BAE Submarine Solutions is a business unit of BAE SYSTEMS' naval systems program. BAE Systems Submarines is the prime contractor for the Astute class of nuclear-powered attack submarines being built for the UK Ministry of Defence. Touted as being more flexible, mobile, and stealthy than previous submarines, the Astute is also powered by an onboard nuclear power plant that requires no refueling. Submarine Solutions is also providing initial in-service support. In addition to building submarines, the unit has reactivated four Upholder-class submarines for the Canadian Navy and built the HMS Bulwark assault ship for the Royal Navy.