A detailed MCNPX (version 2.4.0) model of the CROCUS zero-power critical facility was used to investigate the effect of nuclear data on the analysis of reactivity perturbations caused by variations in the moderator level. The analysis was performed using the ENDF/B-VI (Release 0), ENDF/B-V, JEF-2.2 and JENDL-3.2 cross-section libraries, and compared with experimental values derived from period measurements. The results of this study demonstrate that predictions of the reactivity effects of moderator-level variations are largely independent of the nuclear data evaluation even when the associated keff values vary significantly.