The Energy Department has decided to make a closer study of environmental risks posed by a plutonium refinery being built at the department's Lawrence Livermore National Laboratory near San Francisco, a House panel chairman said. The project is a prototype for a permanent refinery, called the Special Isotope Separation plant, that the department wants to build at the Idaho National Engineering Laboratory, near Idaho Falls. The Livermore study is expected to delay the start of construction in Idaho for at least two years. The Energy Department has called the plant a key to modernization of the nation's nuclear weapons production complex. It would use a new laser technology to convert existing military stockpiles of fuel-grade plutonium to weapon-grade material. Rep. John Spratt, (D-S.C.), chairman of a special Armed Services Committee panel, said Energy Secretary James D. Watkins made the decision in response to an environmental group's threat to sue to force such a study.