One of the main research topics of the Chair for Reactor Safety and Technology at RWTH Aachen University and the Institute for Nuclear Waste Management and Reactor Safety (IEK-6) deals with accident scenarios of gas-cooled High-Temperature Reactors, especially the air ingress scenario. Two experimental facilities have just started operation providing experimental data for the validation and improvement of fluid mechanics codes being developed and applied at IEK-6. The INDEX (INDuction EXperiment) facility is able to heat up single spheres inductively up to 1200°C while exposed to defined gas atmospheres and gas flow conditions. This experimental setup is well suited to study pebble / gas flow interactions as well as graphite corrosion phenomena in detail. The NACOK II (NAturzug im COre mit Korrosion) facility is an integral experiment for fluid mechanics and graphite corrosion processes under natural convection effects. It will examine spherical fuel element samples as well as prismatic blocks. In addition the instrumentation is suitable for CFD validation calculations, for example because PIV (particle image velocity) is applied. The data obtained are used to validate and improve computational fluid dynamics (CFD) models for pebble bed reactors or reactor dynamics code like MGT-3D, which are able to simulate air ingress scenarios. The CFD model shall be able to simulate the fluid mechanics as well as the corrosion processes of after a total pressure release. In this paper we report on the status of the experimental facilities as well as on advances in modelling the fluid mechanics of HTR pebble cores.

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