The predictions, in reports initially suppressed and only released after a legal challenge, explode the case for the giant pounds 1.8bn dump, which is supposed to keep the waste away from people for hundreds of thousands of years.
Their revelation comes just as the Environment Secretary John Gummer is poised to take the crucial decision on planning permission for the dump.
The problem with the planned dump, or repository, is the leaching to the surface of groundwater contaminated by the 275,000 cubic metres of dangerous nuclear waste it will contain. The firm planning it, Nirex, says its own computer models show the radioactivity would leach out into the Irish Sea and be enormously diluted.
But leading scientists say that its models are "over-simplified and idealised" - two dimensional - based on the assumption that the earth is flat.
Two other reports by Her Majesty's Inspectorate of Pollution (HMIP) both say that detailed, three-dimensional computer modelling of the radioactive water show it returning to the surface in two places - directly above the dump and to the south, near the River Irt.
Dr Shaun Salmon, senior hydrologist at environmental consultancy Aspinwalls, says: "The world is not flat. You cannot model what happens in a three- dimensional world in two dimensions." David Smythe, Professor of Geophysics at Glasgow University, in an e-mail he sent last week to Friends of the Earth from a research ship off Antarctica, said: "Nirex's use of a two- dimensional 'flat earth' approach to model the water flow at the site is completely unacceptable" and that its presentation of the geological structure "is so over-simplified and idealised as to be incredible".
Nirex originally funded HMIP's work, but withdrew its cash in April 1994, while it was still going on. HMIP originally refused to give Friends of the Earth the reports because Nirex had placed restrictions on the publication of data. After the pressure group took legal advice, HMIP persuaded Nirex to allow it to release them "in the light of possible public interest".
Dr Rachel Western, of Friends of the Earth, said yesterday: "These hard- won reports destroy the credibility of the dump. Mr Gummer must now reject this disastrous and absurd plan."Reuse content