Spatial Mesh Effects on Three-Dimensional x-y-z Neutron Diffusion Calculations in Fast Breeder Reactors

Spatial mesh effects are studied for three-dimensional x-y-z neutron diffusion calculations. By applying the perturbation theory, it is analytically predicted that the errors in eigenvalue and control rod worth due to the mesh effect vary with the square of the mesh spacing, or inversely with the square of the mesh number, along the x, y, and z axes. The relationships are confirmed numerically by three-dimensional diffusion calculations in ZPPR-10A. Exact solutions are obtained from extrapolation to infinitely fine mesh spacing using these relations and results of coarse-mesh calculations.