Study on Coolant Void Reactivity of Pressure-Tube-Type Heavy Water Lattice by the Substitution Method

Abstract

Using the substitution method combined with the pulsed neutron techniques, coolant void reactivities of PuO/sub 2/-UO/sub 2/ fuel lattices in pressure-tube-type heavy water reactors have been determined as functions of PuO/sub 2/ enrichment in PuO/sub 2/-UO/sub 2/(0.54 and 0.87 wt%), fissile content of plutonium (91 and 75% fissile plutonium), lattice pitch (V/sub m//V/sub f/: 7.4 and 9.9), and coolant void fraction (0, 30, 70, 87 and 100%). The reference loading of 1.2 wt% enriched UO/sub 2/ clusters was progressively replaced by PuO/sub 2/-UO/sub 2/ test clusters. The void reactivities were obtained from Simmons King's formula in which correction was made for a change of the prompt generation time. As decay constants can be maintained invariable due to substitution, buckling differences were analyzed by the first-order perturbation method, on the assumption that lattices are homogeneous and no difference in diffusion coefficients exists between the two lattices. Void reactivities of test lattices were determined with an accuracy of --10% when the minimum number of test fuel clusters was -- 5% of the total. The void reactivity shifted farther to the negative side as the proportion of fissile plutonium was increasingly in the PuO/sub 2/-UO/sub 2/ fuel of the same enrichment of plutonium.