In contrast to the large pressure vessel of a light water reactor (LWR) containing long vertical fuel assemblies, the core of a CANDU reactor consists of ~380 horizontal fuel channels, each of which typically contains 12 fuel bundles.

A fuel channel consists primarily of a Zr-2.5%Nb (Zirconium-Niobium) alloy pressure tube, surrounded by a Zircaloy-2 calandria tube, kept apart by spacers, and with end fittings to allow on-power refuelling. The pressure tubes operate at full system temperature and pressure, and are insulated from the cool moderator by a gas gap.

CANDU fuel channels are based on a proven design and configuration. Incremental improvements—based on a fundamental understanding of ageing mechanisms at the micro-chemical and micro-structural scale—are continually incorporated into the design. By understanding the various mechanisms affecting such phenomena as corrosion, fracture toughness, and dimensional changes, AECL researchers continue to extend the life of CANDU fuel channels.

AECL's strategy is to maximize the life of our fuel channels (>30 years, with a target of 40 years), develop diagnostic technology capable of rapidly determining the state of in-service channels, and develop fast channel replacement technology to minimize reactor downtime. Our objective is to extend the life of CANDU reactors to 60 years or beyond. In the longer term, AECL is developing advanced fuel channel concepts that will increase safety margins, enable higher power operation, allow higher thermodynamic efficiencies, be more resistant to degradation, and will have a high degree of passive safety.