BTS | HARDING PRIZE COMPETITION 2024
TSC OVERVIEW & CONSTRAINTS
HPC is a new build nuclear power station under construction in Somerset, UK. It is the first nuclear power station to be constructed in the UK since Sizewell B opened in 1995. Due to the recent closure of the adjacent Hinkley Point B and other aging nuclear facilities, the timely completion of HPC, including the MW tunnels and shafts, is critical to ensuring future energy demands of the UK can be met, and in a sustainable manner. As these are the first nuclear-classified tunnels and
shafts to be designed and constructed in the UK, I have helped BB to develop the documentation processes required to meet NNB quality assurance requirements. Due to my extensive involvement on the project, to help share knowledge I have participated in multiple lessons learnt workshops covering all aspects of the project. This information sharing will help to improve the buildability of future nuclear projects such as Sizewell C, and the wider industry. The layout of the MW includes: There are three
segmental tunnels (2 No. 6m-dia. Intake Tunnels and a 7m-dia. Outfall Tunnel) and at the ends are six TSC (two per tunnel) at depths of up to 27m below the seabed, and approx. 1.8km-3.5km offshore from the main HPC site (see Figure 1). Located within the Bristol Channel, the TSC are in an environment with an extreme tidal range (up to 17m). The TSC is a critical element of the MW cooling water
system (see Figure 2). They act to connect the segmental tunnels with the heads that have been prefabricated and previously placed on the seabed.4
During operation these
structures will allow the transfer of both intake and outfall water to and from the power station. There are six similar TSC, approx. 20m. deep and with 4.2m final internal diameter. Within the TSC, several temporary works (TW)
Below, figure 1: TSC location in relation to HPC
PHOTO CREDIT: JACOBS
techniques are used. Vertical shafts are located offline from the main segmental tunnels (see Figure 2). The shafts are formed using prefabricated external steel liners, taken offshore and lowered to tunnel level, each
into a drilled socket to be grouted into place. Using an isolation cap, the working environment is sealed from the Bristol Channel and enables mining out from the segmental tunnels, then to form horizontal adits and 90° ‘elbows’, employing rock bolting and sprayed concrete lining (SCL) support techniques. The PW are formed using cast in-situ reinforced
concrete (RC) designed for a working life of at least 85 years. After considerable optioneering, an offline
arrangement for the vertical shaft and adit in relation to the main segmental tunnels was selected. The choice was primarily to minimise the identified risks from seawater inundation, and also constructability for an inline connection and to provide an optimised hydraulic arrangement. To achieve the hydraulic requirements and minimise system head loss, the geometry of the TSC connection was optimised for the final PW arrangement connection to the segmental tunnels.
TSC TEMPORARY WORKS To allow the final PW structures to be constructed it is necessary to first create a temporary space proofed conduit within the bedrock that must remain stable and secure until the PW have been built. The minimum planned working life of the TSC TW is two years (note: it will be left in-situ behind the PW). An overview of the TW procedures and techniques used is given in this section. Other elements of TW, such as the primary/secondary
bulkheads and lifting arrangements, are not covered by this paper due to being designed by others.
TSC design & construction approval Owing to the extremely complicated and innovative nature and the nuclear safety functional requirements of the TSC, this necessitated the highest level of design verification to be aligned and integrated with the planned construction methodologies. The overall design process also included an Expert Panel review of the proposals. The Expert Panel of industry leading independent
TUNNEL HEADS
technical experts scrutinised the planned works and designs. They provided assurance that the design and construction plan was robust and adequately mitigated the project risks associated with inundation of the works by the Bristol Channel, thus supporting NNB in clearing the critical hold point to start construction of the TSC. The TW design was also CAT III checked by an
OUTFALL TUNNEL 1.8 km INTAKE TUNNEL 3.5km
HINKLEY POINT C
independent designer. It was therefore important that construction changes were managed and not enacted without prior consideration of the potential impact, including category of change, and that unplanned deviations from the accepted design do not occur in the field.
Communication & collaboration Once construction works began, to facilitate enhanced collaboration between the Jacobs back office and site- based design team, and the BB site-based construction
10 | May 2024
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