Transactions of the Korean Nuclear Society Spring Meeting Jeju, Korea, May 17-18, 2018 Multi-physics Coupled Reactor Core Analysis System of RAST-K2.0 with CTF and FRAPCON Hanjoo Kim a , Jinsu Park a , Jiwon Choe a , Jiankai Yu a , and Deokjung Lee a* a Department of Nuclear Engineering, Ulsan National Institute of Science and Technology 50 UNIST-gil, Ulsan, 44919, Republic of Korea * Corresponding author: deokjung@unist.ac.kr 1. Introduction In the past decades, the importance of best estimate analysis of nuclear reactors by a multi-scale, multi- physics code system has increased for nuclear safety analysis in nuclear engineering. (Consortium for Advanced Simulation of Light water reactors) CASL [1] project has been conducted by the US DOE to develop the capability of advanced modeling and simulation tools for improved performance of currently operating Light Water Reactors (LWR), and the MPACT [2] code has been coupled with CTF as a part of the CASL project. Seoul National University (SNU) has established a multi-physics coupled code system based on nTRACER [3]. For nuclear reactor design and safety analysis of LWRs, Ulsan National Institute of Science and Technology (UNIST) CORE group has developed a two- step approach nuclear reactor analysis code system called STREAM/RAST-K2.0 [4], [5]. The nodal code (RAST-K2.0) was coupled with a thermal/hydraulics (TH) code and a fuel performance (FP) code to construct a multi-scale, multi-physics analysis code system. The sub-channel TH code CTF [6] and the FP code FRAPCON [7] were selected to be coupled. Chapter 2 will describe the computational codes representing neutronic (N), TH, and FP. Chapter 3 will describe the coupled calculation scheme. Chapter 4 will present numerical results for the first cycle of the OPR-1000 reactor. 2. Computational Codes 2.1. Neutronic code RAST-K 2.0 [4] code is a reactor core analysis code being developed at UNIST for in-core fuel management study, core design calculation, load follow simulation, and transient analysis in neutronics. It solves a nodal diffusion equation by using a 3-dimensional 2-group UNM (unified nodal method), and it adopts CRAM (Chebyshev Rational Approximation Method) with a micro depletion method for the depletion calculation. The 2-group cross section and group constant data are provided by STREAM, which is a lattice physics code also developed at UNIST. Due to the functionalized cross section model in RAST-K2.0, cross section feedback is utilizable. STREAM [5] solves the transport equation to generate the nuclear data of fuel assembly and reflector models used in RAST-K2.0 by 2-dimensional MOC. By adopting a Pin based Slowing down Method (PSM) as the resonance treatment method, STREAM can obtain higher accuracy of numerical results. STORA code links STREAM and RAST-K2.0 by gathering STN files containing cross section and group constant data calculated by STREAM and reformatting it for use in RAST-K2.0. Fig. 1 presents a flowchart of the STREAM/RAST-K2.0 code system. Fig. 1. Two-step flowchart of ST/R2 code system. 2.2. Thermal/Hydraulics code CTF [6], originally developed by Northwest Laboratory in 1980, is a TH simulation code designed for LWR vessel analysis. It is available for solving sub- channel forms of 9 conservation equations by using a two-fluids, three-field (fluid film, fluid drops, and vapor) modeling approach. Because CTF models multi-rod arrays in the reactor core, channel to channel flow (cross- flow) can be considered in the simulation, giving more accurate coolant properties than other TH codes which simulate single fuel rods. Because CTF provides a module that converts channel-centered channel index to rod-centered channel index and a coupling interface module, it can be easily coupled with a neutronics code. MPI based parallelization accelerates the simulation. 2.3. Fuel performance code FRAPCON [7] calculates the steady-state, thermal mechanical response of oxide fuel rods in LWRs during long-term burnup conditions which occur during normal power reactor operation. For each time step, 1) heat conduction in the axial direction is calculated by using