THERMAL-HYDRAULICS AND NEUTRONICS COUPLING CALCULATION AND VALIDATION OF NECP-PANDA: A COMPUTATIONAL CODE FOR PEBBLE-BED HIGH TEMPERATURE GAS-COOLED REACTORS

In recent years, the Pebble-bed High Temperature Gas Cooled Reactor (PB-HTGR) has become a hot area in the research of the fourth-generation nuclear power system. The calculation of temperature distribution in the core and fuel sphere is one of the key tasks in the thermal design and safety analysis of the core. Due to the cylindrical coordinate characteristics of HTGR and the uncertainty of pebble flow in core, the complexity of coolant flow paths and the need to cover both the core and reflector regions in thermal calculations, accurate simulation of thermal hydraulics poses challenges. The mainstream PB-HTGR design and analysis codes use the fine-mesh finite difference method to solve the core physics and thermal hydraulic problems, and only has the ability of two-dimensional calculation and analysis. In order to address challenges in the core design and optimization of PB-HTGR and improve the accuracy of simulating the core state of the PB-HTGR during high temperature operation, the Nuclear Engineering Computational Physics (NECP) laboratory of Xi’an Jiaotong University has independently developed NECP-Panda, a physical and thermal hydraulics calculation code specially designed for PB-HTGR. In the thermal hydraulic calculation of the core, NECP-Panda solves the solid heat conduction equation by using the coarse mesh nodal expansion method (NEM) to obtain the three-dimensional temperature distribution of the solid. The fine-mesh finite difference method is used to solve the convection diffusion equation, and the three-dimensional pressure field distribution and the three-dimensional mass flow field distribution of gas are obtained. To ensure convergence, NECP-Panda incorporates the exponential approach method to handle gas energy conservation, and employs an assumption of linear solid temperature distribution between nodes to solve the gas temperature distribution in the core and flow channels. The three-dimensional temperature field and three-dimensional pressure field are iteratively solved using the overrelaxation method until convergence. The cross section of the physical calculation is updated according to the three-dimensional temperature distribution to obtain more accurate calculation results. To validate the computational capability and accuracy of NECP-Panda for nuclear-thermal coupling, core power and temperature distribution were computed using the High Temperature Reactor Pebble-bed Module (HTR-PM) model of a HTGR nuclear power plant demonstration project. The power and temperature distributions obtained from NECP-Panda exhibit good agreement with those calculated by VSOP, a computer code system for the comprehensive numerical simulation of the physics of thermal reactors. Therefore, NECP-Panda has good computational power and accuracy for the thermal hydraulics calculation of PB-HTGR.