Cameco, the world's largest uranium producer, has told the Canadian nuclear regulator that its refinery might have leaked uranium, arsenic and fluorides into Lake Ontario. A section of the Port Hope, Ontario, plant of Cameco, the world's largest uranium producer. The plant at Port Hope, Ontario, across the lake from Rochester and down the shore from Toronto, first refined uranium for the Manhattan Project during World War II. It has been temporarily closed since July to remove contaminated soil. A spokesman for Cameco, Lyle Krahn, said Wednesday that a computer model created for the cleanup, which is several months behind schedule, indicated that the radioactive and toxic materials have been polluting a harbor adjacent to the factory. The harbor leads directly to the lake. The company notified the regulatory agency, the Canadian Nuclear Safety Commission, about the finding at a meeting last week and now plans drilling tests to confirm the contamination and to measure its extent. "We're anticipating that material may have been entering the harbor,' Mr. Krahn said, adding that Cameco did not know how long it would take to confirm any possible pollution. A spokesman for the agency, Aur