MODELING OF CENTRAL REACTIVITY WORTH MEASUREMENTS IN LADY-GODIVA

The central reactivity worth measurements performed in Lady Godiva wer e duplicated using TWODANT, a deterministic neutron transport code, an d 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 spherica l uranium metal (93.7 wt% U-235) critical assembly with a neutron flux distribution dominant in the first five groups of the Hansen-Roach en ergy structure (0.1 MeV and up). Provided that the cross sections of t he replacement material do not undergo large variations (less than an order of magnitude) in any of the aforementioned groups, the calculate d reactivities were within 10% of the experimental values. For cases w here the reactivities Mere outside this range, a large variation in th e cross section was found to exist in one of the groups, which was not fully accounted for in the Hansen-Roach group structure. However, eve n 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.