Projects Jan 30, 2025

Supporting at Hinkley Point C Nuclear Power Station – Tunnelling Works

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Worker in orange his vis workwear walking in tunnel

Image: Balfour Beatty

Through a series of tunnels designed by Jacobs, the Hinkley Point C nuclear power station will keep its two reactors cool and operational by drawing in and flushing out enough water to fill 4,200 Olympic swimming pools every day.

EDF is building two U.K. European pressurized reactors at Hinkley Point C as part of a 3.2 GWe nuclear power station in Somerset that will meet approximately 7% of the U.K.’s electricity needs. This will provide low-carbon electricity for around six million homes, increasing energy security, creating thousands of jobs and bringing lasting benefits to the U.K. economy. As the U.K.’s first new nuclear power station for nearly 30 years, it is a critical program that will help the U.K. to meet its net-zero commitments and the United Nations Sustainable Development Goals.

Hinkley Point C will keep itself cool by drawing in and flushing out enough water to fill 4,200 Olympic swimming pools every day. This requires 5.5 miles (8.8 kilometers) of tunnels extending from the power station nearly 100 feet (30 meters) underneath the Bristol Channel with six connections between the tunnels and heads on the channel bed. The Bristol Channel is notable as it has the second highest tidal range in the world.

““These are the first nuclear qualified tunnels to be designed in the U.K. They will have the capacity to transfer 2.7 billion U.S. gallons (10.4 million cubic meters) of cooling water a day.” ”

David Maddison

Offshore Connections Design Manager, Jacobs

Collaborating on complex engineering challenges

Supporting the Balfour Beatty-led team, Jacobs provides detailed design for the cooling water circuit for the two reactors consisting of six onshore mined galleries, three shafts and two tunnel boring machine (TBM) launch adits. The majority of the structures, including each of the 38,000 segments lining the TBM driven tunnels, are nuclear safety classified, a first for a nuclear power plant in the U.K. The main segmentally lined intake and outfall tunnels consist of:

Two intake tunnels, each 2.1-miles (3.4km) long with an internal diameter of 19.7 feet (6m)
One outfall tunnel, 1.1-mile (1.8km) long, with an internal diameter of 23 feet (7m).

Jacobs’ tunneling and ground engineering team and other specialists have provided over 200,000 hours of support from the U.K., U.S. and Australia over a 12-year period. This work included early contractor involvement, detailed design, gaining approval from the ultimate client, Nuclear New Build Generation Company (NNB GenCo), for more than 2,000 deliverables, and providing site coordination and support during the construction phase.

10.4 M

cubic meters of water needed to cool the power station every day - enough to fill 4,200 Olympic swimming pools
9 km

of tunnels, lined by 38,000 nuclear-qualified concrete segments, to take cooling water from and to the Bristol Channel
9 M

metric tons of carbon dioxide emissions Hinkley Point C will offset every year
6 M

homes will be powered with low-carbon electricity by each of Hinkley Point C’s twin 1.6 gigawatt reactors

On site integrated team

Drawing on our global experience in drilling offshore shafts and rock tunneling, our Jacobs team included geologists, hydrologists, ground engineers, seismologists, tunnel designers and more. We designed the permanent works for the onshore galleries, shafts and TBM launch adits, the precast segments for the TBM tunnels, and the temporary and permanent works for the unique tunnel to shaft connections.

Jacobs developed a groundwater risk management strategy to control groundwater ingress, enabling safe, efficient excavation of the tunnel to shaft connections.

The connections between the heads and the segmental tunnels are among the most complex aspects of the design. They posed a unique challenge at a scale that is without precedent anywhere else in the world.

Balfour Beatty lowered six 5,000-metric ton head structures onto the Bristol Channel seabed in 2022. Tunneling works followed to connect the offshore shafts to the tunnels creating a seamless route for the essential cooling water to travel to and from the power station. The team designed and supervised the sequential ground investigation, water ingress assessment and analysis, and subsequent rock fissure grouting works to control ground water ingress into the tunnel excavations, executed by Balfour Beatty and BAM Ritchies. We also designed the temporary excavation sequence and support system to enable the construction of the permanent works. The complex construction sequence was developed collaboratively with Balfour Beatty’s construction team.

The tunnels will soon be ready for the connection to the Bristol Channel to be completed – marking a major step in the construction of the power station’s cooling water system.

Tunnel boring machine lift. Image: Balfour Beatty.
Image: Balfour Beatty.
Offshore shaft construction. Image: Balfour Beatty.

Key team members

David Maddison, Offshore Connections Design Manager

David has been involved in the project for nearly a decade, from a summer student to the design manager of the offshore tunnel to shaft connections. Part of Jacobs’ design team embedded with Balfour Beatty’s construction team, David helped develop the documentation processes required to meet new nuclear build quality assurance requirements.
Oliver Rose, Engineering Geologist

Oliver worked on the groundwater management in the Hinkley Point C offshore tunnel to shaft connections between 2021 to 2023. While on site, Oliver oversaw the ground probing, reviewed results and defined subsequent ground grouting patterns to reduce water ingress to manageable magnitudes to safeguard the tunneling temporary support and personnel.
Phillip Lea, Tunnels and Shafts Design Manager for main Marine Works project

Phillip oversaw the detailed design and nuclear safety substantiation for a range of requirements including the onshore permanent works structures, main segmental tunnels, detailed temporary works design of the offshore connections between the segments and offshore heads, and other works.