Development and verification of thermal hydraulic model of the pebble bed high-temperature gas-cooled reactor core analysis code NECP-Panda

The pebble bed high-temperature gas-cooled reactor (PB-HTGR) has become a research hotspot in recent studies. Calculating the temperature distribution of the pebble bed and within the fuel pebbles is one of the key tasks in the thermal–hydraulic analysis. The VSOP code is used in the design for the HTGR Pebble Bed Module (HTR-PM) of China, in which THERMIX is the thermal–hydraulic calculation module. However, some calculation models of THERMIX are relatively simplified, such as the homogenized approach used in the heat conduction calculation of the fuel pebble scale, neglecting the thermal characteristics of the TRISO particle scale. To facilitate the localization and autonomy of core design code and achieve more accurate simulations, a three-dimensional thermal–hydraulic code for PB-HTGR, named NECP-Panda-TH, has been developed. For helium flow calculation, the code adopts the finite difference method (FDM) to solve the three-dimensional fluid momentum and mass conservation equations, and applies an exponential approximation to obtain fluid temperature distributions. In terms of solid heat conduction calculation, the Nodal Expansion Method (NEM) is applied to solve the solid heat conduction equation of the pebble bed. In addition, a multi-scale model is applied to obtain the accurate temperature distribution of fuel pebbles and even TRISO particles. The verification is then carried out based on HTR-PM, and the results are compared with both the THERMIX code and the commercial software COMSOL Multiphysics. The comparisons indicate that NECP-Panda-TH is capable of the thermal–hydraulic simulations of the HTR-PM.