Steve Connor: How a money-making strategy from the 1960s left behind a toxic legacy

Instead of producing 120 tonnes of Mox fuel each year, the plant has produced just 13.8 tonnes since 2002

Bad decisions, poor performance and government subsidies have set the nuclear industry apart from any sector in Britain, except perhaps for banking.

The reason Britain has the biggest waste mountain of civilian plutonium in the world is down to a bad decision in the 1960s when the nuclear industry proposed turning nuclear waste from civilian reactors into plutonium for burning in fast-breeder reactors.

Technical problems meant that these reactors were never developed commercially – the project was finally abandoned in 1994 – but the plutonium stockpile kept growing. It currently stands at 112 tonnes, including 28 tonnes from foreign reactors that must at some point be returned to their owners.

The stockpile from British-owned reactors is currently 84 tonnes but will reach 109 tonnes by the time all the British-made spent fuel at Sellafield has been dealt with by the Thermal Oxide Reprocessing Plant (Thorp) – the biggest building on the Cumbrian site.

Plutonium is one of the most deadly substances known to man. Minuscule amounts can kill if ingested. It can also be used to make nuclear weapons, which is why governments around the world are so concerned about where and how it is stored.

In the 1990s, the industry proposed dealing with the growing plutonium stockpile by "recycling" it as mixed oxide (Mox) nuclear fuel for burning in thermal reactors. The idea, like so many in the nuclear industry, looked good on paper. By mixing relatively small amounts of plutonium dioxide with bigger amounts of uranium oxide from spent nuclear fuel, it would be possible to fabricate Mox fuel rods containing about 7 per cent "recycled" plutonium from reprocessed reactor fuel.

The industry was so confident that it proposed dealing not just with Britain's plutonium waste but with the waste of other countries, in particular Japan which relied heavily on nuclear power.

Sellafield, then operated by BNFL, started making Mox fuel rods at a demonstration facility while it built a new £498m Sellafield Mox Plant (SMP), specifically for foreign customers. The SMP was completed in 1996 but was only given a licence to open in 2002 after Helen Liddell, the then energy minister, had visited Japan to secure a "statement of intent" from Japanese customers who had been spooked by a scandal over falsified data at BNFL's Mox demonstration facility.

Before it opened, BNFL said that the SMP would be able to produce some 120 tonnes of Mox fuel a year and confidently predicted that it would earn millions in foreign exchange for Britain. But critics warned that this was "voodoo economics", essentially because conventional uranium fuel mined from the ground was at least 25 per cent cheaper than Mox.

But the problems were even greater than anyone had predicted. The SMP was dogged by technical failures. An independent investigation by consultants Arthur D Little – the company that had originally said that SMP made sense – found that the plant suffered something like 37,000 breakdowns a year.

Instead of producing 120 tonnes of Mox fuel each year, the plant has to date produced just 13.8 tonnes since it was opened in 2002. It has supplied just one Swiss company with one batch of fuel, fabricated another batch for a German customer that has yet to be delivered and, most importantly, has not made a single fuel rod for its main customers in Japan.

The cost of the SMP to the taxpayer ballooned to £1.34bn and now the Nuclear Decommissioning Authority, which took over the Sellafield site from BNFL in 2005, said that no Mox fuel will be made or delivered to the Japanese power companies until at least the end of this decade. Meanwhile, the SMP is costing the authority nearly £100m a year in ongoing costs.

The total failure of the SMP has had serious repercussions. The Nuclear Decommissioning Authority has had to subcontract some of the Mox fabrication to Areva, the French nuclear power company that operates a Mox plant at Marcoule. This subcontracted work has led to the transport of half a tonne of highly dangerous plutonium dioxide powder from Sellafield to France under armed guard. According to leaked cables from the US Embassy in London, the UK Government believes the Sellafield Mox Plant is one of the biggest failures in British industrial history.