Scale effects methodology applied to uprising jets in ASTRID

Abstract

CEA is involved in the development of the 4 th generation of nuclear reactors, among which are the Sodium Cooled Fast Reactors (SFR). To support their design and safety, specific codes were developed and should be validated using experimental results from relevant mock-ups. Due to the complexity of building a fullsized prototype in the nuclear field, most of the experiments are performed on reduced-sized models but it may lead to scale effects. Such scale effects are under study in this paper, focusing on a critical issue in SFR reactor, which is the rising of the jet outgoing the core at low power. For this purpose, a scale effects methodology is detailed, using the SFR's reduced scale mock-up MICAS as the reference scale. To achieve that, dimensionless Navier-Stokes equations under Boussinesq's approximation are considered and the Vaschy-Buckingham theorem is applied to determine the most relevant dimensionless numbers, among them the densimetric Froude number. Experimental campaigns have been performed to measure velocity fields using the Particle Image Velocimetry (PIV). Their analysis clearly shown dependency of mean pathway of the jet on this dimensionless number. Particularly, plotting the jet angle as a function of the normalized densimetric Froude number, a change of behaviour for a threshold value of 0.45 has been observed. This result also shown negligible effects on the jet rise happening before jet impingement on the Upper Core Structure (UCS).