Improving the viability of modern methods of construction for nuclear facilities
Steel concrete composite structures are unlocking the potential of modern methods of construction for nuclear, offering high impact resistance and faster, safer delivery.
Our research shows this modular construction form can outperform traditional reinforced concrete in extreme scenarios, including aircraft impact.
By validating performance through advanced modelling and testing, we’re helping regulators and technology providers build confidence in next-generation nuclear infrastructure.
Steel concrete composite structures advance nuclear facilities
Embracing modern methods of construction for new nuclear facilities is a critical step forward in improving efficiency and lowering costs within the industry. Previous generations of nuclear facilities have relied on reinforced concrete to mitigate structural risks, but the need for complex reinforcement cages and in-situ concrete pours limit its potential for modularisation and offsite construction. Many forms of modular structural forms are unable to meet the high-risk criteria that nuclear structures are designed for, including earthquakes and major impacts; but steel concrete composite structures are different. They offer high impact resistance, and can also be designed for manufacture and assembly, reducing the duration of onsite work and improving health and safety.
Regulatory assurance
Aware of these benefits, technology providers are increasingly seeking the use of modular steel concrete composite in their designs, particularly for small modular reactors (SMRs). As a novel structural form in the nuclear arena, the nuclear regulators globally, such as UK’s Office for Nuclear Regulation (ONR), must be convinced that sufficient testing and analysis has been carried out to demonstrate the safety of these alternative methods.
To support industry best practice and prevent the potential for delays in design approval, Mott MacDonald’s nuclear team launched a self-funded research project using state of the art analysis to investigate the performance of steel concrete composite structures against high-risk impacts. The team also analysed the structures' thermal-mechanical response to solar gain. These were compared directly to traditional reinforced concrete structures to give the industry and regulators clarity and confidence in the findings.
High impact
Designing nuclear facilities to withstand extreme events, such as large commercial aircraft impact, is a key parameter of the design process. Given the severity of the consequences, this can be a scenario that drives many design features, such as the thickness of walls or the arrangement of the rebar within a traditional reinforced concrete structure.
A typical wall for a standard steel concrete composite structure differs in that it consists of two parallel steel plates joined with tie bars or diaphragm plates and filled with concrete. Composite action between steel and concrete is achieved through the presence of welded steel studs on the inside face of the plates. This combination of steel and concrete is expected to provide higher impact resistance than a reinforced concrete element of the same size, but there has been limited detailed research into this until now.
To improve our understanding, we used finite element modelling techniques to generate the aircraft threat and applied associated impact forces and pressures to reinforced concrete and steel concrete composite structures. The models were non-linear, meaning that they could account for failure modes beyond the structure’s elastic behaviour, and as a method previously used for assessing reinforced concrete structures, it has already been verified and accepted by the regulator. To increase confidence, the analysis results were compared against available test data.
Our analysis also included a proprietary steel concrete composite product called Steel Bricks. Steel Bricks is a fully modular solution that allows for offsite fabrication, trial assembly, testing and optimisation before installation. It provides a more rigid diaphragm than typical steel concrete and removes the need for tie bars or reinforcing bars.
Fail safe
For each of the three types of structures, two potential failure mechanisms were examined – perforation and scabbing. Perforation is penetration through the wall; and scabbing is the potential ejection of materials from the non-impacted face, typically into areas housing nuclear safety related equipment. All structures were tested assuming a 1.8m wall thickness and a variety of tie rod spacings. Analysis was then run on the steel concrete composite and Steel Bricks structures for a 1.4m thick wall to see how slimmer steel concrete walls performed compared to thicker reinforced concrete walls.
Faced with the potential of impact from a large commercial aircraft, none of the 1.8m thick structures were perforated and both 1.4m thick steel concrete and Steel Bricks structures showed no perforation either. The steel plates inherent in the steel concrete composite and Steel Bricks structures also prevented scabbing. Our findings show that the slimmer steel concrete and Steel Bricks performance is within international design code (IAEA SR87) acceptability criteria, meaning that there is a potential saving in overall thickness associated with the use of steel concrete composite and Steel Bricks, even beyond the economics of design for manufacture and assembly.
Thermal regulation
Another important area where more evidence of the behaviour of steel concrete composite would support the industry is in understanding its thermal response to solar gain. Due to the exposed nature of the steel plates and the lower specific heat capacity of steel, there is more potential for heat induced stresses. Running our finite element modelling analysis for the hottest day ever recorded in the UK, told us that compression stresses induced on the external face, and tension stresses on the internal face, were higher than in reinforced concrete structures; but, importantly, these were within the safe design limits as set out in international design codes.
Conducting independent research that supports the development of the industry is important to Mott MacDonald as a company that has over 60 years of experience in nuclear energy, and over a century of experience in structural design. This expertise is also supporting us in our work with the nuclear regulator ONR to evaluate new reactor designs and civil and environmental hazards; and with technology providers (e.g., Holtec Britain) to achieve design approval.
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