MOX is a means to "burn" the plutonium remaining in spent reactor fuel to provide energy and make electricity.
MOX provides about 2 percent of the new fuel used today, but this proportion is increasing.
MOX also provides a means of burning weapons-grade plutonium (from military sources) to produce electricity.
In every nuclear reactor 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. (Very small quantities of Pu-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 that discharged from a reactor is plutonium, and some two thirds of this is plutonium, and some two thirds of this is Pu-239. Worldwide, almost 100 tones of plutonium in spent fuel is generated each year.
A single recycle of plutonium increases the energy derived from the original uranium by some 17 percent, and if the uranium is also recycled this becomes about 30 percent.
Recycling Fuel:
The first step is separating the plutonium from the remaining uranium (about 96 percent of the spent fuel) and the fission products with other wastes (together about 3 percent). 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, UO2+PuO2).
MOX fuel, consisting of about 7 percent plutonium mixed with depleted uranium, is equivalent to uranium oxide fuel enriched to about 4.5 percent U-235, assuming that the plutonium has about 65 percent Pu-239.
If weapons plutonium were used (>90 percent Pu-239), only about 5 percent Pu would be needed in the mix.
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 Am-241 which is a strong 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.
The Am-241 level in stored plutonium increases about 0.5 percent per year. Pu-239, Pu-240 and Pu-242 are long-lived and hence little changed with prolonged storage.
Pu-238 becomes significant in high-burnup fuel. It is a strong alpha emitter and a source of spontaneous neutrons.
While fast neutron reactors allow unlimited recycle of plutonium, since all isotopes there are fissionable, in thermal reactors isotopic degradation limits the plutonium recycle potential.
(Recycled uranium from a reprocessing plant is re-enriched on its own for use as fresh fuel. Because it contains some neutron-absorbing U-234 and U-236, the enrichment level is slightly greater than for mined uranium providing equivalent fuel.)
MOX Use:
MOX was first used in a thermal reactor in 1963, but did not come into general use until the 1980s.
Today MOX is widely used in Europe and is planned to be used in Japan. it is loaded in over 30 European nuclear reactors, while 20 more are licensed for it or are in the process of being licensed.
Most use it as about one third of their core, but some will accept up to 50 percent MOX assemblies. France 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 which will have a complete fuel loading of MOX.
The use of MOX does not change the operating: characteristics of a reactor, though the plant must be designed or adapted slightly to take it. More control rods are needed.
For more than 50 percent MOX loading, significant changes are necessary and a reactor needs to be designed accordingly.
An advantage of MOX is that the fissile concentration of the fuel can be increased easily by adding a bit more plutonium, whereas enriching uranium to higher levels of U-235 is relatively expensive.
As reactor operators seek to burn fuel harder and longer, increasing burn up from around 30,000 MW days per tone a few years ago to over 50,000 MWd/t now, MOX use becomes more attractive.
With low uranium prices, reprocessing to separate plutonium for recycle as MOX is not itself economic, but coupled with reducing the volume of spent fuel to be managed, it becomes so.
Seven UO2 fuel assemblies give rise to one MOX assembly plus some vitrified high-level waste, resulting in only about 35 percent of the volume, mass and cost of disposal.
MOX Production:
Three plants currently produce commercial quantities of MOX fuel. Two are in France, one in Belgium, and a fourth (120 t/yr) is being commissioned in UK.
Currently about 190 tones per year of MOX is produced, incorporating 10-12 tones of plutonium. MOX production capacity is expected to reach 400 t/yr soon after 2000, using 25-30 tones of plutonium. Since 1963, about 400 tones of plutonium has been used in MOX.
A multinational consortium is building a MOX plant in Russia, particularly to utilise weapons-grade plutonium (at 2 yr/yr), and a similar plant is proposed in USA.
These will enable ex military plutonium to be permanently destroyed as it is "burned" in reactors.
MOX is also used in fast neutron reactors in several countries, particularly France and Russia. It was first developed for this purpose, with experimental work being done in USA, UK, France, Germany, Belgium and Japan.
At present the output of reprocessing plants exceeds the rate of plutonium usage in MOX, resulting in inventories of plutonium in several countries.
These stocks are expected to reach nearly 200 yr before they start to decline after 2005 as MOX use increases. By 2010 production and use of plutonium in MOX are expected to be more in balance unless MOX fuel is itself reprocessed.
Source: www.uic.com.au
© 2001 Mena Report (www.menareport.com)
