Experimental activities at KALLA on heavy-liquid metal heat transfer for fast reactors

Heavy liquid metals (HLMs), such as lead-bismuth eutectic (LBE) and pure lead are prominent candidate coolants for fast neutron systems, both critical assemblies and accelerator-driven systems. One of the key questions for the safety of these systems is the fundamental understanding of thermal-hydraulic aspects, such as the cooling of fuel assemblies under normal and accidental conditions. For several reasons, this problem is particularly challenging, compared to reactors cooled with conventional coolants. Extensive efforts are currently devoted worldwide to improving the understanding of these flows. Reliable experimental data are essential for developing advanced predicting models, but scarce. This issue has been experimentally investigated at the Karlsruhe Liquid Metal Laboratory KALLA for over a decade, in a comprehensive series of experiments of progressively increasing complexity. This contribution presents the main results from these experiments, e.g. heat transfer correlations, local temperature distributions in the fluid as well as in the rod cladding. Lessons learned in view of the experimental setup, instrumentation, procedures and usefulness of the results for development and validation of simulation codes are also presented. Finally, the main open issues which require further research are identified.