Introduction

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

Normally, with the fuel being changed every three years or so, about half of the ²³⁹Pu is "burned" in the reactor, providing about one-third of the total energy. It behaves like ²³⁵U and its fission releases a similar amount of energy. The higher the burn-up, the less fissile plutonium remains in the used fuel. Typically, about one percent of the used fuel discharged from a reactor is plutonium, and some two-thirds of this is fissile (~ 50% ²³⁹, 15% ²⁴¹Pu). Worldwide, some 70 tonnes of plutonium contained in spent fuel arises each year.

A single recycle of plutonium in the form of mixed oxide fuel (MOX) increases the energy derived from the original uranium by some 12%, and if the uranium is also recycled this becomes about 22%.

Recycling Fuel

The first step in recycling fuel 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 about 7-9% plutonium mixed with depleted uranium, is equivalent to uranium oxide fuel enriched to about 4.5% ²³⁵U, assuming that the plutonium has about two-thirds fissile isotopes. If weapons-grade plutonium were used (>90% ²³⁹Pu), only about 5% plutonium 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 plutonium isotopes. In particular, ²⁴¹Pu (half-life 14 years) decays to americium-241 (²⁴¹Am), a strong gamma ray emitter, giving rise to a potential occupational health hazard if separated plutonium over five years old is used in a normal MOX plant. The ²⁴¹Am level in stored plutonium increases about 0.5% per year, with corresponding decrease in fissile value of the plutonium. ²³⁸Pu (half-life 88 years) is increased in high-burnup fuel. It is a strong alpha ray emitter and a source of spontaneous neutrons. ²³⁹Pu, ²⁴⁰Pu and ²⁴²Pu are long-lived and hence little-changed with prolonged storage.

Fast neutron reactors allow multiple recyclings of plutonium, since all transuranic isotopes there are fissionable, but in thermal reactors isotopic degradation limits the plutonium recycling potential and most spent MOX fuel is stored pending the deployment of more effective fast reactors. The plutonium isotopic composition of used MOX fuel at 45 GWd/tU burnup is about 37% ²³⁹Pu, 32% ²⁴⁰Pu, 16% ²⁴¹Pu, 12% ²⁴²Pu and 4% ²³⁸Pu.

Recycled uranium from a reprocessing plant is re-enriched on its own for use as fresh fuel. Because it contains some neutron-absorbing ²³⁴U and ²³⁶U, the enrichment level is slightly greater than for mined uranium providing an equivalent fuel source.

MOX Use

A MOX fuel assembly for a pressurized water reactor.

MOX was first used in a thermal reactor in 1963, but did not come into commercial use until the 1980s.

Today, MOX is widely used in Europe and is planned for future use in Japan. Currently over 30 reactors in Europe (Belgium, Switzerland, Germany and France) are using MOX. Japan also plans to use MOX in up to 20 of its nuclear reactors. Most reactors use it as about one-third of their core, but some will accept up to 50% MOX assemblies. France aims to have all its 900 MWe series of reactors running with at least one-third MOX in the future. 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. Advanced light water reactors, such as the European Pressurized Water Reactor (EPR) or Westinghouse Electric Company's AP1000, will be able to accept complete fuel loadings 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 allow for its use. For example, more control rods are needed, and for more than 50% MOX loading, significant changes are necessary and a reactor must be designed accordingly.

An advantage of powering a nuclear reactor with 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 ²³⁵U is relatively expensive. As reactor operators seek to burn fuel harder and longer, increasing burnup from around 30,000 MW days per tonne a few years ago to over 50,000 MWd/t now, MOX use becomes more attractive.

When uranium prices were low, reprocessing to separate plutonium for recycle as MOX was not itself economic, but with the rise in uranium prices coupled with reducing the volume of spent fuel to be managed, it is becoming so. Seven UO₂ fuel assemblies give rise to one MOX assembly, as well as vitrified high-level waste, resulting in only about 35% of the volume, mass and cost of disposal.

MOX Production

Four plants currently produce commercial quantities of MOX fuel. Two are in France, one in Belgium, and a fourth (intended to be 128 t/yr) was commissioned in UK in 2001. In 2005, about 200 tonnes of MOX was produced, incorporating 12-14 tonnes of plutonium. MOX production capacity is presently around 225t/yr, using 15-18 tonnes of plutonium. Since 1963, over 400 tonnes of plutonium has been used in the production of MOX.

In 2006, the Belgian plant is expected to close and the French Melox plant is expected to be licensed to 195 t/yr capacity. Also, the Sellafield MOX plant in UK was downrated from 128 to 40 t/yr. Japan is planning to start up a major MOX plant by 2011.

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, Russia, UK, France, Germany, Belgium and Japan. Today, Russia leads the way in fast reactor development and has long-term plans to build a new generation of fast reactors fuelled by MOX. The world's largest fast reactor, the 800 MWe BN-800, is currently under construction at Beloyarsk, in the Ural Mountains of Russia, and due to start up in 2010.

At present, the output of reprocessing plants exceeds the rate of plutonium usage in MOX, resulting in inventories of (civil) plutonium in several countries. These stocks are expected to exceed 250 tonnes before they start to decline after 2010 as MOX use increases, with MOX then expected to supply about 5% of world nuclear reactor fuel requirements.

MOX reprocessing

Used MOX reprocessing has been demonstrated since 1992 at the La Hague nuclear facility in France. In 2004, the first reprocessing of used MOX fuel was undertaken on a larger scale with a continuous process. Ten tonnes of MOX irradiated to about 35,000 MWd/t and a plutonium content of about 4% was involved. In this process, the main problem of fully dissolving PuO₂ was overcome.

MOX & disposition of weapons plutonium

For disposal of weapons-grade plutonium, special arrangements are envisaged. A multinational consortium is to finance the construction of a MOX plant in Russia (Tomsk, Siberia), particularly to utilize Russian weapons-grade plutonium (at 2 t/yr), and a similar plant is to be built in USA (at the Savannah River Site, South Carolina). These facilities will enable ex-military plutonium from disarmament to be permanently destroyed as it is "burned" in reactors as MOX. Meanwhile, the first fuel assemblies with MOX from US weapons plutonium and fabricated at the Melox plant in France are being burned on a trial basis in USA.

The objective of this MOX fuel program is to destroy one-third of weapons-grade plutonium and convert the remainder to reactor-grade plutonium (with significant levels of plutonium-238 & plutonium-240), radically degrading its isotopic quality and potential for weapons.

Plutonium-thorium fuel

Since the early 1990s, Russia has operated a program to develop a thorium-uranium fuel, with more recent emphasis on the utilization of weapons-grade plutonium in a thorium-plutonium fuel. The program is based at Moscow's Kurchatov Institute and involves the US company Thorium Power and US government funding to design fuel for Russian VVER-1000 reactors.

Further Reading

• European Nuclear Society. The European Pressurized Reactor (EPR).
• International Nuclear Safety Center. Third-Generation VVERs.
• International Nuclear Safety Center. Overview of Fast Reactors in Russia and the Former Soviet Union.
• U.S. Energy Information Administration. New Reactor Designs.
• Westinghouse Electric Company. The AP1000 Nuclear Reactor.