CFD and Experiments for Wire-wrapped Fuel Assemblies

Liquid metal reactors typically employ wire wraps as spacers between the fuel pins. In the past, design and safety calculations were largely one-dimensional and based on experimental data. Nowadays, with modern state-of-the-art computer power and tools, three-dimensional Computational Fluid Dynamics (CFD) simulations allow designers and safety specialists to obtain much more detailed information on the flow and heat transport in liquid metal cooled fuel assemblies, obviously in close collaboration with experimental campaigns. This may lead to new insights possibly decreasing the safety margins. This paper intends to provide an overview on the activities in the frame of design and safety support for wire-wrapped fuel assemblies. It all starts with validation. Therefore, validation efforts will be shown for fuel assemblies as they are designed on the drawing board for ‘cold’ conditions. Such analyses will profit from the quantification of uncertainties and determination of most influencing parameters. Nevertheless, in reality a fuel assembly will not be employed as designed in ‘cold’ conditions. Therefore, they will probably deform. This requires an assessment of the effect of deformations. Another aspect possibly occurring during operational conditions is vibrations. State-of-the-art coupled CFD and finite element method fluid structure interaction techniques have been developed and applied to a wire wrapped fuel pins, providing insights in the vibration behavior of such assemblies. Apart from that, vibration experiments have been performed at complete fuel assembly scale providing important insights to safety analysts and designers. However, design and safety analysts will not only have to cope with operational conditions, but also have to show the heat transport behavior under accident conditions. Assessments of the effect and formation of blockages are necessary. In all above cases, it should be clear that experiments and numerical simulations go hand-inhand. Numerical simulations are used to design the experiment, the experiment is used to validate the simulations, and the simulations are used to interpret the experimental results.