Criticality calculations with MCNP4A using correlated sampling methods

Two methods of calculating reactivity effects due to small changes in reactor configurations have been implemented in MCNP4A. The pseudo-correlated sampling method makes use of the correlation of particle tracks in problems with well localized perturbations. The true correlated sampling method forces a perturbed particle on the track of a reference particle and adjusts its weight for compensation. It works well for problems of small changes in the composition or density of materials and is not restricted to a localized region of the reactor arrangement. Both methods are using the fission matrix approach for calculating the eigenvalue of the multiplying system rather than the source iteration method. These methods of correlated sampling have been applied to problems in the FRM-II research reactor design, to study small reactivity changes due to installations in the moderator pool and to verify reactivity coefficients due to density changes of the cooling water and the heavy water moderator.