Earlier disposal studies by federal agencies concluded the Canadian Shield's granite was the only rock formation stable enough for the necessary millennia of storage.
But a just-released report from the industry-led Nuclear Waste Management Organization gives a tentative green light to locations like Toronto, London, Hamilton-Niagara, Windsor-Sarnia, Kingston, Ottawa and a swath running from Kitchener-Waterloo to Barrie.
Nuclear mausoleums could be buried a kilometre deep in these areas because the underlying sedimentary rock is low risk for fractures and water seepage, says the NWMO's annual report to the federal government.
"Safety cases for repositories in sedimentary rock appear to be quite strong," the report says.
The NWMO will not study a specific location for the mausoleum until the federal government okays its favoured form of long-term storage.
9 million bundles of waste uranium fuel are now stored temporarily on site at Canada's 22 nuclear reactors, largely in concrete casks inside ordinary metal sheds on the surface. The waste can remain dangerously radioactive for as long as 100 centuries.
More than $800 million has already been spent by Atomic Energy of Canada Ltd., Ontario Power Generation and others since 1978 on research into storing waste fuel bundles deep in the shield. A federal environment assessment a decade ago also backed the shield's granite as the safest place.
But a May 2005 NWMO report to the federal government first raised the distant prospect of instead burying the waste fuel in a type of sedimentary rock underneath southern Ontario and Quebec and along the western shore of James and Hudson Bay.
The 2006 annual report touts this idea more forcefully, saying this is where the organization focused its technical research over the year. The report was released on the NWMO website last week with minimal publicity.
"Work being done in several countries is showing that sedimentary rock is a potentially suitable host rock formation for a deep repository. Switzerland and France are among countries which are focusing their research efforts on sedimentary formations," the report says.
The layered rock is between 470 million and 430 million years old and was created through pressure on dirt and other sediments deposited from lakes and rivers.
By contrast, the crystalline rock of the Canadian Shield was forged under intense heat and pressure.
The nuclear waste organization was set up under federal law to recommend the best long-term management for the waste fuel, almost 90 per cent of which is stored at the Pickering, Darlington and Bruce power stations in Ontario.
But the federal government has not yet responded to the organization's formal November 2005 recommendation for a $24 billion mausoleum, consisting of caverns dug up to a kilometre below ground.
The NWMO did not comment then on a location for the burial grounds, saying only that the emphasis should be on finding a "willing community."
In a statement accompanying the annual report, NWMO president Ken Nash and chair Gary Kluger said that storage plans cannot move ahead until the federal government responds to the proposed solution.
The organization's preferred storage relies on steel-and-copper capsules each holding 324 bundles of waste nuclear fuel, designed to last at least 100,000 years.
The capsules would be sealed in the underground vaults behind water-resistant clay barriers.