The vessel, one of the UK's four Vanguard Class nuclear submarines, was officially handed back to the Navy for operational use on Thursday 21 June 2012. Known as a Long Overhaul Period (Refuel), the work package was an enormous engineering task, with the submarine emerging as effectively a new boat inside. Around 2.2 million man-hours were involved in the overhaul under a £350m contract with Babcock Marine at Devonport Dockyard in Plymouth. The work that was carried out included the installation of improved strategic weapons equipment and the integration of the tactical weapons command systems. A new reactor core - which provides the power for the 15,000-tonne vessel - was also fitted, meaning that HMS Vigilant will be able to function until the end of her operational life without ever having to refuel again.