Extensive fuel performance data obtained from Canadian Deuterium Uranium (CANDU) power reactors and experimental reactors are analyzed. Various modes of power ramping that cause fuel defects are identified, and fuel defect criteria are derived. The probability of a defect occurring in a power ramp is found to depend on the magnitude of the ramped power, the power increase, the fuel burnup, and the time fuel dwells at the ramped power.

The fuel defect rate in CANDU power reactors based an the last 10 years of operation and due to all causes has been only 0.27%. The understanding gained through the development of the CANDU power ramp defect criteria has caused the defect rate to decrease progressively.

Physical interpretations are offered in terms of mechanisms believed to account for power ramp defects. The dominant mechanism is considered to be stress corrosion cracking of the Zircaloy clad, induced by the release of such fission products as iodine from the fuel.