MOX fuel
From Free net encyclopedia
Current revision
Mixed oxide, or MOX fuel, is a blend of plutonium and natural uranium, reprocessed uranium, 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.
Contents |
Overview
In every nuclear reactor core there is both fission of isotopes such as uranium-235, and the formation of new, heavier isotopes due to neutron capture, primarily by U-238. Most of the fuel mass in a reactor is U-238. This can become plutonium-239 and by successive neutron capture Pu-240, Pu-241 and Pu-242 as well as other transuranic or actinide isotopes. Pu-239 is fissile, like U-235. (Small quantities of U-236 and 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 and its fission releases a similar amount of energy. The higher the burn-up, the less plutonium remains in the spent fuel, but 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 recycle of plutonium increases the energy derived from the original uranium by some 17%, and if the uranium is also recycled this becomes about 30%. With additional recycling the percentage of fissile Plutonium in the mix decreases requiring the total plutonium percentage to be increased.
Re-licensing precedes the introduction of MOX fuel into existing 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 power plant near Pheonix, AZ was designed for 100% MOX core compatibility but has so far always operated on fresh slightly enriched uranium.
CANDU reactors could use 100 % MOX cores. Atomic Energy of Canada Limited (AECL), reported to the US NAS committee on plutonium disposition that it has extensive experience in testing the use of MOX fuel containing from 0.5 to 3 % plutonium. According to the AECL, CANDU reactors can use 100 % MOX cores without physical modification.
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 reactors in Europe (Belgium, Switzerland, Germany and France) are using MOX and a further 20 have been licensed to do so. Japan also plans to use MOX in around a third of its reactors by 2010. 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 changes to favour fission for almost all of the actinides (yes even 238U can undergo neutron induced fission with very fast neutrons (fusion neutrons) a fast reactor is best for using plutonium. Depending on how the reactor is fueled it can either be used as a plutonium fast breeder or a fast burner.
All plutinium isotopes are either fissile or fertile, in thermal reactors isotopic degradation limits the plutonium recycle potential. Along with about 40% Pu-239, there may be 32% Pu-240, 18% Pu-241, 8% Pu-242 and 2% Pu-238.
These fast reactors are better suited for the transmutation of other actinides than the thermal reactors.
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 reprocessing plant.
The plutonium, as an oxide, is then mixed with depleted uranium left over from an enrichment plant to form fresh mixed oxide fuel (MOX, which is UO2+PuO2). 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 plutonium were used (>90% Pu-239), only about 5% plutonium would be needed in the mix.
While MOX fuel can be made by grinding together UO2 and PuO2 before the mixed oxide is pressed into pellets, but this process has the disadvantage of forming lots of radioactive dust. An alternative is to mix a solution of uranyl nitrate and plutonium nitrate in nitric acid. This can be converted using a base into a solid. The solid can then be calcined into a mixed uranium plutonium oxide.
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 Pu. In particular, Pu-241 decays to 241Am which is a gamma 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 241Am is a gamma emitter most of the photons it emitts 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 241Pu it contains decays with a short 14.1 year half-life into more radioactive 241Am which makes the fuel difficult to handle in a production plant. Within about 5 years typical reactor-grade plutonium would contain too much 241Am (about 3%) [1].
But it is possible to purify the plutionium 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. Note that the isotopic quaility of the plutonium obtained by this separation process will be better than the original plutonium (which existed before the americium was formed by the beta decay of 241Pu, because much of the 241Pu will have decayed away. Hence in this way it might be possible to obtain better good quaility (with regards to the ingrowth of americium) 239Pu by a repurification of a very old sample of plutonium but unless one was willing to wait for a very long time this sit and wait method will not remove the 240Pu. 240Pu is a neutron poison which can be activated to form 241Pu which leads to further 241Am.
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 irradation of curium generates the higher actinides 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