Numerical Investigation of Mass Mixing in the Reactor Core of a Pebble Bed HTGR

In the reactor core of a pebble bed HTGR, fuel pebbles are in random arrangement. The coolant flow passages are complicated. Under the air and water ingress accident conditions, the partially enriched oxidizing gas corrodes the fuel pebbles by oxidation reaction. How the oxidizing gases (oxygen and water vapor) diffuse in the pebble bed will greatly influence the oxidation process. Analysis of the mass mixing is important for analyzing the fuel element graphite corrosion and safety of HTGR. In this paper, a three-dimensional simple cubic structure packed cells of pebble bed core model with surface contact method was established. The flow, and mass mixing processes were numerically and theoretically studied using numerical simulation and porous media methods. The effects of molecular diffusion, turbulent diffusion and mass dispersion on mass mixing effect were discussed respectively.