Influence of Burnable Gadolinia Poison on Coolant Void Reactivity in a Pressure-Tube-Type Heavy Water Reactor

The influence of burnable gadolinia poison in fuel assemblies on coolant void reactivity for a pressure-tube-type boiling light-water-cooled heavy water reactor is investigated in critical experiments using the Deuterium Critical Assembly (DCA) and theoretical analyses by the WIMS-D4/ CITATION code system. The experimental and the calculated void reactivities agree within ±0.2 $., A number of gadolinia-poisoned fuel assemblies are dispersively loaded in the central region of  the DCA core together with unpoisoned fuel assemblies. Each gadolinia-poisoned assembly is composed of three or four Gd₂O₃-poisoned UO₂fuel rods as well as unpoisoned fuel rods. The gadolinia concentration is varied from 0.0 to 1.0 wt%., The void reactivity in the core becomes less negative with the addition of gadolinia but become saturated at a Gd₂O₃ concentration of ≈0.5 wt%. The void reactivity becomes much less negative with a higher loading ratio of the gadolinia-poisoned assemblies in the core and as the gadolinia-poisoned fuel rods are arranged in the outer layers of the assembly. When the fissile nuclide in the pellets of the unpoisoned fuel assembly is changed from uranium to plutonium, the incremental positive shift of the void reactivity can be reduced because of the increase in the 0.3-eV thermal resonance absorption of ²³⁹Pu and ²⁴¹ Pu.