Britain on Monday namedengineering contractors Babcock and U.S. group Fluor as the preferred bidders for a14-year, 7 billion pound ($11.65 billion) contract to manage the decommissioning ofits nuclear sites, Reuters reported.The contract, which includes some of Britain's oldest nuclear power sites such asHinkley, Sizewell and Dungeness, is one of Britain's largest and most valuable publiccontracts put out to tender.Shares in Babcock were the biggest gainer on the FTSE 100 Index at 0936 GMT, up 3.5percent.