Scaling of thermal stratification or mixing in outlet plena of SFRs

Sodium-cooled fast reactors (SFRs) have a tendency for thermal stratification in the hot or outlet plena especially during shut-down and loss of forced convection transients. The formation of thermally stratified zones in the outlet plena can: lead to thermal fatigue in adjoining solid structures; and impact passive heat removal capabilities. There are limited number of experimentally validated models in literature for thermal stratification or mixing in liquid metal pools. A high fidelity scaled-down experimental facility is required to address these needs and capture the physical phenomenon close to what is expected in a real system. The design of experimental facility and instrumentation becomes complicated with use of liquid sodium, so a surrogate fluid can simplify the design and operation considerably, providing flexibility to obtain high quality measurements. Similarity analysis based scaling methodology is used to design a scaled-down model of outlet plenum with liquid gallium (Pr∼ 0.025) as a surrogate for liquid sodium (Pr∼0.005). However, reduction in size and different choice of coolant must be handled carefully to maintain relevance and reliability of the experimental facility. The possible differences in thermal stratification or mixing physics in the scaled-down model and the prototype SFR are evaluated by quantifying eddy thermal diffusivity and mixing efficiency. Empirical models are used to correlate these quantities as a function of Richardson number and turbulent Reynolds number, and verified using computational fluid dynamics simulations. The verified empirical model is used to construct mixing efficiency for the prototypical conditions and the scaled-down model conditions. For low flow rates in both geometries, the results show thermal profiles dominated by molecular thermal diffusivity resulting into a fully stratified interface. The full flow cases for both geometries show energetic mixing, characteristic of eddy thermal diffusivity dominated, mixed temperature profiles. These results provide evidence of accurate scaling analysis by capturing the thermal stratification or mixing physics representing the dominant physical modes of those of the prototype SFR.