Decommissioning the UK's ageing nuclear power stations will cost billions of pounds more than originally expected.
Sellafield's age and lack of records will make decommissioning difficult
In its first report, the Nuclear Decommissioning Authority estimates that £56bn will have to be spent cleaning up 20 sites.
NDA wants to speed up the clean-up, including that of the ageing Magnox plants, from 125 years to 25 years.
It has published consultation plans which will be open until 11 November, with a proposal finalised in December.
NDA chairman Sir Anthony Cleaver said the £56bn estimate was higher than the £48bn figure the four-month-old agency had inherited.
The higher estimate is based on the costs over the whole lifetime of the sites, calculated by the UK Atomic Energy Authority and British Nuclear Fuels.
"It's important to recognise that we're talking about a programme that's running just over a century, so those changes in estimates don't seem very surprising," Sir Anthony said.
He said the consultation document was the "first time anyone has had the opportunity to put together a coherent programme for all 20 of the sites".
The decommissioning of the Sellafield and Dounreay sites was described as "our number one decommissioning priority" by the NDA.
A leak was discovered in April at the Sellafield, in Cumbria, but it could have occurred as long ago as August 2004.
NDA'S 20 NUCLEAR SITES
Calder Hall, Cumbria
Chapelcross, Dumfries and Galloway
Trawsfynydd, north Wales
Wylfa, north Wales
An investigation in June found "significant deficiencies", and ordered improvements to be introduced by October.
Inadequate historic records mean that the precise contents of Sellafield and Dounreay are unknown.
"Consequently, we have yet to choose the best way to retrieve the materials safely and without endangering the environment," NDA said.
Sir Anthony, speaking on BBC Radio 4's Today programme, highlighted the four operating Magnox stations - Dungeness in Kent, Oldbury in Gloucestershire, Sizewell in Suffolk and Wylfa in north Wales - which will all be closed by 2010.
Current plans assume that after initial work for 10 or 15 years, the "more difficult pieces" are dealt with some 70 years later.
"That's leaving the problem for future generations and throughout that period you've got the problem of storage, safety, security," Sir Anthony said.
"We believe it ought to be possible to complete the decommissioning of these stations over a 25-year period."
During the consultation period, NDA staff will host and attend meetings to explain the document and answer questions, the NDA said.
A final strategy is expected to be published next spring.
Among issues that the authority is looking at is the need for an alternative for Drigg in Cumbria, which is the only place in the country where low-level nuclear waste can be stored in perpetuity.
Drigg is about 1km from a shoreline that is eroding at the rate of 1 metre a year.
There is a risk it could flood between 500 and 5,000 years after it is closed.
Former environment minister Michael Meacher expressed concern at the cost of the decommissioning and questioned plans for future nuclear builds.
"Nuclear is neither necessary nor desirable to meet our climate change targets.
"It involves huge economic, military and environmental risks that should be avoided. The risks are the long-term risks of storage.
"Despite strenuous efforts over the last 30 years no government has yet solved the problem of safe long-term storage."
The NDA was set up in April this year under the Energy Act 2004 to take responsibility for the UK's nuclear legacy.
Environmental group Greenpeace said NDA's timeline was shorter than that given as feasible for building a repository for waste.
"The NDA claims a speedier clean-up of sites would lead to cost reductions," nuclear campaigner Jean McSorley said.
"But we're worried that one of the real motives behind the NDA's move for quicker clean-up is to make way for new reactors.
"In our view it would be madness to push for cleaning up sites only to make way for new plants which will create even more waste for decades to come."
THE NDA'S 20 NUCLEAR SITES
Source: Nuclear Decommissioning Authority