Fuel Condition in Canadian CANDU 6 Reactors

Abstract

The cornerstone of the CANDU concept is its natural uranium fuel, and the success of its reactor operation hinges on the fuel condition in the reactor. Neutron economy, on power refuelling and simple fuel design are among the unique characteristics of CANDU fuel.

In Canadian CANDU 6 reactors (Gentilly 2 and Point Lepreau), the 37-element fuel has provided an enviable record of safe, economic and reliable plant operation for 29 reactor years to date. The fuelling cost is among the lowest in the world - a corollary of high neutron economy, simple fuel design, and judicial fuelling scheme. The reliability of fuel is high: only 21 of the 60000 bundles discharged from Gentilly 2 were confirmed defective and the five-year period from March 1992 to February 1997 saw no defect at all at Gentilly-2. Also, thanks to the inherent on-power refuelling capability and an effective defect detection and removal system, the primary coolant loops are kept extremely clean (very low activity level) - benefitting both maintenance and safety. Moreover, the inventories of fission products in the core and in the channel are maintained within the safety analysis envelope, due to on-power fuelling and sophisticated fuel management.

In this paper, CANDU 6 fuel performance is reviewed against the feedback from post-irradiation examinations, and the findings from our ongoing R and D program. The results suggest that the fuel behavior in reactor are basically as originally anticipated, despite an evolutionary 3% increase in bundle uranium mass in the 1980's For operating conditions within the CANDU 6 37-elementexperience, the average strains are typically 0.09%; and fission gas release, 2.7% The UO₂ fuel remains stoichiometric after irradiation In-core measurements of pressure tube fretting are generally low. All these observations are consistent with the excellent fuel performance statistics coming out of the two Canadian CANDU 6 reactors.

Additionally, this paper will briefly discuss our experience in some situations which are not normally encountered by the fuel, such as return to full power after a long period of low power operation, response to the loss of electric power, and sustained shim-mode operation.