Thermohydraulic Analysis of Coolant Entry Patterns in a High-Temperature Pebble Bed Reactor: Insights and Characterization

Conducting three-dimensional thermohydraulic analysis of an entire nuclear reactor poses significant challenges due to the considerable geometric volume and complex internal structures involved. The top reflector is one of the internal structures found in high-temperature pebble bed Small Modular Reactors (SMR). This structure serves several critical functions, including neutron reflection, control and distribution of helium inlet into the core, neutron and thermal shielding, among others. In this kind of system, the detailed representation of the top reflector includes the representation of more than 460 channels of 2.5cm of diameter. Considering that the reactor has almost a ten of meters then dimension scales of various orders must be represented, which is a challenge. In this sense, a three-dimensional Computational Fluid Dynamics (CFD) thermohydraulic analysis of the entry pattern to the core of a High Temperature SMR using ANSYS CFX has been done. This study presents a comparison between five coolant entry patterns into the core. Initially, two prototype models of 460x2.5cm, one with vertical channels and another with inclined channels, are modeled. Additionally, two prototype models of 20x12cm of equivalent area, with vertical and inclined channels are also included. Finally, a simplified porous media model with the same equivalent area is considered. The thermohydraulic behavior of the coolant before and after passing through the top reflector was then analyzed for these five patterns. An analysis of fuel elements temperature in the core was conducted. It is important to highlight that this study is qualitative and has the goal of identifying and characterizing the impact that the coolant entry pattern into the reactor core has on the main thermohydraulic parameters in this region. The study exposes a strong correlation between the porous media model and all prototype models in terms of the maximum fuel temperature, average fuel temperature, and helium velocity. In this study, the potential applicability of the porous media models for an integral full-scale reactor simulation in the future was demonstrated. As a benefit, the porous media model reduces the mesh quantity compared to a prototypic model.