Much more rests on the £19.6bn project to construct the Hinkley Point C nuclear power station in Somerset than the ability to power 6M homes.
Everyone is bearing the pressure to get it right – from the shovel drivers at a nearby Somerset quarry who wash the wheels of their vehicles to ensure the quality of the raw materials being delivered to Hinkley is as pure as possible, to the team of 48 planners painstakingly mapping out each element of work on the construction site.
This is because the political, economic and historic context of the construction of Hinkley Point C, means that successful delivery could pave the way for more nuclear new builds in the UK and overseas.
Economic case
Some UK politicians are doubtful about Hinkley Point C’s economic case. French energy giant EDF is delivering the scheme in conjunction with the China General Nuclear Power Group (CGN). Going over budget or time could damage EDF’s plans for another nuclear power station, Sizewell C in Suffolk.
In France, where more than 70% of electricity generated comes from nuclear power, the energy firm needs investment to overhaul the country’s ageing fleet of reactors. The Flamanville 3 new build in Normandy is now notorious for having been delayed by several years and has gone over budget by hundreds of millions of pounds.
So it comes as no surprise that EDF and its contractors are under more than the usual amount of pressure to get Hinkley Point C right.
“One of the biggest challenges we had was take best practice from the industry and some of the key lessons learned from other projects and make sure that they were embedded here,” says EDF Hinkley Point C delivery director Nigel Cann.
In detail, this means looking carefully at what triggered the delays and cost increases on Hinkley’s predecessors and heading them off. The result is a series of construction innovations that have so far resulted in the project having a 97% right first-time rate.
Hinkley Point C is on the north Somerset coast. The 176ha construction site for two European Pressurised Water Reactors (EPRs) – Units 1 and 2 – sits alongside an operating nuclear power station, and one being decommissioned.
Construction of the new power station started at the end of 2016. Enabling works are substantially complete and excavations for Unit 2 are nearing the finish line. A Kier Bam Nuttall joint venture (JV) has the £203M enabling works contract, which involves excavating over
5.5M.m3 of material, building terracing structures and creating a road network. Costain is due to finish the 500m long jetty, which will handle raw materials delivered by ship in March, while Balfour Beatty has the marine works contract to build the three tunnels needed for water intake and outfall.
Milestones for the main civils construction work are now being ticked off – December’s concrete pour on the first part of Unit 1’s 4,500t base was a major one. The Bylor contractor JV of Bouygues and Laing O’Rourke has the deal worth over £2.8bn for the reactor buildings and main civils works.
So where is EDF and its tier 1 teams looking to learn lessons and what is being done differently?
Construction on site in August 2018, showing the Unit 1 base in the foreground
Huge importance has been placed on concrete production – 3M.t is needed. Problems with concrete at other nuclear power station builds have caused costly delays. Flamanville suffered from cracking or poorly compacted concrete. Finland’s Olkiluoto 3 – not an EDF project – had too much water in some of its concrete mix impacting strength.
Consistency and quality are the two key drivers for concrete production at Hinkley Point C. Engineers have been working on mixes since 2012, with testing at the Bouygues lab in Paris resulting in a 150 page technical report for each. An 18 month programme to mock up and test the batching plants was also undertaken.
“What we’re trying to develop is a product that’s so consistent, that when the [workers] on site are placing it, compacting it, finishing it, they do the same over and over again over the next six years,” says Bylor chief materials engineer Peter Abel. “So, if there’s a change, for instance if we’re losing workability too quickly or it’s a bit sticky or heavy, [the workers] might change the way they’re placing and curing it, which is exactly what we don’t want.”
Aggregates come from Hanson quarries, many of them local. Abel says that it is less the mix of concrete that makes it suitable for nuclear, and more the way it is handled. “What is nuclear concrete? Nuclear concrete is everyone involved knowing what they’re doing, why they’re doing it and maintaining it. The actual mix design isn’t that special, but it’s the way it’s handled and treated and tested and maintained which is the nuclear bit.”
Concrete testing
There are simple tests and surveillance methods throughout the process. “One of the biggest things you can do wrong is put the wrong powder in the wrong silos,” says Abel.
In addition, high performance microwave moisture probes are used to monitor water content. There are also probes within the mixer, and the mix is controlled by a machine, not an operator.
The batching plant operator normally uses experience and judgement in the final mix, tempering adjustments before completing the batch, but at Hinkley Point C the adjustments are by a specially calibrated machine.
“You lose a little bit on output, but you gain massively on consistency,” says Abel.
The result is that the plants at Hinkley Point C are producing the lowest standard deviation concrete in the UK, according to Abel. He says a good UK batching plant will produce concrete with deviations of about 5, but at Hinkley it is typically producing concretes below 2.
Standard deviations are the result of tests on the ingredients, and strength of the concrete. Higher values indicate changes in the composition of the concrete.
“All the quarries know what they need to produce because we treat them as the same team as here. If we didn’t have that control, there would be variability. I have never had such good control over the inbound aggregates,” he says.
