CESAR experiment: Sodium boiling during Transient OverPower

•Data of sodium boiling during transient overpower.•Description of the sodium Thermalhydraulics loop CESAR.•Main learnings on fast transient sodium boiling. In the framework of nuclear revival under the constraint of global warming and in the context of the emergence of new small modular reactor concepts, sodium cooled fast breeders seem to be a good candidate for maximizing the energy yield of the fuel and burning actinides. In the case of rapid reactivity insertion, characterizing transient sodium boiling is essential, as it can lead to dry out and rupture of fuel pins, but also as it has an impact on core reactivity. As sodium is a metal with a low Prandtl number, its boiling behavior is different from that of water. In order to provide insights and data on boiling of sodium during transient overpower, past experimental results obtained in the frame of the CESAR program (Caracterisation de l’Ebullition du Sodium en Accident de Réactivité) performed at CEA Grenoble in the 70th are described and main learnings are given. The CESAR test loop and experimental procedure are described, and results of seven transient tests with different inlet flow rates, temperatures, pressures and transient powers are described and interpreted on the basis of a comparison with single-phase simulations. The phenomenon of sodium superheat at boiling onset is studied. These data, which are rare in the literature, can be used to validate simulation tools.