Treatment of cavities in three-dimensional diffusion calculations based on the finite element method

The finite element method is a flexible tool to solve the neutron diffusion equation for complex three-dimensional problems like in HTR pebble bed reactors with inserted control rods into core and reflector. For the adequate treatment of regions for which the diffusion theory is not valid (e.g. for cavities or strong absorbers) a coupled transport/diffusion method was implemented into the FE-code DIFGEN which was successfully used also in 2D-finite difference diffusion calculations. It could be demonstrated that the coupled method allows to solve streaming problems in diffusion calculations for arbitrary 3D-FE-nets