Image copyright Sellafield LTD Image caption Estimates suggest that the taxpayer spends £80m a year to store it safely and stop it from falling into the wrong hands

The UK is sitting on a plutonium stockpile that represents "thousands of years" of energy in the bank, according to a leading nuclear scientist.

Tim Abram, professor of nuclear fuel technology at the University of Manchester, made the comments at a briefing to discuss the fate of the UK's plutonium.

The Sellafield nuclear plant in Cumbria has around 140 tonnes of the material.

It is now the largest stockpile of civil plutonium in the world.

The government is yet to decide on its fate, although the Department of Energy and Climate Change (DECC) stated in 2013 that rather than being disposed of, its preference was that it should be reused as fuel.

This is not, however, a straightforward process; it requires new nuclear reactors to be built that are capable of using plutonium as fuel.

Full circle

Plutonium is extracted from reprocessed nuclear waste and was originally stockpiled as a source of fuel for a new breed of experimental nuclear reactors.

But in the 1990s, the government-backed programme of research to develop these new reactors was cancelled, on both cost and safety grounds.

This left Sellafield storing plutonium with no long-term plan for it.

Image copyright Sellafield Ltd Image caption Barrels containing nuclear waste are stored in a pool to keep them cool before they are reprocessed at Sellafield

It also, somewhat ironically, put new nuclear reactor technology back on the government's list of priorities.

DECC tasked the Nuclear Decommissioning Authority (NDA) with assessing the technical, safety and economic pros and cons of the three "credible" types of new generation nuclear reactor that would allow the plutonium to be used as fuel.

The NDA said it was still "in the middle" of this complicated consultation.

"A decision is expected to be made by ministers on how to proceed during 2015/16," the authority said in a statement.

"However, only when the Government is confident that its preferred option could be implemented safely and securely, in a way that is affordable, deliverable and offers value for money, will it be in a position to proceed."

Professor Abram added: "Having [a store of] separated plutonium without a declared end use represents a poor international example.

"We should at least keep the process moving forward and not give the impression to the world that we have stalled."