Scale Effects Methodology for Buoyant Impinging Jets in Sodium Fast Reactors

The CEA is involved in the development of 4th generation sodium-cooled fast reactors for which specific codes were developed. However, they need to be validated using experimental results obtained on relevant mock-ups. Due to the complexity of building a full-sized prototype in the nuclear field, most of the experiments are performed on reduced-sized models but it may lead to scale effects. We have to ensure that the validation carried out at small scale is effective at the reactor one. A critical issue in SFR reactor is the rising of the jets outgoing the core at low operating power: this phenomenon induces thermal fluctuations in the vessel, leading to thermal fatigue of the components. Therefore, this scale effect study is focused on the behaviour of the jets. Previous studies on this issue were performed on a 1:6 scaled mock-up representative of a SFR's hot plenum named MICAS. Theoretical study, thanks to the Vaschy-Buckingham theorem and dimensionless Navier-Stokes equation under Boussinesq's approximation, led to relevant dimensionless numbers such as the densimetric Froude number and the Euler number. The scale effect methodology based on the scale series is detailed, using MICAS as a reference scale. A new experimental mock-up called MOJIT-Eau has been designed to be representative of the smaller scales: this mock-up can be modified to allow scale effects investigation from 1:4 to 1:2.5 of MICAS's scale, and allows a phenomena investigation to quantify the influence of the complex geometry on the raise of the jets. Due to the size reduction, some geometrical distortions were made in order to ensure the turbulence of the flow. Other distortion devices were also designed to quantify the relative influence of the Euler number compared to the densimetric Froude number. This work is a preliminary study for a later experimental investigation with numerical comparison. This mock-up is built in a transparent polymer vessel to carry out optical measurements as laser velocimetry. Temperature fields are measured with optical fibre and PT100 probes in the vessel.