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.