The natural uranium fuel cycle has worked well in the CANDU reactor: both front — and back-end of the fuel cycle are economical, fuel performance has been excellent and uranium utilization is high. The advantages of introducing an advanced fuel cycle into the CANDU reactor would need to be compelling in order to offset the costs of fuel qualification, increased fuel costs and the analysis costs for reactor safety and licensing. The considerations would include not only economics, but local, national and strategic aspects such as security, diversity and availability of energy and fuel supply. The features that enable the use of natural uranium fuel facilitate the use of other advanced fuel cycles such as RU, LEU, MOX, minor actinides and a variety of thorium fuel cycles.

Before describing specific fuel cycles, some general characteristics will be listed: [22]

• The simple, small fuel bundle design facilitates remote processing and fabrication for highly radioactive recycle fuels.

• On-line refuelling in a pressure tube reactor provides flexibility in fuel management to accommodate both high and low reactivity fuel and to shape the axial and radial power profiles.

• An extensive array of flux detectors in the core ensures knowledge of the flux and power distributions in the core, regardless of fuel type.

LEU

The optimal fuel enrichment in the CANDU reactor from the perspective of uranium utilization is around 1.2% U-235, which would result in a burnup of ~21 MWd/kg HE and uranium utilization almost double that of an LWR (Boczar et al., 1996). This enrichment could be accommodated by a 2-bundle shift, bi-directional refuelling scheme (Younis and Boczar, 1989a). The economics of the use of LEU in CANDU will depend of course on whether the increased costs for enrichment, conversion, fuel fabrication, fuel qualification and safety and licensing are offset by the higher fuel burnup. Enrichment can also be used in the CANDU reactor to tailor reactivity coefficients. For instance, in the Low Void Reactivity Fuel (LVRF) bundle, a BNA material in the centre element of the fuel is compensated by enriched uranium in the outer elements (Boczar et al., 1992). BNA content and enrichment can be chosen to give desired values of coolant void reactivity and fuel burnup. This of course is at the expense of fuel utilization. Enrichment can also be used to achieve other objectives, such as power uprating by flattening the radial power distributions across the core (without exceeding maximum bundle or channel powers).