It is possible to actually design fission reactors to efficiently "breed" Pu-239 fuel while "burning" U-235. Such reactors in essence create nuclear fuel as they consume it. Although a number of experimental breeder reactors were built in this country they were plagued by problems, not the least of which were concerns regarding the safety of using highly toxic plutonium in commercial settings. Currently breeder technology is not in use in the United States. There are a few operating commercial breeder power reactors abroad.