The project "Case Study: Simulation of AGR Core Flows and Carbon Deposition", delivered for EDF in 2015, addressed a practical nuclear energy challenge through computational fluid dynamics (CFD). Rather than producing simulation images in isolation, the study was organised around the client's design questions: what controlled performance, where the principal risks or losses occurred, and how the design or operating strategy could be improved.

Advanced Gas-cooled Nuclear Reactors (AGRs) are the second generation of British gas-cooled reactors and there are currently seven of them in the UK, all owned and operated by EDF Energy. The core of AGRs consists of hundreds of fuel pins containing the nuclear fuel pellets. The surfaces of these fuel pins are rib-roughened to enhance heat transfer. One of the major operational problems with the AGR fuel pins is associated with the carbon particle deposition between the rib which results in heat transfer impairment and in turn higher fuel pin temperatures. The aim of this project was to analyse the thermal-hydraulic effects of carbon depositions on fuel pins within AGR cores. These depositions represent a major operational problem and result in a reduction in AGR’s efficiencies. The outcome of this project led to several journal publications and played an important role in British Energy’s understanding of the problem and taking appropriate actions which led to a significant life extension of the AGR stations.

The methodology centred on high-fidelity thermal-hydraulic CFD of Advanced Gas-cooled Reactor fuel channels and coolant passages. The representation retained rib-roughened fuel-pin surfaces, relevant bypass paths, local restrictions and deposit geometries that influence coolant distribution and heat transfer. Boundary and operating conditions covered representative reactor flow and thermal conditions, changes in roughness or carbon deposition, partial blockage and plausible flow imbalance across the operating envelope, with material, fluid and equipment properties assigned from the available design information. Resolution was concentrated in regions where steep velocity, thermal, concentration or phase gradients were expected, while the overall model remained efficient enough to compare several credible configurations. This balance allowed system-level performance to be linked to the local mechanisms responsible for it.

The assessment compared operational responses and geometry or surface-condition variants intended to protect thermal margin. Performance was judged using coolant distribution, pressure loss, heat-transfer coefficient, wall and fuel temperatures, flow maldistribution, local hot spots and margin-to-limit indicators. Results were reviewed through quantitative summaries and engineering visualisations, such as contours, vectors, streamlines, sections and time histories, selected to suit the physics. Important assumptions and operating uncertainties were considered so that the recommendations relied on repeatable comparative trends rather than a single nominal case.

For EDF, the principal value was a stronger technical basis for safe operation, inspection priorities, mitigation planning and life-extension decisions. The final evidence linked each recommendation to the relevant model or process output, making it suitable for internal design reviews, supplier or contractor discussions and, where applicable, planning, safety or regulatory dialogue. The work also created a reusable baseline that can be updated as geometry, operating data or test results become available, reducing the cost and risk of later design iterations.