The warning comes from the Government's Radioactive Waste Management Advisory Committee, which completed its report more than four months ago and sent it to ministers. But despite repeated requests for publication, the findings have been kept secret.
This is the latest twist to an intense debate going on behind closed doors in Whitehall over whether to grant British Nuclear Fuels Limited (BNFL) permission to start operating its new pounds 2.8bn Thermal Oxide Reprocessing Plant (Thorp).
BNFL maintains that Thorp will not make Britain the world's nuclear dustbin because, since 1976, clauses in its contracts with overseas customers have made them take back radioactive wastes, after the plutonium and uranium has been extracted. However, BNFL intended to start Thorp before it had reached agreement with its customers on how to handle the return of waste.
Unless the British government accepts the need to bury the plutonium-contaminated waste at Sellafield, BNFL's foreign customers will face an extra bill of more than pounds 200m to transport the material back for disposal in their own countries.
BNFL's customers are so confident that they will not need to take the plutonium-contaminated material that they have not bothered to build storage and handling facilities. Even if Britain insisted on returning the material, BNFL would have to store it at least until 2010 to allow its customers time to build facilities.
BNFL has contracts to reprocess nearly 4,700 tonnes of foreign fuel in the first 10 years of Thorp's operations. But a third of this fuel is not covered by return-of-waste contracts, and all the resulting high-, intermediate- and low-level wastes will have to be disposed of in the UK.
For the 3,200 tonnes which are covered by return-of-waste options, BNFL proposes to send back as soon as it can all the high level wastes, in the form of glassified blocks, for disposal by their customers. But instead of returning low- and intermediate-level wastes, the company wants its customers to accept more high-level waste, calculated to be an 'equivalent quantity' in radiological terms.
About 200 cubic metres of high-level wastes from the first 10 years of Thorp will have to be disposed of in Britain anyway. The waste substitution arrangements could significantly reduce this stockpile - by between 18 and 36 cubic metres.