In reactor criticality calculations the most usual approach is to start with an elementary cell
of the fuel lattice (this cell contains fuel and moderator separated). In this elementary cell the neutron slowing down and thermalization problems can be treated with optical reflecting
or isotropic reflecting (white boundary condition
). However, even this is too complicated in the resonance energy range. In a closely packed lattice the in-current of resonance neutrons into the fuel is reduced
, as compared to the in-current into a single fuel rod in an infinite moderator, because of the shadowing effect
of adjacent rods.
As a first approximation in the resonance self-shielding calculations, a single fuel lump (usually a fuel rod) in an infinite moderator is considered, and the presence of other fuel rods is taken into account by applying a certain correction, generally called the Dancoff correction.