New Dreadnought-class submarines are set to replace the ageing Vanguard fleet as the future of the UK’s Continuous At Sea Deterrent (CASD).The Royal Navy has maintained an unbroken at-sea deterrent patrol for more than half a century, but the submarines that currently carry the UK’s nuclear warheads are nearing the end of their service lives.The Dreadnought-class will take over that role.