Capabilities of Reynolds stress turbulence model in applications to thermal stratification

In the safety analysis of advanced fast breeder reactors, licensing authorities require that inherent safety capabilities be proved by numerical simulation with well-validated computer programs. Even in the worst case of loss of power to the primary pumps, natural convection circulation must provide, through intermediate heat exchangers, a heat sink sufficient to prevent coolant temperatures from reaching saturation and triggering development of a two-phase flow domain with subsequent reduction of coolant capabilities and loss of core integrity. Numerical simulations of reactor coolant behavior require the modeling of turbulent flows in the critical transition phase between forced and natural convection. Also required is the modeling of impact of coolant temperature stratification on turbulence, which can be enhanced or suppressed according to unstable or stable stratification patterns. Therefore the accuracy of computer codes depends on the mathematical modeling, particularly on the modeling of turbulence in thermal stratification. A Reynolds stress turbulence model (RSM) has been implemented in the COMMIX code, together with transport equations describing turbulent heat fluxes, variance of temperature fluctuations, and dissipation of turbulence kinetic energy. This article outlines the model, explains the verifications performed thus far, and discusses potential applications of the RSM in the analysis of thermal stratification in engineering systems. The problem of analyzing thermal stratification and minimizing the impact of thermal stresses on structures is of concern in both nuclear and conventional industries