Study of the neutronic and thermal coupling effect on VHTR fuel pebble using OpenMC and OpenFOAM.

• A coupled OpenMC/OpenFOAM code system has been developed and applied to the VHTR fuel pebble. • The coupled code system based on Monte Carlo and CFD method includes the TRISO double heterogeneity and multiscale effects. • The neutronic and thermal coupling effect of a VHTR fuel pebble was evaluated. • The traditional maximum temperature calculation method without neutronic feedback is slightly conservative. Pebble bed very high-temperature gas-cooled reactor (VHTR) has the advantages of inherent safety and high outlet temperature. It adopts a pebble fuel element with a double heterogeneity structure. This paper presents the thermal evaluations of the VHTR fuel pebble using the OpenMC/OpenFOAM code system. Firstly, the neutronic model using Cell-Based Method (CBM) and the multiscale heat conduction model are established respectively. The multiscale heat conduction model decomposes the fuel pebble on two different scales: 1) the mesoscale, defined as the fuel pebble scale; and 2) the microscale, defined the TRISO fuel particle scale. The neutronic and thermal (N/T) calculations are coupled by Picard method. The results of the steady-state coupling study of the VHTR fuel pebble show that the spatial self-shielding effect has a greater impact on the temperature of the center of the fuel pebble compared with other locations. For different fuel pebble power and pebble boundary wall temperature, the maximum mesoscale temperature and microscale temperature calculated with N/T coupling are lower than those calculated without N/T coupling. Therefore, the traditional maximum temperature calculation method without N/T coupling is conservative. And the conservative degree increases with the operating parameters of the fuel pebble. [ABSTRACT FROM AUTHOR]