In the Advanced Gas-cooled Reactors (AGRs) a fuel element contains 36 fuel pins that hold the fuel pellets. The 36 fuel pins are contained within a cylindrical graphite sleeve. The fuel pins have helical ribs in order to increase the rate of heat transfer from the pins and to improve gas mixing in the fuel cluster. A fuel assembly is made out of 8 such fuel elements, stacked vertically end to end. As the graphite bricks making up the core of the reactor age, there is a possibility that mechanical interaction between the graphite sleeves and the bricks could cause small axial gaps to open between the fuel element sleeves. The gap could allow ingress of gas at a different temperature to occur. Currently it is assumed that all gas mixes before reaching the top of the assembly, due to the swirling motion imposed by the helical ribs. Recent studies using Computational Fluid Dynamics (CFD) have shown that the effect of the ribs might be limited to a region close to pins and therefore it is possible that the two streams of cold and hot gas do not mix completely before exiting the assembly. No flow periodicity can be set due to the varying boundary conditions for the temperature along the fuel assembly. Therefore there is the need for a very large scale computation. This has been carried out using the High Performance Computing facilities at STFC. The total number of cells is close to one billion to be able to model the mixing of the two streams.
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