The report, obtained by the Independent on Sunday, was prepared by a respected firm of technical consultants for HM Inspectorate of Pollution (HMIP), and casts grave doubt on the long-term safety of the underground repository project.
It is certain to embarrass UK Nirex, the nuclear industry's waste disposal company, which picked Cumbria at the end of a desperate search for an acceptable site for a store. A public inquiry is under way into the first stage of the project, the building of an underground laboratory to test the rocks at depth.
The new calculations, carried out by consultants W S Atkins, show that even 25,000 years into the future, so much plutonium could have leaked out of the repository to contaminate drinking water that local inhabitants could receive a radiation dose of 9 Sieverts each a year. The current limit set by the Government for the general public is one- thousandth of a Sievert a year, and no single new installation should expose people to more than a third of that.
Yet in a submission to the inquiry, Nirex's manager for science, Alan Hooper, claims that "in assessment studies, plutonium-239 is found to return to the surface environment in such low concentrations as to make no significant contribution to radiological risk".
According to Mark Tyrer, one of the authors of the consultants' report, Nirex has failed to make allowance for an important chemical process known as colloidal transport. Although plutonium will not dissolve in water, it can be transported by underground water as part of a colloidal suspension - a dispersion of extremely small particles in the liquid. Common starch solution is one example of a colloid.
"The Nirex case glosses over the possibility of colloidal transport," Dr Tyrer said. At least two chemicals known to give rise to colloids would be present in the repository, he pointed out: ferric flocs, removed from heat exchanger plant, and saccaric acid, a breakdown product of cellulose.
"I believe this is an important and significant finding. The importance is not that in 25,000 years' time there will be 9 Sieverts at Grange-over- Sands, the important thing is that we believe there is a mechanism by which the plutonium dose could be raised significantly. We must study this further," he said.
Tom Curtin, on behalf of Nirex, said the company had received a copy of the plutonium study in August, after Dr Hooper's written evidence had been submitted to the inquiry.
"As far as we are concerned it's been deliberately slanted to find an unrealistic case," he said. "It has no basis in reality. We take deep issue with this particular report. They did this exercise knowing the results they expected to get."
A spokesman for HMIP said: "These studies and reports were not designed to inform HMIP as to the suitability of the site. We cannot proceed to consider that until a formal application has been made, under the Radioactive Substances Act, for the repository itself."Reuse content