Isotopic and Energy Groupwise Dependence of Fuel Temperature Coefficient of Reactivity in Natural Uranium-Fueled Pressurized Heavy Water Reactors

We have studied the individual effect of the temperature dependence of the multigroup cross sections of 238U, 235U, 239Pu, 240Pu, and 16O on the calculated fuel temperature coefficient (FTC) by performing detailed sensitivity studies. The thermal contribution and the Doppler contribution of the FTC have been estimated for the above isotopes for the 19-element UO2-fueled heavy water lattice of the pressurized heavy water reactor (PHWR). The groupwise breakdown of the FTC due to 238U resonances has also been obtained. The FTC of Canada deuterium uranium reactor (CANDU)-type pressurized heavy water moderated lattices using UO2 fuel becomes less negative with burnup and changes sign at high burnups. Our studies clearly demonstrate that the positive component of the FTC in natural UO2-fueled PHWRs arises primarily because of the temperature dependence of scattering cross sections of 16O in agreement with the earlier findings of Stammler. In this paper, we have calculated the reactivity due to the change in fuel temperature, and all our discussions are based on this fuel temperature reactivity rather than the FTC itself.