Simulating turbulent mixing in nuclear reactor pressure vessels

FZR has 10 years of experience using ANSYS CFX software for CFD applications focused on nuclear reactor applications and more recently also chemical process simulations. These CFD activities are very closely connected to experimental investigations at facilities in the institute. Using advanced two-phase flow measurement techniques, a comprehensive data base for CFD code verification is being created. So in addition to single-phase flow nuclear reactor engineering applications, development and verification of physical models for two-phase flows can be performed. FZR is participating in the German initiative on CFD applications in nuclear reactor safety research in close co-operation with the Gesellschaft für Anlagen- und Reaktorsicherheit (GRS, the German nuclear safety authority) and ANSYS. The group of young and highly motivated CFD scientists that has been established at the institute also has a significant portion of the 100 processor Linux cluster of the FZR at its disposal. The FZR coordinated the experimental and analytical investigations of turbulent mixing inside pressurized water reactors (PWRs) performed within the EC project FLOMIX-R (Fluid mixing and flow distribution in the reactor circuit). The purpose of this project was to describe coolant mixing phenomena, particularly for severe accident scenarios. Such scenarios include steam line breaks and boron dilution, where mixing of coolant from different loops moderates the inflow of water with insufficient boron content or temperature into the reactor core. These changes can lead to reactor power excursions (rapid increase in reactor power) due to positive reactivity effects. An example of a typical boron dilution scenario is during start-up of the first main coolant pump after a slug of low borated water has formed in one of the cooling loops, where mixing is largely forced by the momentum introduced by the pump starting. Another safety issue arises during emergency core cooling (ECC) situations, when cold water is injected into a hot cooling loop. In this case, buoyancy-driven mixing is influenced by density differences in the fluid and is typical for so-called pressurized thermal shock (PTS) scenarios. When a streak of cold ECC water touches the reactor pressure vessel (RPV) wall, unacceptable thermal stresses can occur. Measurement data from several sets of mixing experiments, using advanced measurement techniques with enhanced temporal and spatial resolution, improved the basic understanding of turbulent mixing and provided data for CFD code verification.
Selected experiments were then simulated using ANSYS CFX and applying best practice guidelines (BPGs), a set of systematic procedures for quantifying and reducing numerical errors. BPGs were applied when considering computational grid resolution and time step, turbulence models, internal geometry modelling, boundary conditions, numerical schemes, and convergence criteria. These investigations highlighted the importance of grid quality, and also the need to minimize numerical diffusion by using second order discretization. In fact, first order schemes were found
to sometimes even provide physically incorrect results. ANSYS CFX was well able to predict the experimental flow patterns and mixing phenomena for both buoyancy-driven and momentum-driven flows. Two-equation turbulence models like k-ω or SST were found to be suitable for momentum-driven slug mixing, while Reynolds stress models provided better results for buoyancy driven mixing.
Comprehensive multiphase flow models, advanced turbulence models, second order discretization, and scalable parallel performance all combine to make ANSYS CFX a valuable tool at FZR. ANSYS CFX software has been instrumental in the development and verification of best practices guidelines for the use of CFD in nuclear safety analyses.