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.