The USS Henry Clay is in Plymouth Sound for servicing by the Royal Fleet Auxiliary’s RMAS Waterfowl. The attack submarine is a United States Navy Lafayette-class boat developed during the Cold War as a response to the threat of a potential Soviet nuclear missile attack. All NATO nuclear-powered submarines use the SSN and SSBN tags and carry on-board nuclear reactors. Only hunter-killer SSBNs are capable of firing nuclear-tipped ballistic missiles.