Moderator displacers for reducing coolant void reactivity in CANDU reactors: a neutronics scoping study.
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When the coolant is voided in a CANDU lattice, 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 does not 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 to reduce the moderator-to-coolant volume ratio in existing CANDU reactors by packing the moderator with displacers in the shape of close-packed hollow spheres. Results show that it is possible to reduce the coolant void reactivity through the use of spherical moderator displacers, albeit at a cost in discharge burn-up.