CANDU fuel: past, present and future

The direction of CANDU (CANada Deuterium Uranium) fuel development was set in 1957 with the decision to build pressure tube reactors. Short - 50 cm long - rodded bundles of natural UO2 clad in Zircaloy was adopted to facilitate on-power fuelling to improve uranium utilization. Progressive improvements were made during 25 years of development, involving 650 man-years and 180 million dollars. Thermalhydraulic studies showed that more power could be extracted by increasing the number of fuel rods to the present 37. Mixing between coolant subchannels, originally promoted by a spiral wire wrap, was found to be satisfactory if brazed spacers were inserted to maintain rod-to-rod spacing. The latter change, combined with the use of resistance force-welds, rather than fusion-welding, to attach end caps, reduced the amount of zirconium and led to manufacturing economies. Today's CANDU bundle is based on the knowledge gained from extensive irradiation testing and experience in power reactors. Over 1000 individual rods, some highly instrumented, and 400 bundles were irradiated and inspected. Pioneering achievements were made in defining UO2 operating temperatures, UO2 structural changes, fission gas release, rod dimensional changes, Zircaloy corrosion and hydriding, and the mechanisms and prevention of power ramping defects. The latter led to the present CANLUB design that features a thin graphite layer between cladding and pellet. Fuel performance in power reactors has been excellent; less than 0.2% of the bundles irradiated have had defects