Spherical fuel for high temperature nuclear reactors; thermal hydraulic analysis

High temperature nuclear reactors use gas as cooling fluid, graphite as moderator and spherical fuels. In this work, a spherical fuel such as those used in PBMR or HTR-PM nuclear reactors, is studied. The calculations are carried out for a gas flowing around the spherical fuel. Four inert gases, namely air, helium, carbon dioxide and nitrogen are investigated in three dimensions of space. The equation set-up is based on the resolution of the conservation equations of mass, momentum and energy. The k − ω SST turbulence model is used for the evaluation of turbulent viscosity. The main results obtained deal with the evolution of the pressure and the shear stresses around the fuel sphere as well as the evolution of the Nusselt number and the surface heat transfer coefficient. From the obtained results, one can see that the greatest rate of heat transferred from the spherical fuel is the one when helium is used. Following this, a parametric study is made to know the optimum gas velocity to use safely. This study shows also that the other three gases can also be used even in emergency situations, as the melting temperature of clad is never reached.