Nuclear fuel: Encyclopedia II - Nuclear fuel - Common chemical forms of nuclear fuel

Nuclear fuel - Common chemical forms of nuclear fuel

Nuclear fuel - UOX

Nuclear fuel - MOX

Mixed oxide, or MOX fuel, is a blend of plutonium and natural or depleted uranium which behaves similarly (though not identically) to the enriched uranium feed for which most nuclear reactors were designed. MOX fuel is an alternative to Low enriched uranium (LEU) fuel used in the light water reactors which predominate nuclear power generation.

An attraction of MOX fuel is that it is a way of disposing of surplus weapons-grade plutonium, which otherwise would have to be handled as a difficult-to-store nuclear waste product, and a nuclear proliferation risk. Old reactor-grade plutonium can be difficult to use in a MOX fuel plant, as the plutonium-241 it contains decays with a short 14.1 year half-life into more radioactive americium-241 which makes the fuel difficult to handle in a production plant. Within about 5 years typical reactor-grade plutonium would contain too much americium-241 (about 3%) [1].

Relicensing precedes the introduction of MOX fuel into existing reactors. Often only a third to half of the fuel load is switched to MOX.

Some concern has been expressed that used MOX cores will introduce new disposal challenges, though MOX is itself a means to dispose of surplus plutonium by transmutation.

Currently (March, 2005) reprocessing of commercial nuclear fuel to make MOX is done in England and France, and to a lesser extent in Russia, India and Japan. China plans to develop fast breeder reactors and reprocessing. Reprocessing of spent commercial-reactor nuclear fuel is not permitted in the United States due to nonproliferation considerations. (All of these nations have long had nuclear weapons from military-focused "research"-reactor fuels except Japan, which wants no such weapons.)

Nuclear fuel - Spent fuel

Spent low enriched uranium fuel contains:

• 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) 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.

Spent fuel is an example of a nanomaterial which existed before the term nano became fashionable, in the oxide fuel intense temperture gradients exist which cause fission products to migrate. The zirconium tends to move to the centre of the fuel pellet where the temperture 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. Also metallic particles of an alloy of Mo-Tc-Ru-Pd tends to form in the fuel.

• 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 [2]. 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 0.4% of the mass, making this product depleted uranium. If it contains more ²³⁵U than natural uranium (0.7%) it is still enriched, and could be further enriched to be reused.
• Traces of the minor actinides. In present in used reactor fuel are the minor actinides, 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 [3].

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.

Adapted from the Wikipedia article "Common chemical forms of nuclear fuel", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki