HOUSTON, Jan 14 (Reuters) - A hydrocracking unit and a jetfuel hydrotreating unit were shut on Monday night after a smallfire at BP Plc's 240,000 barrel per day (bpd) LosAngeles-area refinery in Carson, California, according to WestCoast refined products trade sources. A BP spokesman declined to discuss operations at the Carsonrefinery. A hydrocracker uses hydrogen under high pressure to makemotor fuels, increasing a refinery's output, especially fromcheaper, heavy, sour crude oil. A hydrotreater uses hydrogen toreduce sulfur content in motor fuel. (Reporting by Erwin Seba; Editing by Kim Coghill)