News

Aug 02, 2023

Future of Energy

Significant milestones have been achieved on the Hinkley Point C project over

Significant milestones have been achieved on the Hinkley Point C project over the past six months, with offshore works having reached the final stage.

The first new nuclear reactor vessel for a UK power station for more than 30 years arrived in February on the Somerset construction site of the Hinkley Point C power station.

The 13m long, 5.5m diameter, 500t reactor pressure vessel is a high strength steel cylinder. It will contain the nuclear fuel and the chain reaction needed to make the heat which will produce the steam to drive one of the plant's turbines.

EDF Energy's Hinkley Point C, which is now estimated to cost £32.7bn, will have two European pressurised reactors with a combined capacity of 3.26GW. It is being built and will be operated by EDF Energy's subsidiary Nuclear New Build Generation Company.

The power station will produce low carbon electricity for around 6M homes over its 60 year lifespan. Construction started in 2017 and it is now expected to finish in 2028.

The arrival of the reactor pressure vessel was one of many milestones celebrated by EDF and its project team in the past six months.

Nuclear reactor vessel for Unit 1 arrived on site earlier this year

"Activity at Hinkley Point C has moved up another gear and continues to progress well," says EDF Energy nuclear island delivery director Simon Parsons.

The reactor pressure vessel will be housed in Unit 1, one of the two units – also called reactor buildings – being built as part of the project.

"The first reactor building is progressing at pace, after the [third and] final 304t steel liner ring was lifted into place in December," says Parsons.

The world's largest crane, Sarens’ 5,000t capacity SGC-250 which is named Big Carl, was used to lift it into position.

The liner ring, an element of the reactor building's structure, was prefabricated in a factory on site. It features supporting brackets for the beam of the polar crane, an internal crane that will rotate 360° above the reactor and will be used for refuelling.

Big Carl was also put to work this March, when it installed the 768t pool for Unit 1. The pool is a concrete and stainless steel water tank which will lie in the heart of the reactor building. It covers the reactor and is filled with water during refuelling and maintenance for safety.

"The unit now stands at 44m high and we are busy completing the work that will allow us to fit the reactor building crane, a key step before we lift the dome onto Unit 1," Parsons adds.

Elsewhere, work on the first Turbine Hall is progressing. Parsons says that it will be handed over to turbine manufacturer General Electric later this year for the installation of the world's largest turbine. Its power train – including the generator – is 70m long and it will rotate at 1,500 revolutions per minute.

Progress is not only being made onshore. In April, offshore work for the project entered its final stages when two jack-up vessels arrived off the Somerset coast to install components for the power station's cooling system.

Nuclear New Build Generation Company awarded the marine and tunnelling contract to Balfour Beatty in 2017. It involves the construction of the cooling system.

The cooling water system will supply the nuclear plant with water at a rate of 120,000litres/s.

It consists of several parts. Water will enter the system through intake heads on the seabed and will then flow through shafts connected by adits to one of two intake tunnels before entering the station's onshore galleries and systems.

Once the water has performed its cooling function, it will be returned to the sea via the outfall tunnel, adits and shafts and exit from the two outfall heads.

In terms of complexities, it's how we can get the casing to a safe depth where it's stable

Balfour Beatty offshore delivery manager Luke Cooke says offshore work started in 2018 with dredging. Floating cranes then placed the specially designed six 5,000t concrete heads on the seabed last summer.

Two heads have been fabricated for each tunnel, with the system having two intake tunnels and an outfall tunnel. The four intake heads are 44m long, 16m wide and 8m high, while the two outfall heads are 16m long, 16m wide and 8m high. The heads have a hole in the middle to facilitate the construction of the shafts below them. Balfour Beatty will drill through those holes this summer to construct 5.5m diameter shafts to a depth of 25m below the seabed.

Cooke says: "We’ve got six different locations and quite varied [ground] conditions across each. So in terms of complexities, it's how we can get the casing to a safe depth where it's stable."

The two jack-up vessels, which arrived in April, have a combined lifting capacity of 1,500t and will help the contractor install the six shaft liners.

The steel shaft liners were fabricated by Global Energy Group and Balfour Beatty carried out concrete fitout works and isolation cap installation.

The isolation caps are made of steel and incorporate a series of valves which will enable the project team to control when water flows into the cooling system. This will create a dry and safe working zone for the excavation of the adits.

Once each liner is installed, the excavation of a 16m to 17m long adit to connect the bottom of each shaft with one of the tunnels will begin. Balfour Beatty completed the construction of the tunnels, which will transfer the cooling water to and from the power station, in the summer of 2021. The two intake tunnels are 3.5km long and have a diameter of 6m, while the outfall tunnel is 1.8km long and has a diameter of 7m.

Preparations for the construction of the adits involved ground investigation and grouting. Balfour Beatty appointed Bam Ritchies to carry out this work, which was completed in May.

"The aim is to reduce the water ingress when Balfour Beatty does the break-out works between the [intake/outfall] tunnel and the shaft," says Bam Ritchies senior geotechnical engineer Hollie Colville.

"So the initial phase for us was doing the ground investigation. It was a series of probe holes at various elevations and depths set by the designer Jacobs in conjunction with Balfour Beatty.

"At each connection initially seven probe holes were drilled using a water hammer and high pressure pumps." Rotary coring was used to gain rock samples for laboratory analysis.

Bam Ritchies contract manager David Lindfield says that through the ground investigations the sizes of the fissures in the rock mass were identified. The ground investigation information was provided to Jacobs to develop the grout design.

The grouting scheme developed was executed from inside the cooling system tunnels.

The grouting scheme consisted of an array of boreholes going out around the proposed adit locations

"The grouting scheme consisted of an array of boreholes going out around the proposed adit locations which allowed us to grout an envelope of ground around the adits to seal the rock mass," says Lindfield.

With the cooling system tunnels handed back to Balfour Beatty, adit construction is a step closer.

"The tunnel/shaft connections will start when we’ve got the first shaft liner placed and grouted," says Selby.

Sprayed concrete lining will be used for the adits. "The [excavation] advances will be fairly minimal, we are looking at the order of around 1m to 1.5m," says Balfour Beatty engineering manager Gareth Harris.

He adds that there will be a tricky geometry to execute at the start of the adit excavation operation. "We have to form the break-out perpendicular to the tunnel axis, then execute a 45° turn over a 5m length of adit. We do this to align the adit with the vertical shaft liners," Harris explains.

Balfour Beatty estimates that it will take nine months to complete. It will then take another 10 months to complete the permanent secondary lining concrete works. The contractor expects the adit excavation to finish in May 2025. When the adits are complete, the tunnels will be flooded. The valves in the insulation caps on the intake heads will be opened to allow water to flow through all the parts of the cooling system.

Electricity generation at Unit 1 is expected to start in June 2027, with the project team working hard to meet this target. Work to complete this unit includes lifting the building's dome and installing the polar crane, reactor pressure vessel and steam generators.

Construction of Unit 2 has also begun. This is expected to be operational 12 months after the first unit. Parsons says that the second unit is being delivered at a faster rate.

"Experience from building the first unit is enabling us to build the second unit faster and more efficiently. Efficiency gains of 20% are being seen in some repeated tasks, such as the large concrete pours. This will also benefit the follow-on nuclear plant at Sizewell C in Suffolk," Parsons says.

Like what you've read? To receive New Civil Engineer's daily and weekly newsletters click here.