NEAMS IRP challenge problem 3: Mixing in large enclosures and thermal stratification

Mixing in large enclosures and thermal stratification play critical roles in advanced reactor designs, including liquid metal-cooled and high-temperature gas reactors. Lessons from a recent international benchmark (IAEA, 2017), using system-level codes for Sodium-Cooled Fast Reactors (SFRs) highlight the need for improved models to accurately capture mixing and thermal stratification in the reactor hot pool upper plenum. These improvements are essential for predicting the propagation of stratification fronts and the effects on natural circulation and heat transfer between primary and intermediate loops. Current computational dynamics (CFD) codes, particularly those relying on Reynolds-averaged Navier-Stokes (RANS)-based turbulence models and the Simple Gradient-Diffusion Hypothesis (SGDH), underperform in simulating buoyancy-driven flows, leading to inaccurate predictions of stratified fronts. The NEAMS IRP Challenge Problem 3 (CP3) aims to develop multi-fidelity, multi-scale models for mixing and stratification in large enclosures. This includes models ranging from high-fidelity Large Eddy Simulations / Direct Numerical Simulations (DNS/LES) to system-level code models. High-resolution experiments and LES/DNS inform the development of these models, providing accurate and computationally affordable predictions. This paper provides an overview of ongoing experimental and modeling activities within CP3, showcasing advancements in understanding and predicting mixing and stratification in large enclosures for advanced reactor applications.