MOX fuel

Mixed oxide, or MOX fuel, is a blend of oxides of plutonium and natural uranium, reprocessed uranium, or depleted uranium which behaves similarly (though not identically) to the low 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 that predominate nuclear power generation.

Thorium-plutonium fuel has some attractive characteristics such as much lower production of and more complete burning of transuranics and has been used in a few reactors to date, but is usually referred to specifically as thorium-plutonium rather than MOX to avoid confusion with uranium-plutonium MOX fuel.

One attraction of MOX fuel is that it is a way of disposing of surplus weapons-grade plutonium, which otherwise would have to be disposed as nuclear waste, and would remain a nuclear proliferation risk.

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Overview

In every uranium-based nuclear reactor core there is both fission of isotopes such as uranium-235 (U-235), and the formation of new, heavier isotopes due to neutron capture, primarily by uranium-238 (U-238). Most of the fuel mass in a reactor is U-238. This can become plutonium-239 (Pu-239) and by successive neutron capture plutonium-240 (Pu-240), plutonium-241 (Pu-241), plutonium-242 (Pu-242) and other transuranic or actinide isotopes. Pu-239 and Pu-241 are fissile, like U-235. Small quantities of uranium-236 (U-236), neptunium-237 (Np-237) and plutonium-238 (Pu-238) are formed similarly from U-235.

Normally, with the fuel being changed every three years or so, most of the Pu-239 is "burned" in the reactor. It behaves like U-235, with a slightly higher cross section for fission, and its fission releases a similar amount of energy. Typically about one percent of the spent fuel discharged from a reactor is plutonium, and some two thirds of the plutonium is Pu-239. Worldwide, almost 100 tonnes of plutonium in spent fuel arises each year. A single recycling of plutonium increases the energy derived from the original uranium by some 17%, and if the uranium-235 is also recycled by re-enrichment, this becomes about 30%. With additional recycling the percentage of fissile (usually meaning odd-mass number isotopes) in the mix decreases and even-mass number neutron-absorbing isotopes increase, requiring the total plutonium and/or enriched uranium percentage to be increased. Today in thermal reactors plutonium is only recycled once as MOX fuel, and spent MOX fuel, with a high proportion of minor actinides and even-mass plutonium isotopes, is stored as waste.

Re-licensing precedes the introduction of MOX fuel into existing nuclear reactors. Often only a third to half of the fuel load is switched to MOX. The use of MOX does change the operating characteristics of a reactor, and the plant must be designed or adapted slightly to take it. More control rods are needed. For more than 50% MOX loading, significant changes are necessary and a reactor needs to be designed accordingly. The Palo Verde Nuclear Generating Station near Phoenix, Arizona was designed for 100% MOX core compatibility but so far have always operated on fresh low enriched uranium. In theory the three Palo Verde reactors could use the MOX arising from seven conventionally fueled reactors each year and would no longer require fresh Uranium fuel.

According to the AECL, CANDU reactors could use 100% MOX cores without physical modification. Atomic Energy of Canada Limited (AECL), reported to the United States National Academy of Sciences committee on plutonium disposition that it has extensive experience in testing the use of MOX fuel containing from 0.5 to 3% plutonium.

Current applications

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.

Thermal reactors

Over 30 thermal reactors in Europe (Belgium, Switzerland, Germany and France) are using MOX and a further 20 have been licensed to do so. Most reactors use it as about one third of their core, but some will accept up to 50% MOX assemblies. In France, EDF aims to have all its 900 MWe series of reactors running with at least one-third MOX. Japan aims to have one third of its reactors using MOX by 2010, and has approved construction of a new reactor with a complete fuel loading of MOX.

Fast reactors

Because the fission to capture-neutron cross-section with high energy or fast neutrons changes to favour fission for almost all of the actinides, including U-238 fast reactors can use all of them for fuel. All TRU or TRans-Uranium actinides can undergo neutron induced fission with unmoderated or fast neutrons. A fast reactor is more efficient for using plutonium and higher actinides as fuel. Depending on how the reactor is fueled it can either be used as a plutonium breeder or as a fast burner.

These fast reactors are better suited for the transmutation of other actinides than are thermal reactors. Because Thermal Reactors use slow or moderated neutrons the actinides which are not fissionable with thermal neutrons tend to absorb the neutrons instead of fissioning. This leads to build up of higher isotope actinides and lowers the number of thermal neutrons available to continue the chain reaction.

