BNEN Investigation of thermal stratification in low Prandtl number fluids.

LEANDREA is a lead cooled fast spectrum test reactor under development at SCK•CEN. This reactor will be the test and demo reactor for the European lead cooled SMR and validate the fuel for this class of reactors.

The safety of the EU LFR SMR relies on the natural circulation characteristics under loss of forced flow conditions. During this type of accidental scenario’s, the cooling of the decay heat in the core is guaranteed by natural circulation. At the lower mass flow rates encountered during the transients mixing becomes less intense and stratification can occur. This stratification can lead to increased thermo-mechanical loads on the reactor structures due to important temperature gradients.

The goal of this thesis project is to investigate thermal stratification in liquid metals under transient conditions. Liquid metals are characterized by low Prandtl numbers due to their high thermal conductivity. This induces particular challenges in the simulations of these fluids.

Therefore, comparison with detailed experimental data is necessary to validate computer models.

In the 1980s, the SUPERCAVNA test facility was built and operated at the CEA Grenoble research center. The experimental campaigns investigated the onset of thermal stratification in a rectangular pool, (reference: 10.1016/j.ijheatmasstransfer.2025.127277)

In this thesis, the SUPERCAVNA facility will be simulated by means of a CFD and compared to the experimental results in order to validate the computer models.

As sodium has a Pr number one order of magnitude lower than Pb, the Pr number effects are more pronounced. This allows to make the best choices to model the low Pr fluid behaviour.

Validation of the models based on experimental data is a necessary requirement to enable the use of these models into safety evaluations of the reactor.