Decommissioning Britain's 19 former power stations is likely to cost £73bn, rather than the estimated £63bn, a new National Audit Office report has found.
BBC Wales Sustainability Correspondent Iolo ap Dafydd visited a new nuclear storage site at the former Trawsfynydd nuclear power plant.
Hidden behind the two hulking 70m (230ft) high old reactors at the Trawsfynydd site, is a brand new
Later this year it will used to store concrete crates of intermediate nuclear waste from when electricity was produced at the former atomic power station.
Up to 360 such crates, weighing up to 42 tonnes can be stored there.
On the other side of this expensive nuclear warehouse, there will be room for 2,400 drums of resin waste which was contaminated in the cooling process of producing power.
Since 1993 the decommissioning work at the site has continued.
The 45-acre area won't be totally cleaned until 2098.
The contractors, Magnox North who carry out the clean-up for the Nuclear Decommissioning Authority (NDA), used 30 tonnes of reinforced concrete, and dug down to the bedrock to secure the storage's foundation.
The storage unit is almost the size of a football field
Simon Lewis, the senior project manager on site, said: "The facility itself is incredibly safe. It's...a very, very large concrete structure, the walls are between 2ft - 3ft thick.
"We've put something like 30,000 tonnes of reinforced concrete in here. It's incredibly safe.
"It's gone through an enormous amount of stringent design, it complies with all the legislation that's required. It's designed to last up to 150 years."
Right in the middle of the Snowdonia National Park, the storage design had to conform to its environment. Severe weather and a higher than average rainfall are other considerations.
Other nuclear waste storage units are being built elsewhere in Britain, but this one at Trawsfynydd will be the first to actually receive and store local waste.
Almost the size of a football field - 92m (302ft) long and almost 30m (98ft) high, all of the former nuclear power's intermediate and low-level radioactive waste can be stored here for up to 40 years.
Or at least until a national nuclear repository centre can be found, and then be prepared to store all of Britain's dangerous waste.
Hywel Madog Jones who has worked at the Trawsfynydd site since the 1970s, is the site's safety case officer.
"In order to build this building, we went through a public inquiry and one of the planning conditions we have on this store is that we're not allowed to import waste from other sites.
"It's for use for Trawsfynydd waste only and that's a legal condition."
Earlier this week, the actual costs of nuclear decommissioning were highlighted in the National Audit Office report to the NDA.
There is room for 2,400 drums of resin waste
With costs between 2005 and 2007 estimated costs will be £12bn pounds higher than expected.
In the audit office report, the Trawsfynydd site's decommissioning costs are 23% higher and the final clean-up bill will be closer to £1.5bn.
Haf Elgar works for Friends of the Earth Cymru. Asked about the £12bn rise identified in the report, she said: "The cost was high in the first place, and has risen each year.
"I believe that this shows that we do not know what the future holds. It is going to take years to deal with nuclear waste."
The consultation for new nuclear power stations has already been announced by the Business Secretary John Hutton, as one proposal of producing enough power for Britain in the next 10 to 20 years, in a clean and carbon-free way.