A leak of what was believed to be hundreds of gallons of water containing tritium was discovered this week from a temporary fix at the Monticello Nuclear Generating Plant, where 400,000 gallons (1.5 million liters) of. water with tritium leaked in November, Xcel Energy said in a statement Thursday. The plant about 38 miles (61 kilometers) northwest of Minneapolis is scheduled to power down Friday so permanent repairs can begin, the company said.