Power flattening in the fuel bundle of a CANDU reactor

Abstract

The strong non-uniformity of the fission power production density in the CANDU fuel bundle could have been mitigated to a great degree. A satisfactory power flattening has been achieved through an appropriately evaluated method by varying the composition of the LWR spent fuel/ThO 2 mixture in a CANDU fuel bundle in radial direction and keeping fuel rod dimensions unchanged. This will help also to greatly simplify fuel rod fabrication and allow a higher degree of quality assurance standardization. Three different bundle fuel charges are investigated: (1) the reference case, uniformly fueled with natural UO 2, (2) a bundle uniformly fueled with LWR spent fuel, and (3) a bundle fueled with variable mixed fuel composition in radial direction leading to a flat power profile (100% LWR spent fuel in the central rod, 80% LWR + 20% ThO 2 in the second row, 60% LWR + 40% ThO 2 in the third row and finally 40% LWR + 60% ThO 2 in the peripheral fourth row). Burn-up grades for these three different bundle types are calculated as ∼7700, ∼27,300, and 10,000 MW.D/MT until reaching a lowest bundle criticality limit of k ∞ = 1.06. The corresponding plant operation periods are 170, 660, and 240 days, respectively.