Temperature dependence of unshielded cross-sections in multigroup cross-section sets

The self-shielding factor method in the multigroup approach is well known in Reactor Physics. The temperature and background dependent neutron cross-sections are conventionally represented in a problem-independent multigroup cross-section set by specifying for each group and reaction the unshielded cross-section (at 0 K) along with a set of self-shielding factors for various back- ground cross-sections and temperatures. Usually the unshielded group cross-section is taken for granted to be independent of temperature. This paper illustrates that the temperature dependence of the unshielded cross-section could be signi®cant depending on the relative positions of the group boundaries and the energies of the resonance peaks and also that the Doppler changes in the e€ective cross-sections could be signi®cantly a€ected by the convention. For internal consistency in a multigroup cross-section set the de®nition of self-shielding factor must be relative to the unshielded average at 0 K