The Verification, Validation and Application of a Hybrid Domain Overlapping Coupling between System Thermal Hydraulics and Computational Fluid Dynamics Codes

Abstract

One important problem in nuclear reactor safety analysis is how to compute the evolution of accident events and accurately estimate reactor safety margins. Currently, regulatory authorities accept simulation results obtained from System Thermal Hydraulics (STH) codes if the application's conditions fall within the code's range of validation. STH codes rely on a simplified one-dimensional (1D) representation of the power plant, and the simplified representation utilizes several 1D closure correlations informed by experimental data or high fidelity simulations. However, when three-dimensional (3D) flow effects are significant, the simplifying 1D assumptions of STH codes breakdown and may lead to non-conservative results. In current and next-generation nuclear reactor systems, components such as containment compartments, reactor vessel and pools contain 3D effects that can play an important role in the evolution of accident scenarios. This necessitates the use of Computational Fluid Dynamics (CFD) codes that can better-predict 3D flow and mixing phenomena. However, modeling an entire reactor system with CFD remains prohibitively computationally expensive. As a result, the coupling of CFD with STH codes is an important undertaking.

The majority of STH/CFD coupling efforts have utilized the traditional domain decomposition method, where CFD models a region of the system where 3D effects are relevant and STH models the rest. Recently, Grunloh et al. proposed a domain overlapping method where the STH code models the entire system, including the region modeled by CFD. However, the work was only focused on demonstrating the method for coupling mass and momentum.

In this thesis, the method is extended to include the coupling of energy and scalar transport; and it is further generalized to produce a much-simpler implementation, referred to as the hybrid domain overlapping method. The new coupling method is verified using canonical open and closed flow loops, and the coupling method's numerical stability and convergence show favorable behavior compared to the domain decomposition method. The new method is first validated against an isothermal double T-junction experiment from open literature. The experiment was designed such that the transport of an injected tracer is strongly affected by 3D mixing effects. Here, the STH/CFD coupled model matches experimental data much better than the STH standalone model. Then, the new method is validated against the nonisothermal, TALL-3D experimental STH/CFD coupling benchmarking facility: a liquid-metal facility with a pool-type enclosure for CFD modeling. The STH/CFD coupled model is validated against six steady states, followed by two transients from forced circulation to natural circulation. The STH/CFD coupled model is able to reproduce flow reversal observed during the first transient as well as the limit cycle oscillations observed during the second transient. Lastly, the coupling method is applied to the safety analysis of a sodium-cooled fast reactor undergoing a protected loss of flow event, where CFD models the reactor's hot pool. The STH/CFD coupled model reveals a possible safety concern considering the predicted maximum fuel cladding temperature that is more severe than in the STH standalone model.

From this thesis, the hybrid domain overlapping coupling method emerges as a simple-to-implement, numerically robust and validated STH/CFD coupling scheme for the safety analysis of nuclear reactor systems. The new coupling method stands on firm ground to aid in the advancement of safer, more efficient and robust nuclear energy systems.