A nuclear plant built at a cost of £470m to provide atomic fuel to be used in foreign power stations has produced almost nothing since it was opened six years ago, the government has admitted.

The mixed oxide (Mox) facility at Sellafield in Cumbria - which was opposed by green groups as uneconomic - was originally predicted to have an annual throughput of 120 tonnes of fuel.

The energy minister, Malcolm Wicks, has admitted in response to a parliamentary question that it had managed only 2.6 tonnes in any one 12-month period between 2002 and 2006-07.

In the four years before 2002, the plant had produced annual figures respectively of 2.3 tonnes, 0.3 tonnes, 0 tonnes and 0 tonnes.

The technical difficulties at the facility and the failure to get anywhere close to its financial targets will add to concerns about the economics of nuclear power, following the government's decision to give the green light to a new generation of atomic reactors.

Wicks described the Sellafield Mox plant (SMP) as being based on "largely unproven technology" and pointed out that its estimated annual output had been reduced by 2001 to 72m tonnes.

British Nuclear Group (BNG), which operates the Sellafield site, said a range of improvements were being made to the facility but it admitted that the 2007-08 period had again seen production disrupted by various problems.

The SMP was designed to make new fuel from the recycled uranium and plutonium recovered from used nuclear fuel, which had been reprocessed by the nearby thermal oxide reprocessing plant (Thorp) at Sellafield. A Mox demonstration complex was opened in 1998 but was hit by a scandal involving quality control and the falsification of documents, which led to the resignation of John Taylor, chief executive of BNFL.

Attempts to open the main SMP facility led to high court challenges by Greenpeace and Friends of the Earth, which argued that the government's decision to allow BNG's parent group, BNFL, to proceed with opening the facility was unlawful under European law. The Irish government also took unsuccessful legal action to stop the SMP opening over concerns about radioactive effluent from the plant polluting the Irish Sea.

Jean McSorley, a nuclear campaigner at Greenpeace, said the Mox plant was - along with Thorp - "another great failure of British nuclear engineering" and pointed to the dangers of accepting the industry's economic models and promises. She pointed out that Thorp had been shut for the past three years because of an accident and continual attempts to reopen it had been thwarted by further problems.

A spokeswoman for BNG, the operating division of BNFL, said production was still being "ramped up" and the performance of the plant was the subject of a detailed improvement plan. She said the problems at the SMP were not related to the difficulties at Thorp.

"We had to overcome a number of technical issues and make engineering improvements to the [SMP] plant as part of the normal commissioning process. We made improvements to plant maintenance, equipment reliability and have installed upgraded equipment as required," she said.

"We are awaiting suitable plant availability to demonstrate the benefits of these capacity enhancements. Throughput in 2007-08 was adversely affected by the extended outage for the fuel campaign change. We remain committed to meeting our customers' Mox fuel requirements."

BNG has been forced to meet the needs of Swiss and other contracted customers for Mox fuel through buying alternative supplies from France and Belgium.

With the £470m construction costs written off, the plant was assessed by government-appointed consultants in 2001 to have a net positive value of only £216m - a value that was partly based on winning back Japanese business, which proved hard after the falsification of quality-assurance data in 1999.