By David Shukman
BBC News environment correspondent
Four nuclear sites in west Cumbria are being decommissioned
The cost of cleaning up the UK's ageing nuclear facilities, including some described as "dangerous", looks set to rise above £73bn, the BBC has learned.
A senior official at the Nuclear Decommissioning Authority said the bill would rise by billions of pounds.
Nineteen sites across the country, some dating from the 1950s, are due to be dismantled in the coming decades.
A spokesman for the Department for Business said it was ready for an adjustment in the clean-up costs.
In January, the National Audit Office said that the cost of decommissioning ageing power sites had risen from £12bn to £73bn.
Radioactive decay
At the largest site, Sellafield, on the Cumbrian coast, I saw for myself one of the "ponds" in which an unknown mass of radioactive material was dumped in the 1950s.
Beneath the unruffled surface of the water lies an unrecorded collection of rusting metal containers holding radioactive waste, including spent fuel rods.
Beside it, workers are constructing a vast new building to handle the material when a retrieval operation eventually gets under way.
Speaking to the BBC, Jim Morse, a senior director at the authority, said of the projected cost: "I think it's a high probability that in the short term it will undoubtedly go up.
"We've still a lot to discover, we haven't started waste retrieval in those parts of the estate where the degradation and radioactive decay has been at its greatest."
When asked if the cost increases could run into billions of pounds, Mr Morse said: "I'm sure it'll be some billions, I really don't know.
"No-one's done this before. It's very difficult to find another measure. There's nothing in engineering terms that allows you to extrapolate from what you have today."
A pilot project to investigate the task of dismantling has been under way for years at one of the most distinctive landmarks at Sellafield - the giant sphere of the 1950s-era Windscale Advanced Gas-cooled Reactor.
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Discovering the extent of Sellafield's nuclear legacy
We were allowed inside - dressed in hard hats and white overalls - passing through a steel corridor into the vast dome of the reactor's containment housing.
Down below us, behind heavy shielding, robotic tools have been used to break up the most contaminated elements which are then extracted and sealed inside giant concrete blocks.
For the moment the blocks - along with all nuclear waste - are stored in temporary facilities while the government decides where and when to build a long-term underground dump.
A spokesman for the Department for Business, Enterprise and Regulatory Reform suggested the government was ready for the clean-up's costs to rise.
"As the NDA continues its work to establish - for the first time - the scale of the challenge, the assessment of the costs involved will naturally need adjusting," he said.
The spokesman also added that the next generation of nuclear power stations would produce less waste than the older ones, and that the power generators would be obliged to pay for the costs.
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