Hinkley Point C is often billed as Europe’s largest building site.

The mammoth undertaking to build a pair of nuclear reactors involves an enormous number of key steps along a narrow critical path, with safety and quality paramount throughout.

German specialist Kaefer is one of the many tier two contractors bringing expertise to the ambitious megaproject. It is tackling an access and surface-protection project for principal contractor Bylor (a joint venture between Bouygues and Laing O’Rourke, working for client EDF).

Like most activities at the site, Kaefer’s task – essentially a high-quality paint job – is somewhat larger than ordinary life. It must blast, prime and faultlessly coat more than 30,000 square metres of steel, a job expected to require 150,000 hours of effort.

The first steel containment ring is 17m tall and weighs 382 tonnes – around the weight of a 747 Jumbo jet

The steel in question has a particularly critical job, forming the reinforcement liner of the eventual concrete containment building that will surround the nuclear reactor.

The containment liner is formed from five gigantic prefabricated parts: a curved cup at the bottom; a stack of three cylindrical walls, each 17 metres high and 50 metres in diameter; and a rounded steel dome at the top.

Each of these major components is fashioned on site in Somerset but under offsite factory conditions, with the high-precision work carried out inside two 60-metre wide cylindrical buildings at the Hinkley site: Bunkers 5 and 6. The interior of each bunker shelters beneath a removable domed roof. As each piece of the liner is completed, the project’s mammoth crane – dubbed Big Carl – lifts off the bunker lid, hoists out the completed piece, and carefully lowers it into place at the nearby location where the reactor building is taking shape.

Work on the containment liner for Hinkley’s second reactor saw the transfer of its completed 170-tonne cup section in September last year, followed by the 382-tonne first liner ring in December (pictured above).

The key surface protection job for this liner is being carried out while components are still inside the bunkers. Safe and secure access was needed to treat the sides of the cup in Bunker 5, and to work on the 17-metre-high walls of the liner segments in Bunker 6, both inside and outside the ring.

Kaefer UK design manager Jonathan Leyland says: “Scaffolding has to provide access to both vertical and horizontal welds for each ring – a total of some 150 linear metres.”

The task was complicated by the fact that the ring walls are not smooth, but are punctuated at many different points by protrusions, marking where the nuclear innards of the reactor will eventually be connected to critical external systems.

“There is little commonality in the layout of these protrusions on each structure and also a wide variation in dimensions, from diameters of 2.7 metres to just 50mm,” Leyland explains.

“So it is vital that the scaffold layout is sufficiently adaptable to meet this variation.”

“Scaffolding has to provide access to both vertical and horizontal welds for each ring – a total of some 150 linear metres”

Achieving that tricky goal meant creating a scaffold design digitally, and proving that the proposed structure could cope with the constraints long before the first scaffold poles arrived on site. Kaefer worked with strategic partner Layher to establish the required level of certainty.

“That’s how we and Kaefer won this particular project,” explains Layher UK managing director Sean Pike. “Through our system we were able to draw a model and then animate the model, to show that all the clash collisions had been detected.”

Layher, a Germany-based modular scaffolding system manufacturer, deployed its own LayPlan software to design and verify the required scaffold structures. Layher describes its system as a scaffolding information modelling (SIM) suite that can directly integrate with established building information modelling (BIM) packages. Integration allows scaffolding design work to be done directly against building models.

The suite supports 5D models, catering for variations in the 3D structure through time, with consequent costs. This allows the scaffolding design to adapt to support changing needs as work progresses. The resulting layouts can be experienced as virtual-reality models.

Layher says this VR capability means virtual structures can be optimised both on screen and using immersive headsets, allowing assessment of both the practical layout and movement through the structure. It estimates that at Hinkley, this refinement process reduced the eventual amount of scaffolding required by about 15 per cent, yielding cost savings as well as other benefits. “This saves time and helps to optimise logistics and labour – and the customer sees what they are getting beforehand,” Pike summarises.

As a manufacturer, Layher was able to innovate to support the peculiarities of the job. To provide decking throughout the large-diameter, curved scaffold, it developed an adjustable splay deck component, an extendable corner unit that did away with the need to overlap boards to accommodate the curvature.

Pike says LayPlan works as a bolt-on to AutoCAD. “Navisworks integration then allows you to do collision checking,” he adds. “So you’ve got the BIM drawing, our SIM drawing over the top. You’re then able to animate through Navisworks to see if what you’ve drawn clashes with anything.”

