The central reactivity worth measurements performed in Lady Godiva were duplicated using TWODANT, a deterministic neutron transport code, and the 16-group Hansen-Roach cross-section library. The purpose of this work was to determine how well the Hansen-Roach library predicts the reactivity worths for a fast neutron system. Lady Godiva is a spherical uranium metal (93.7 wt% 235U) critical assembly with a neutron flux distribution dominant in the first five groups of the Hansen-Roach energy structure (0.1 MeV and up). Provided that the cross sections of the replacement material do not undergo large variations (less than an order of magnitude) in any of the aforementioned groups, the calculated reactivities were within 10% of the experimental values. For cases where the reactivities were outside this range, a large variation in the cross section was found to exist in one of the groups, which was not fully accounted for in the Hansen-Roach group structure. However, even in the cases where the agreement between calculation and experiment was not good, the calculated reactivity appeared to be an extremum in that the effect was found to be either more negative or more positive than the experimental value.
