The Integrity of CANDU Fuel at Extended Burnups

Abstract

The performance of CANDU fuel has been excellent to date. Planned evolutions of the CANDU reactor include the CANDU 3 design and the high-burnup version of the CANDU 6 design. These are expected to place increased demands on the 37-element CANDU fuel, such as a combination of: peak element rating of 60 kW/m; element burnups of 700-4300 MWgU; power-ramps at high burnup; and in-reactor residence time of 800-900 hot-coolant days. Corrosion is expected to remain very low in CANDU fuel sheaths under these conditions. Readily-available engineered solutions like plenums and bigger chamfers keep the gas pressure below the coolant pressure. The stresses and the strains in the fuel sheath at circumferential ridges can be reduced significantly by improving the internal design of the fuel element. The following are some illustrative examples of fuel design features that achieve this: lower initial density of U02; smaller pellets; bigger lands; shallower dishes; central holes; and graphite discs. The CANFLEX design, which subdivides the fuel bundle into 43 elements compared to the 37 elements used in the current design, can reduce the gas pressure and the stresses/strains even further.