Spent nuclear fuel

Used nuclear fuel (often called spent nuclear fuel) is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant) to the point that it is no longer useful in sustaining a nuclear reaction. If not reprocessed to retrieve the remaining usable uranium and plutonium, it is a form of radioactive waste.

Used nuclear fuel is currently planned for disposal in deep geological formations, such as Yucca Mountain.

Nature of used fuel

Nanomaterial properties

Used 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 caesium, hence many of these bubbles contain a lot of Cs-137. 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.

For details of how to make a nonradioactive (uranium active) simulation of spent oxide fuel see: 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. This SIMFUEL has been used in a large number of experiments in which attempts are made to predict the long term behaviour of used nuclear fuels within a waste store. One leading worker in this area is David Shoesmith[1][2] who is an electrochemist working in Canada, he tends to use many of the electrochemical experiments normally used for the study of galvanic corrosion to investigate the galvanic of uranium dioxide.

According to Dave Shoesmith's work the nanoparticles of Mo-Tc-Ru-Pd have a strong effect on the corrosion of uranium dioxide fuel. He 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.

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.

Table of chemical data

The chemical forms of fission products in uranium dioxide [3]

Element	Gas	Metal	Oxide	Solid solution
Br	Yes	-	-	-
Kr	Yes	-	-	-
Rb	Yes	-	Yes	-
Sr	-	-	Yes	Yes
Y	-	-	-	Yes
Zr	-	-	Yes	Yes
Nb	-	-	Yes	-
Mo	-	Yes	Yes	-
Tc	-	Yes	-	-
Ru	-	Yes	-	-
Rh	-	Yes	-	-
Pd	-	Yes	-	-
Ag	-	Yes	-	-
Cd	-	Yes	-	-
In	-	Yes	-	-
Sn	-	Yes	-	-
Sb	-	Yes	-	-
Te	Yes	Yes	Yes	Yes
I	Yes	-	-	-
Xe	Yes	-	-	-
Cs	Yes	-	Yes	-
Ba	-	-	Yes	Yes
La	-	-	-	Yes
Ce	-	-	-	Yes
Pr	-	-	-	Yes
Nd	-	-	-	Yes
Pm	-	-	-	Yes
Sm	-	-	-	Yes
Eu	-	-	-	Yes

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

Minor actinides

• Traces of the minor actinides are present in used reactor fuel. These are actinides other than uranium and plutonium. These include 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 than the use of a uranium/thorium based fuel (²³³U in a ²³²Th matrix). Also present as a minor actinide is ²³⁷Np, this neptunium isotope is fissile but also can be converted into ²³⁸Pu by neutron bombardment.

For natural uranium fuel: Fissile component starts at 0.71% ²³⁵U concentration in natural uranium). At discharge, total fissile component still 0.50% (0.23% ²³⁵U, 0.27% fissile ²³⁹Pu, ²⁴¹Pu) Fuel is discharged not because it is fully used-up, but because the neutron-absorbing fission products have built up and the fuel then 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 preasure. This has been used to make a layer of the uranium oxide on a gold surface which was then studied with AC impedance spectroscopy.

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