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.