All plutonium isotopes are either fissile or fertile; in thermal reactors isotopic degradation limits the plutonium recycle potential. Along with Uranium about 1% of spent fuel is plutonium broken down as 40% Pu-239, and about 32% Pu-240, 18% Pu-241, 8% Pu-242 and 2% Pu-238 when the fuel is first removed from the reactor.

Fabrication

The first step is separating the plutonium from the remaining uranium (about 96% of the spent fuel) and the fission products with other wastes (together about 3%). This is undertaken at a nuclear reprocessing plant.

Dry mixing

MOX fuel can be made by grinding together uranium oxide (UO₂) and plutonium oxide (PuO₂) before the mixed oxide is pressed into pellets, but this process has the disadvantage of forming lots of radioactive dust. MOX fuel, consisting of 7% plutonium mixed with depleted uranium, is equivalent to uranium oxide fuel enriched to about 4.5% U-235, assuming that the plutonium has about 60- 65% Pu-239. If weapons-grade plutonium were used (>90% Pu-239), only about 5% plutonium would be needed in the mix.

Coprecipitation

A mixture of uranyl nitrate and plutonium nitrate in nitric acid is converted by treatment with a base such as ammonia to form a mixture of ammonium diuranate and plutonium hydroxide. This after heating in 5% hydrogen in argon will form a mixture of uranium dioxide and plutonium dioxide. The resulting powder can be converted using a base into green pellets using a press. The green pellet can then be sintered into mixed uranium and plutonium oxide pellet. While this second type of fuel is more homogenous on the microscopic scale (scanning electron microscope) it is possible to see plutonium rich areas and plutonium poor areas. It can be helpful to think of the solid as being like a salami (more than one solid material present in the pellet). In the following picture of MOX voids are seen in the plutonium rich phases, these are voids which formed during irradiation.

Americium content

Plutonium from reprocessed fuel is usually fabricated into MOX as soon as possible to avoid problems with the decay of short-lived isotopes of plutonium. In particular, Pu-241 decays to americium-241 which is a gamma ray emitter, giving rise to a potential occupational health hazard if the separated plutonium over five years old is used in a normal MOX plant. While Am-241 is a gamma emitter most of the photons it emits are low in energy, so 1 mm of lead, or thick glass on a glovebox will give the operators a great deal of protection to their torsos. When working with large amounts of americium in a glovebox, the potential exists for a high dose of radiation to be delivered to the hands.

As a result old reactor-grade plutonium can be difficult to use in a MOX fuel plant, as the Pu-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 Am-241 (about 3%). But it is possible to purify the plutonium bearing the americium by a chemical separation process. Even under the worst possible conditions the americium/plutonium mixture will never be as radioactive as a spent-fuel dissolution liquor, so it should be relatively straight forward to recover the plutonium by PUREX or another aqueous reprocessing method.

Also, Pu-241 is fissile while Pu-240 is not (in general thermal neutrons will usually fission the isotopes with an odd number of nucleons (mass number), but rarely those with an even number), so decay of Pu-241 to Am-241 leaves plutonium with a lower proportion of isotopes usable as fuel, and a higher proportion of isotopes that simply capture neutrons (though they may become fissile isotopes after this capture). The decay of Pu-238 to U-234 and subsequent removal of this uranium would have the opposite effect, but Pu-238 both has a longer halflife (87.7 years vs. 14.3) and is a smaller proportion of the spent nuclear fuel. Pu-239, Pu-240, and Pu-242 all have much longer halflives so that decay is negligible. (Pu-244 has an even longer halflife, but is unlikely to be formed by successive neutron capture because Pu-243 quickly decays with a halflife of 5 hours giving Am-243.)

Curium content

It is possible that both americium and curium could be added to a U/Pu MOX fuel before it is loaded into a fast reactor. This is one means of transmutation. Work with curium is much harder than work with americium because curium is a neutron emitter, the MOX production line would need to be shielded with both lead and water to protect the workers.

Also, the neutron irradiation of curium generates the higher actinides, such as californium, which increase the neutron dose associated with the used nuclear fuel; this has the potential to pollute the fuel cycle with strong neutron emitters. As a result, it is likely that curium will be excluded from most MOX fuels.

References

• Technical Aspects of the Use of Weapons Plutonium as Reactor Fuel
• Synergistic Nuclear Fuel Cycles of the Future
• Nuclear Issues Briefing Paper 42
• Burning Weapons Plutonium in CANDU Reactors
• Program to turn plutonium bombs into fuel hits snags

See also

• Nuclear fuel cycle
• Nuclear breeder reactor
• Spent nuclear fuel shipping cask
• Nuclear power
• Nuclear fission
• Nuclear power plant
• Savannah River Site