Development of a coupled CFD---system-code capability (with a modified porous media model) and its applications to simulate current and next generation reactors

Motivated by recent developments in the field of Computational Fluid Dynamics (CFD) and recognizing the limitations on computing power, this dissertation is aimed at combining the desirable features of system codes and CFD codes, thus elevating the nuclear reactor thermal hydraulics simulation capabilities to address problems that cannot be addressed with existing computational tools. The goal is achieved by first implementing improved porous media models in a commercial CFD code and then by judicious coupling of the CFD code with a coarse nuclear system code. Computationally intensive CFD is used in spatial domains where the flow is expected to be three-dimensional; whereas a system code is used to simulate regions where the flow is expected to be one-dimensional or to simulate components such as pumps, etc. Work accomplished in this dissertation can be divided into the following five parts. · Test a commercial CFD code, FLUENT, by solving a nuclear-specific benchmark problem. Extend the porous media turbulence model in the CFD code using User-Defined Functions (UDFs). · Demonstrate the porous media simulating capability by a nuclear system using a combined CFD model of clear flow and porous media flow (for core region). · Propose a hybrid approach to couple a CFD code with a nuclear system code. Develop the coupled CFD---system-code approach. Verify the coupled code using a simple flow in a network of pipes. · Test the large scale application of the coupled CFD---system-code by modeling the Nuclear Steam Supply System (NSSS) of a Pressurized Water Reactor (PWR). · Demonstrate the potential of the coupled CFD---system-code for next generation reactors by applying it to a Gas Turbine - Modular Helium Reactor (GT-MHR).;Part 1 is accomplished by implementing a modified k - epsilon turbulence model for porous media in a CFD code (FLUENT) using UDFs. Transverse flow through porous media is simulated with the extended CFD code. Results are compared with experimental data.;In the second part of this thesis, the International Standard Problem (ISP) No. 43, rapid boron-dilution experiment, is simulated using FLUENT to verify capability to model nuclear systems. Australian Replacement Research Reactor (RRR) is modeled to demonstrate application of CFD, with porous media model for the reactor core. The parameters for the porous media model are obtained through a series of assembly level CFD simulations.;RELAP5-3D is introduced in Part 3 as the nuclear system code for coupled CFD---System-code development. UDF feature of FLUENT is used to develop the interface for this coupling effort. This innovative coupling approach is verified by comparing the results of a simple transient flow problem obtained using the coupled codes with the results from the CFD-only simulation and the system-code-only simulation.;Part 4 is the first large scale application of the coupled CFD---system-code. A simplified PWR NSSS is modeled by the coupled CFD---system-code approach developed in Part 3. Time-dependent three-dimensional reactor power profile is calculated in a PWR transient scenario which investigates the spatial impact of the coolant thermal mixing by using a specially developed discrete reactor kinetic model.;In Part 5 simulation of reactor coolant system in the GT-MHR vessel is carried out using the coupled CFD---system-code, demonstrating the potential of the coupled CFD---system-code approach to Gen IV reactor design and optimization.;Thus, by implementing an improved porous media model in a CFD code, and combining the best features of a CFD code and a nuclear system analysis code, a simulation capability has been developed to model three-dimensional effects in complete integral systems with existing computational resources. The utility of this capability has been demonstrated by applications to a PWR and to a GT-MHR. This coupled CFD---system-code capability will be useful in developing better optimized reactor designs by reducing reliance on conservative models and simulations.