Another constraint restricting the liner-protection task is that the curved scaffold has to be entirely self-supporting. There is no possibility of bracing against, or tying onto, the critical steel liner itself, which needs to be pristine at the end of the coating process.

Structural analysis for this critical question was achieved by exporting the SIM model into Dlubal’s RSTAB package, an established program for calculating load levels and the resulting behaviour of 3D structures. It can cater for external factors such as wind loading.

It’s vital to plan for the worst case, Pike notes. “Sites like Hinkley will use the full [capabilities] of SIM,” he says. “For some large construction projects – like [the restoration of] Big Ben, for example – the structure needs to be scrutinised not just by us but several others, because it’s right in the public domain. If something did go wrong, it could be quite serious and be reported immediately and plain for everybody to see.”

“Sites like Hinkley will use the full [capabilities] of SIM. For some large projects, the structure needs to be scrutinised by several others, because it’s right in the public domain”

While less visible than Big Ben, Hinkley Point C is of course the kind of critical project where it is vital “to apply best practices in construction methodology and digital technology”, says Trevor Woodward, Kaefer UK and Ireland director of business and strategy. He says that this approach extends to all aspects of the job: “Access for productivity and efficiency is essential but more importantly we are able to design, model and plan safety-critical aspects such as ventilation and rescue.”

Pike adds: “We work together to create the scheme and then [refine it]. Is it easy to build or not? Is it safe to build or not? Do we need to put additional platforms in [or] staircases? We’ll have all of these conversations together before the final piece is designed and approved.”

Pike says Layher and Kaefer have worked on many projects together. “You get the trust of one another, and both sides know what’s needed to create the scaffolds and the drawings and technical support,” he explains.

“Working together, certainly on critical projects like this, is quite important,” Pike adds. “[Kaefer is] good at what it does but we only do Layher [scaffolding], so we’ve got the specialist product knowledge, of solutions they might not have considered. [Similarly] they’re the access specialist – we don’t put the stuff up, they do – and so we have to make sure whatever design we come up with has good, solid, safe buildability.”

After many steps of meticulous planning, Kaefer’s team began surface work on reactor two’s liner cup in August 2020, before moving on to the first wall segment ahead of its installation in December.

Woodward says: “The surface protection is applied as a critical part of the nuclear safety system. The process has involved a high level of quality assurance, inspections and health, safety and environmental assessment, so it’s definitely been a long journey to get to the point of applying paint.”

“Generally speaking, system scaffolding is a little bit more expensive in the first instance, when you look at it on paper, but the safety, time and labour benefits far outweigh the cost over conventional scaffolding,” argues Layher UK managing director Sean Pike.

Rather than deploying smooth steel poles clamped together with bolt-closed fittings, Layher’s modular approach employs rosette brackets welded onto the upright standards at regular intervals.

Bracing poles and other components such as horizontal decks, guard rails and toe boards connect and latch onto the rosettes to form the required structure. “Each rosette has eight holes, for making eight independent connections at the same location,” Pike says.

This approach makes the scaffold quick to erect but demands organisation. “There’s more effort required at the start because you really want to take the right kit to the job,” Pike admits. “With conventional tubes and fittings, you can generally make it work – cut your tubes and boards, and things like this. Whereas with [a modular] system, you want the right parts in the right area, so planning is more important. But once you’ve done your planning it’s about 60 per cent quicker [to install].”

He argues that time on site is becoming more valuable as the labour pool of skilled scaffolders is shrinking. “We’re working hard to attract young people,” Pike says. He adds training for system scaffolding is offered within the Construction Industry Scaffolders Record Scheme (CISRS), offering two-day courses for experienced scaffolders or in-depth training for new recruits. “You can do part one, part two, and we’re just in the early stages of a pilot advanced course in system [scaffolding], so a scaffolder can choose the route they want to take [in either conventional or modular skills],” he says.

“For new users of our product, one of the first skills they need to develop is planning,” Pike adds.

“They get the planning right and then it all fits into place [but] it’s only in the last 10 years or so that a lot of scaffold companies have invested in their own technical departments.”

The move to digital planning of complex jobs has “gained a lot of momentum” over the past five years, he adds, particularly over the past year, as the virus has accelerated technology uptake.