Thousands of containers of lethal nuclear waste are likely to fail before being safely sealed away underground, a devastating official report concludes.
The unpublicised report is by the Environment Agency, which has to approve any proposals for getting rid of the waste that remains deadly for tens of thousands of years.
The document effectively destroys Britain's already shaky disposal plans just as ministers are preparing an expansion of nuclear power.
It shows that many containers used to store the waste are made of second-rate materials, are handled carelessly, and are liable to corrode.
The report concludes: "It is cautious to assume a significant proportion will fail." It says computer models suggest up to 40 per cent of them could be at risk.
Britain's leading expert on nuclear waste yesterday called the report "devastating" and Peter Ainsworth, the Conservative environment spokesman, said he would write to ministers to urge them to "make changes to ensure public safety". He added: "Such a warning from the Environment Agency must be taken extremely seriously. The failure of just one container could prove catastrophic."
The report says that "tens of thousands" of containers of immensely dangerous waste, bound in concrete, are simply being stored above ground, mainly at Sellafield, while the Government and the nuclear industry decide what to do with them. On present plans it is assumed they will remain there for up to another 150 years before being placed in a repository underground. It will take another 50 years to fill the repository, which will then remain open for another 300 years, while the waste is monitored, before being sealed up and buried.
Officially, containers are designed to last for the full five centuries before the repository is closed. But the Environment Agency report questions whether this is "realistic" and says there is an "absence of robust arguments which demonstrate that this target is achievable in practice".
It suggests that the containers are not made of the kinds of stainless steel best able to resist corrosion and questions whether the types used are "fit for purpose over an extended time period".
It reveals that their internal surfaces are not treated to remove vulnerabilities to corrosion, and that some have seals "that are not expected to be durable over periods of hundreds of years". It also discloses that some operators have touched the steel drums with their bare hands, although the rules require gloves, depositing sweat that can also lead to corrosion.
Tens of thousands of containers already in store have been produced to less exacting specifications, which do not even attempt to make them safe for the necessary 500 years. The report adds that the implications of this do not seem to have been "fully considered". Some 17,000 containers in storage contain a kind of nuclear waste that reacts with cement and so is expected to fracture the concrete encapsulating it within 140 years.
Computer models show, the report says, that 40 per cent of the containers could fail within 1,000 years, and that under "certain scenarios" this timescale could shrink to "less than 200 years". It concludes: "It is not clear how package integrity during storage can be assured over the extended timescales now being suggested."
Yesterday, Professor Gordon MacKerron, who until recently chaired the Government's official Committee on Radioactive Waste Management, called the report "devastating". He said that it should prove a "nail in the coffin" of proposals to keep the waste accessible for hundreds of years. He said: "If we are going to dispose of the waste, this should be done as quickly as reasonably possible."
The Nuclear Decommissioning Authority will consider the report this week.