One-Dimensional Analysis of Thermal Stratification in AHTR and SFR Coolant Pools

Thermal stratification phenomena are very common in pool type reactor systems, such as the liquid-salt cooled Advanced High Temperature Reactor (AHTR) and liquid-metal cooled fast reactor systems. It is important to accurately predict the temperature and density distributions both for design optimization and accident analysis. Current reactor system analysis codes only provide lumped-volume based models that can give very approximate results and can only handle simple cases with one mixing source. While CFD methods can be used to analyze simple configurations, these methods require very fine grid resolution to resolve thin substructures such as jets and wall boundaries, yet such fine grid resolution is difficult or impossible to provide for studying the reactor response to transients due to computational expense. Therefore, new methods are needed to support design optimization and safety analysis of Generation IV pool type reactor systems. Previous scaling has shown that stratified mixing processes in large stably stratified enclosures can be described using one-dimensional differential equations, with the vertical transport by free and wall jets modeled using standard integral techniques. This allows very large reductions in computational effort compared to three-dimensional numerical modeling of turbulent mixing in large enclosures. The BMIX++ (Berkeley mechanistic MIXing code in C++) code was originally developed at UC Berkeley to implement such ideas. By including liquid salt properties, BMIX++ code is extended to analyze liquid salt pool systems in the current AHTR baseline design, to provide an example of its application. Similar analysis is possible for liquid-metal cooled reactors.