Improved Dancoff factors for cluster fuel bundles by the WIMSD code

The recently developed tracking routine PIJM, for computing improved two-dimensional first-flight collision matrices for the WIMSD code, has been modified to evaluate black Dancoff factors for cluster fuel bundles in unit cells with cylindrical outer boundaries. The routine, similar to the original PIJ but employing an efficient in-plane integration scheme of the Bickley functions appearing in those expressions, is first compared with the accurate Carlvik method in the calculation of Dancoff factors for regular lattices, both with and without a gap surrounding the fuel rods. Dancoff factors for the more general cases of CANDU37 and CANFLEX fuel bundles are next obtained and compared with Monte Carlo based calculations, illustrating the accuracy of the proposed scheme.