Calculation of angle-dependent extrapolation lengths at the control channel edge of absorber assemblies

Based on a model formulated within the framework of two-group diffusion equation for the absorber section of WWER-440 type control elements and the Fourier series expansion of the corresponding solution of the modified one-group diffusion approximation, the extrapolation length on the control channel edge is investigated as a function of angle for an absorber eccentrically positioned in a homogeneous cylindrical reactor core. The angle-depending extrapolation length is shown to be calculable in a sufficient approximation from absorber-specific parameters and a flux distribution resulting from the assumption of a constant extrapolation length. The method of calculating angle-dependent extrapolation lengths is transferred onto the coarse-mesh difference schema method of the one-group diffusion equation and is also appropriate for a more exact consideration of heterogeneous core configurations. (author)