The current work investigates the thermal-hydraulics of the fast core of a HYbrid fast/thermal core configuration Subcritical Testbed (HYST), a new concept for irradiation experiments in fast-reactor-like environment. The Reynolds-Averaged Navier Stokes (RANS) simulations using ANSYS Fluent are performed. A benchmark study is first performed to validate the simulation of liquid LBE flow in a wire-wrapped rod bundle geometry using the experimental data in Pacio et al. (2016), which provides the most detailed data for rod surface temperature, LBE temperature in the subchannel, and pressure drop along the test section. Two turbulence models (Standard k-ε model with Standard Wall Function and Realizable k-ε model with Enhanced Wall Treatment), two approaches to simplify the wire-rod contact and different models for turbulent Prandtl number (Prt) are tested. It is found that the selected models can predict the data well and are therefore used for simulations of the LBE loop in HYST. A mesh independence study is performed in simulating the LBE loop in HYST with three different meshes being generated (number of cells: 14.8 million, 25.8 million, and 45.4 million). It is found that the mesh independency is reached using the mesh with 25.8 million cells. The effect of non-uniform power distribution in the radial direction of the core is investigated. It is found that the temperature profiles change significantly when uniform power distribution changes to non-uniform. An experimental facility is being constructed and newly collected data will be used to evaluate the RANS simulations. Once validated, the current model will be valuable for the design, licensing, and operation of the HYST prototype.