Application of sub-channel modeling to BWR core analysis

The current trends in nuclear science and engineering, related to modeling and simulation, are towards high-fidelity multi-physics multi-scale simulations to address industry challenge and high impact problems. These trends stimulate the utilization of sub-channel modeling approaches to nuclear reactor cores for both operation applications (core follow and cycle depletion evaluations) and safety applications (transient and accident analysis). While sub-channel modeling of Pressurized Water Reactor (PWR) cores has advanced significantly in last few years, the application of sub-channel modeling to Boiling Water Reactor (BWR) cores is under development. CTF is an improved version of COBRA-TF being developed and maintained by the North Carolina State University (NCSU) in cooperation with Oak Ridge National Laboratory (ORNL), and with support of the US Department of Energy (DOE) Consortium for Advanced Simulation of Light Water Reactors (CASL) as well as from the members of the CTF User's Group. CTF uses a two-fluid, three-field representation of the two-phase flow, which makes it capable of modeling the high-void flow conditions expected in BWR operation. This paper focuses on applications of CTF to mini- and whole-core BWR calculations on assembly/channel and pin-cell/sub-channel resolved levels as well as on demonstrating that CTF can properly model bypass flow. To increase the confidence in CTF's BWR modeling capabilities, simulations have been performed using the international Organization for Economic Cooperation and Development (OECD)/US Nuclear Regulatory Commission (NRC) Oskarshamn-2 benchmark, including modeling of a single assembly and a mini-core of 2 × 2 assemblies on a pin-by-pin/sub-channel level, and a full core model on an assembly/channel level. Each model is varied with an increasing amount of detail. Key parameters such as pressure losses and void fraction distribution were analyzed to determine the impact of different levels of detail within a thermal-hydraulic model on the simulation results. The results demonstrated that CTF is capable of modeling BWR core on different spatial resolution levels. The Oskarshamn-2 core simulations was used to further verify and demonstrate CTF's capabilities of modeling BWRs.