Hinkley Point C completes 620 m fish return tunnel beneath Bristol Channel

Tunnelling has been completed on a 620 m fish return tunnel beneath the Bristol Channel at Hinkley Point C, marking another milestone in the marine infrastructure being built for the UK's new nuclear power station.

The tunnel was excavated over five months by the tunnel boring machine Sarah Guppy, which broke through beneath the seabed before being recovered by floating crane. The machine has now been removed for cleaning and future reuse, while work can progress on the remaining hydraulic and mechanical elements needed to bring the fish return system into operation.

The tunnel forms part of a wider marine system designed to reduce the impact of the station's cooling-water intake on fish. Hinkley Point C will draw seawater through offshore intake structures before circulating it through the plant's cooling system. Any fish reaching the screening system will be directed into the return tunnel and carried safely back to the Bristol Channel.

The 1.8 m diameter tunnel was constructed using a pipe-jacking method rather than the much larger TBMs used for the station's main cooling-water tunnels. Hydraulic jacks at the launch site pushed the machine forward while precast concrete pipe sections were progressively installed behind it, meaning excavation and permanent lining advanced together.

The alignment was also more complex than a simple straight drive. The tunnel incorporated vertical curvature and terminated at an offshore concrete structure around 45 m long. Working beneath the Bristol Channel added another layer of difficulty because of its large tidal range and exposed marine conditions, particularly during retrieval of the machine from the seabed.

The fish return tunnel complements two other protection measures planned for the station. Four low-velocity intake heads are designed to allow fish near the openings to swim away from the incoming flow, while an acoustic fish deterrent is intended to guide fish away before they approach the intakes. Together, the three systems are intended to reduce the number of fish entering the cooling-water infrastructure and provide a return route for those that reach the screens.

Sarah Guppy, named by local schoolchildren after the 19th century Bristol engineer and inventor, is considerably smaller than the machines used to excavate the station's principal intake and outfall tunnels. Those larger tunnels extend several kilometres beneath the channel, forming an extensive offshore cooling-water network around the two-reactor development.

With the 620 m drive complete and the TBM removed, attention now shifts to completing the interfaces, equipment and hydraulic connections that will turn the excavated tunnel into an operational part of the cooling-water system. The breakthrough closes one of the more unusual tunnelling packages at Hinkley Point C, where underground construction has extended well beyond the power station itself and several kilometres out beneath the Bristol Channel.