Computational models provide a framework through which to predict impurity in-growth in reactor generated radiological sources. However, the energy group structure and methodology used in these codes can have a significant impact on the accuracy of neutron cross sections and, as a result, on the inventory values calculated. The European Activation SYstem II (EASY-II) partitions neutron data in a number of different standard structures and then uses these to generate energy collapsed cross sections for each neutron reaction of interest. How well these single values represent the true neutron environment of the reactor is key to the codes efficacy for evaluating source impurities for use in material attribution. By comparing EASY-II nuclide inventories for cobalt source materials against analytically derived equivalents, these approximations have been shown to have limited impact. However, of the fission applicable standard structures investigated, only XMAS and CCFE were capable of precisely accounting for the differences in the energies required to simulate all the neutron reactions of potential interest to forensic investigations.