Artist’s impression of Hinkley Point C
Alongside the base of the structures, Bylor’s concrete work will include the housing for the two reactors and prefabricated components for the fuel buildings, which sit adjacent to the reactors. Prefabrication of elements for the fuel buildings aims to cut 18 months off the construction programme and reduce insitu welding by 45%.
The 57m diameter cylindrical reactor buildings are designed with a 1.8m thick reinforced concrete domed roof and will be around 64m tall. The reactor buildings have a double-wall containment structure and a reinforced concrete foundation slab. A cross section of the reactor design shows the outer concrete wall, and then a gap which will be maintained at sub-atmospheric pressure to enable radiation leaks to be collected, within an inner containment wall with a leak-tight steel liner.
The team has developed an app for delivering the concrete to site that produces a running order for concrete production each day. These timings are cascaded across the project, so everyone can see what concrete is being delivered where and when.
We’ve planned our pour timings like an orchestra
All this has contributed to the successful first pour for the foundation slab or nuclear island for Unit 1. The pour was undertaken continuously over 30 hours requiring just under 2,000m³, forming a maximum 3.2m thick slab, reinforced with steel from south Wales.
The main concrete was a C40/50 mix with a cem3b low heat cement.
Workers poured a specially developed low heat concrete mix which could be revibrated up to six hours after placement.
“We’ve planned our pour timings like an orchestra, so by the time we placed this layer and stepped back to put the next layer on top, the concrete is in tune with that movement,” says Abel.
One of the key elements of Hinkley Point C’s concrete mix is ground-granulated blast-furnace slag (GGBS) from Port Talbot in south Wales. A by-product of steel manufacturing, It helps to lower and control adiabatic heat generation in mass concrete.
While GGBS is commonly used in concrete, at Hinkley it will be the first time it has been used in an inner containment building.
Although most of the mixes used in the various Hinkley Point C structures contain constituents from local quarries, the particular mix for the containment building also includes a special low heat cement from the South of France. It has taken five years to fully develop and samples are currently undergoing load testing at the University of Dundee.
Most important concrete
“We need 10,000t, which is a smallish volume, but this concrete is the most important concrete on the project. The cement is very pure, very consistent, and has a natural low heat. By blending with GGBS, it helps to manage heat and hydration,” says Abel.
The concrete for the reactor buildings must be able to withstand everything from a tsunami to an aeroplane collision.
It is with the construction of the reactor buildings that the team is also learning from the steel construction problems at Flamanville. On that project, the reactors’ containment buildings were fabricated on site and comprise 150 steel panels. There, workers were hampered by bad weather and welds were deemed substandard. This caused delays and cost hikes as they were re-done.
As well as concrete, Hinkley Point C’s containment liner includes a 6mm thick steel plate, which will be prefabricated in six major components. These include four huge rings to line the cylindrical reactor building, as well as the dome. Prefabrication will be undertaken at steel manufacturer Tissot’s works near Bordeaux in France. It is there that Tissot hopes to do as much of the welding as possible.
Some welding will still have to be done insitu, and a huge temporary works shelter is being built, so that this work can be done under cover.
“At Flamanville, welding was disrupted by wind and weather. The impact of weather can cost two months per year,” says Bouygues services director Jean-François Persegol.
At Taishan in China, another EDF/CGN nuclear new build, the first four liner rings were also prefabricated, with the results boding well for Hinkley Point C. Engineers there found they took 25 days to construct a ring, compared to 40 at Flamanville.
Vital offsite prefabrication
Persegol explains that offsite prefabrication is vital at Hinkley as it enables work to be done in a more controlled, less time stressed environment.
When it moves to the mechanical, electrical and heating, ventilation and air conditioning installation stage, contractors will have to co-ordinate cabling and pipework in 2,500 rooms throughout each unit of the power station. The MEH Alliance of Altrad, Balfour Beatty Bailey, Cavendish Nuclear and Doosan Babcock is in charge of this work and EDF is again drawing on lessons from Flamanville. Given 235,000t of steel reinforcement bars have to be set in concrete, the Hinkley Point C team does not want to repeat clashing problems at Flamanville, where some of the steel had to be moved to make way for other components later in the project.
To avoid this at Hinkley Point C, there is an extensive 4D modelling programme, showing in detail where every component goes – what Bechtel 4D team manager at Hinkley Point C, Andy Codd, describes as a “Russian doll of Rubik’s Cubes”. Workers can access the full design on tablets and although this has been done before, the team at Hinkley Point C says it has never been done to this scale.
“You’ll have more than [a total of] 4,500 rooms just in the main nuclear island units, which are mainly replicas of each other with some differences. So, if you take that grand scale of the universe looking down, we did a calculation on how much data we’re really handling and it is approximately 1,700 items to be fitted in each of those rooms,” says Codd. “I have never been on such a complex project in terms of the amount of numbers and data there is. We’re trying to organise a digital twin to predict the issues that will come out in the future.”
Source: New Civil Engineer