.H. Wegen and P.D.W. Bottomley, Journal of Nuclear Materials, 2001, 298, 280-290.