Moderator Displacers for Reducing Coolant Void Reactivity in CANDU Reactors: A Scoping Study

When the coolant is voided in the lattice of a Canada deuterium uranium (CANDU) reactor, the net reactivity change is positive, due primarily to the fact that the coolant and moderator are separated and the coolant volume is much smaller than the moderator volume. The modest loss in moderation occurring when coolant is lost is not sufficient to offset the positive reactivity contributions of increased fast fission rate and reduced epithermal absorption. A way to achieve a negative net reactivity effect on coolant voiding is to increase the importance of moderation in the coolant by decreasing the moderator-to-coolant volume ratio. This work proposes reducing the moderator-to-coolant volume ratio in existing CANDU reactors by packing the moderator with displacers in the shape of hollow spheres in a close-packed pattern. Several materials and shell thickness values are investigated for different fuel enrichments. Calculations are performed using the lattice code DRAGON. Results show that it is possible to reduce the coolant void reactivity in a CANDU lattice with spherical moderator displacers arranged in a hexagonal closed-packed array, albeit at a cost in discharge burnup.