The achievement is the latest sign that lessons learned from Britain’s first new nuclear plant in a generation are accelerating delivery of the twin-reactor project.

The giant reactor vessel was lifted into a temporary elevated platform at side of Unit 2 reactor building by Big Carl, the world’s largest crane.

It was then slid along rails through a giant entry hatch into the reactor house before being rotated upright and lowered into its final position with just 40mm clearance on either side.

Once transported into the building through a specially designed portal in the side of the reactor house, the 13m-long vessel was lifted and rotated into a vertical position by the reactor building’s internal polar crane before being carefully lowered onto its support ring.

While the first reactor vessel was installed using a large temporary overhead lifting system, the redesigned Unit 2 operation reduced the need for extensive temporary works, cutting time, cost and disruption on site.

The latest milestone comes less than a year after the steel dome was installed on top of the second reactor building.

EDF said Unit 2 is now progressing 20-30% faster than Unit 1 and is significantly further advanced than the first reactor was at the same stage of construction.

More equipment has already been installed, additional structural steelwork has been completed and the outer containment layer is already in place.

Three major heat exchangers have also been fitted in Unit 2, compared with none at the equivalent stage on Unit 1.

Project leaders say the gains are being driven by construction experience, repeatable processes and innovations developed during the first reactor build.

Simon Parsons, Hinkley Point C’s delivery director, said: “This marks a tremendous achievement by the entire team and one that has taken months of planning and close coordination between the 10 main contractors involved.

“We’ve also seen strong innovation to achieve not just a ‘cut and paste’ from the first reactor’s installation, but using our experience to save time, money and disruption to the site.

“Importantly, we are also applying those lessons to put Unit 2 well ahead of the first unit’s position at the equivalent stage, with more materials in place and more work achieved.”