Simulation of Liquid Mixing and Stratification in Upper Plenum of Pool-Type Sodium Fast Reactors Following Sudden Decrease in Power or Scram

Flow mixing and the oscillation of the stratified liquid layer forming below the free surface in the upper plenum of pool-type sodium fast reactors following a sudden decrease in reactor power or a scram could induce thermal stresses in the reactor core and support structure. Such an event has been investigated in the protected loss of power (PLOP) experiments performed at Purdue University using liquid gallium simulant. The present work performs computational fluid dynamics analyses to simulate liquid mixing and stratification in the upper plenum of the experiments for isothermal and colder liquid injection transients. In addition to investigating the formation of the liquid mixing eddies, the present analyses calculate the temperature and average velocity distributions in the plenum and compare the results for rigid and nonrigid liquid free surfaces. The performed analyses using the large eddy simulation turbulence model predict complex liquid mixing patterns that could not adequately be resolved using the unsteady Reynolds-averaged Navier-Stokes shear stress transport k-ω turbulence model. Results also show that the thickness of the hot liquid stratified layer near the free surface progressively decreases with increasing injection rate of the cooler liquid gallium into the plenum and the time of simulated transients. With a nonrigid gallium free surface, extensive formation of small turbulent mixing eddies occurs in the top plenum section and the thickness of the hot liquid stratified layer decreases faster with time than in the analysis with a rigid free surface. Simulation results of the axial distributions of the liquid temperature and the time-averaged flow velocity in the plenum are in good agreement with reported measurements in the Protected Loss of Power-40 (PLOP40) and Protected Loss of Power-60 (PLOP60) experiments with injection rates of cooler gallium into the upper plenum of 0.524 and 0.786 kg/s, respectively.