MOX is an acronym for Mixed OXide fuel. It is fuel for nuclear reactors, composed of mixed oxides of plutonium and uranium. The two elements are reclaimed from spent fuel that has already been through a reactor cycle, become irradiated and is, conventionally, useless.

The fissile material inside conventional fuel rods in a reactor is uranium with an atomic mass of 235 (U235). This releases neutrons in a self-sustaining or chain reaction and generates heat. Reactor fuel is enriched so that around 5-6% is fissile U235. This compares with weapons grade, highly enriched uranium which contains up to 95% U235. However, most of the fuel mass in a reactor is a different uranium isotope, U238. Some U238 is also formed when nuclei capture neutrons and gain atomic mass. U238 inhibits the chain reaction by absorbing neutrons. This absorption causes it to gain atomic mass.

When it gains a neutron, the U238 can become a different element, plutonium (Pu). As it has an extra neutron, the atomic mass of the new isotope is 239 - Pu239. This is also a fissile material and, for the purposes of a reactor, has similar properties to U235. However, not all of it is 'burnt' in a typical reactor cycle. Around two-thirds of one per cent of spent fuel is Pu239 - it doesn't sound like a lot, but worldwide this adds up to around 100 tonnes of plutonium produced every year.

So what to do with all this plutonium? It has been sitting in a reactor vessel for around three years, so it is highly radioactive. It is top of the shopping list for antisocial non-state actors like Mr. bin Laden and friends. There are large stockpiles of plutonium in many countries, simply as a by-product of normal energy generation.

One option is to reprocess it into MOX fuel, so that it can be used again to generate more electricity. In 2003, there were plants capable of this process in use in three countries; the United Kingdom, Belgium and France. The USA, Russia and Japan expect to have MOX fabrication facilities operational by 2005.

However, the idea of MOX fuel is not new. The earlier fast reactor programmes in the UK, USA and France all used MOX fuel. The thinking then was that using fast reactors (also known as breeder reactors) would produce more Pu than was consumed. This was shown to work in practice, but the reactors were too expensive to be commercially viable. MOX fuel has been produced for these fast reactors since the early 1970s. Indeed, Britain's first MOX plant has already been decommissioned.

The ending of the fast reactor programmes left a stock of Pu which had the potential for large-scale electricity generation. This was reprocessed into a different blend of MOX fuel designed for use in reactors already in service. The design of the reactors means that the MOX will not generate more Pu in a reactor cycle. By 'burning' the plutonium this way, international stockpiles of Pu can be depleted if not eliminated entirely.

The UK's MOX facility is at the Sellafield site in Cumbria. The acronym-heavy site has two areas which produce MOX fuel: the MOX Demonstration Facility (MDF), designed to produce commercial quality fuel for pre-production purposes, and the Sellafield MOX Plant (SMP), intended for bulk supply of MOX fuel.

As of January 2004, BNFL's Sellafield MOX operations have been stricken with catalogues of problems preventing them from reaching full production. In 1999, a BNFL quality control inspector noticed similarities between pellet diameters from two different batches bound for a Japanese customer. When this was duly reported, the Nuclear Installations Inspectorate acted in a manner reminiscent of the proverbial tonne of bricks, and the MDF was shut down pending a full investigation. When it was discovered that plant operators had measured one set of pellets and then copy and pasted the results to the second batch, this resulted in the widely publicised and profoundly embarrassing return of fuel that had already shipped to Japan's Takahama reactor complex. Heads rolled.

Although the £472m Sellafield MOX Plant has a series of orders to fulfil from Japanese, German, Swiss and Swedish customers, it has still to deliver a single successful shipment of MOX fuel.