PIV measurements of natural circulation driven flow inside and around SMR fuel assemblies

• Particle Image Velocimetry system development. • Natural circulation driven flow around a fuel assembly. • Low Reynolds number flow. • Buoyancy driven free jet. • Experimental data appropriate for CFD model validation. Buoyancy-driven circulation has gained popularity in water-cooled Small Modular Reactor (SMR) designs, serving as a means to facilitate heat removal from the fuel assemblies in normal operation conditions. Natural convection cooling has found application not only in SMRs but also in numerous pool-type research reactors like the Training Reactor in operation at Budapest University of Technology and Economics (BME) in Hungary. Heat transfer efficiency depends on flow conditions, so high-accuracy models are needed to predict coolant behaviour around fuel rods. The nuclear industry uses many Computational Fluid Dynamics (CFD) codes for this. However, as in the case of any numerical simulations, the need for experimental validation remains essential. The Particle Image Velocimetry (PIV) method provides a high-accuracy solution to measure flow characteristics in at least two spatial dimensions without intervening in the flow. To facilitate the investigation of thermal-hydraulics inside and around fuel pin bundles of BME’s Training Reactor (EK-10 type fuel) and also of future Small Modular Reactors, a new equipment has been designed and constructed in the PIV Laboratory at the Institute of Nuclear Techniques at BME. The Transparent mOdel for Water-coolEd Reactors (TOWER) system includes a single full scale electrically heated fuel assembly model, comprising 16 fuel rods arranged in a 4x4 square lattice, with adjustable power outputs. PIV measurements have been performed at four different power levels focusing on the flow behaviour inside subchannels and right at the fuel assembly outlet where the buoyancy-induced free jet of lower density warm water exits the bundle.