Neutronics and thermal-hydraulics coupling analysis in accelerator-driven subcritical system

Abstract Accelerator-driven subcritical system (ADS) is a new generation nuclear reactor with various highly coupled physical fields. A typical system is a liquid metal, lead-bismuth eutectic cooled subcritical reactor core coupled to a neutron spallation target. Therefore, ADS simulations require multi-physics coupling among the proton, neutronics, and thermal hydraulics. In this work, GEANT4, RMC, and FLUENT were used to simulate the multi-physics processes in MYRRHA. The GEANT4/RMC was used for the spallation process and the proton-neutron transport calculations, with the power distributions verified against the MCNP6 code with an average difference of about 1.85%. A hybrid RMC/FLUENT coupling scheme was used for the neutronics thermal-hydraulics calculations. The subcritical reactor was simplified using the porous media method for the FLUENT simulations to obtain the coolant temperature and density fields. A temperature-dependent thermal conductivity model used to calculate the temperature filed in typical fuel pellets gave consistent results with the FLUENT predictions within an absolute error of 3 K. The neutronics and thermal-hydraulics coupling took the temperature and the lead-bismuth eutectic density feedback into consideration. Simulations show that thermal-hydraulics feedback has a fairly small effect on the power distribution in the ADS reactor. In this case, the radial power difference between the coupling and coupling-free is within 5%. In addition, the mechanisms of neutronics/thermal-hydraulics are compared for lead-bismuth eutectic cooled reactors and water-cooled reactors. The feedback is much more significant in water-cooled reactors than in the Pb-Bi cooled reactor since the water moderation, and the water density is more sensitive to the temperature.