The paper reports a computational study of the effects of distortions of the moderator cooling passages of an Advanced Gas-cooled Reactor (AGR) on the temperature of the graphite core using a finite element based Computational Fluid Dynamics (CFD) code FEAT. This study is an example of using CFD tackling complex practical engineering problems. Due to the complexity of the problem, significant simplifications were inevitable in order to obtain a manageable CFD model, which were done based on the understanding of the problem by the modellers plus engineering judgements. These were backed-up by sensitivity studies and benchmarking based on relevant simpler problems. The paper discusses the development of the CFD model and its validation, followed by presentation and discussion of the simulation results. (c) 2005 Elsevier B.V. All rights reserved.