The CANDU fuel bundle is relatively simple in structure. The fuel is designed to be compatible with on-power refuelling in a pressure-tube reactor, and like the reactor, to have high neutron economy. The bundle is small (nominally 10 cm in diameter, 50 cm long and weighing around 23 kg). These features facilitate remote handling and would, therefore, be suitable for recycling and fabrication in some advanced fuel cycles.

There are only seven components in a bundle (eight in the CANFLEX® design[21]). See Fig. 11.4 for an illustration of the components of a 37-element CANDU fuel bundle that is currently in use in many CANDU stations.

The fuel ‘meat’ is UO2 , with a density higher than in LWR fuel and with the ends of the pellets contoured (dishes and chamfers) to offset swelling and wheatsheafing (or hour-glassing, see Chapter 14).

The fuel sheath is composed of a zirconium alloy. The low burnup corresponding to the use of natural uranium fuel results in very low corrosion on the inside and outside of the sheath. The fuel sheath is thinner than in LWR fuel, which improves neutron economy and which allows the sheath to contact the fuel pellets under the pressure of the coolant and the thermal expansion of the pellets. This gives good thermal contact and reduces fuel centreline temperatures. The system is designed so that the coolant pressure always exceeds the internal fuel element pressure from gaseous fission products.

A graphite coating (CANLUB) on the inside of the fuel sheath protects against environmentally assisted cracking (EAC), sometimes called stress-corrosion cracking (SCC), which could be initiated by pellet-cladding interaction during on-power refuelling. One of the protection mechanisms of CANLUB is believed to be the gettering of corrosive fission products.

Small zironium-alloy spacer pads are brazed to the fuel sheath to prevent them from touching one another.

Larger zirconium-alloy bearing pads are brazed onto the outermost fuel sheaths to prevent them from touching the pressure tube. Both spacer pads and bearing pads increase coolant turbulence, which improves heat transfer. It is noted that in India, the appendages are welded (rather than brazed) to the fuel sheath.

Endcaps seal the element at each end.

The endcaps are welded to endplates at each end of the bundle, which hold the elements together. The bundle has to have sufficient structural rigidity so as not to fail through vibration or fretting, yet sufficient flexibility to pass through a pressure tube that has sagged as a result of age.

The CANFLEX bundle also contains additional small, non-contacting appendages brazed to the sheath to further promote coolant turbulence.

Two changes have taken place in Canada to enable more power to be obtained from the fuel bundle without exceeding power limits on the individual elements. The first was an increase in the number of fuel elements in a bundle, from 7 to 19, 28 and 37. The CANFLEX bundle, which has been qualified but is not yet in commercial use, has 43 elements arranged in rings of 1, 7, 14 and 21, with the central eight elements having a larger diameter than the outer 35. This results in a flatter rating profile across the CANFLEX bundle so that it becomes possible to generate 20% more power than the 37-element design at the same maximum linear element rating (Inch et al. , 2000). The second change was an increase in pressure tube (and fuel bundle) diameter, in going from the 19-element fuel bundle in the Douglas Point reactor (in which the pressure tube inside diameter was 82.6 cm) to the 28-element and 37-element fuel bundles in the Pickering and later reactors (in which the pressure tube inside diameter was 103 cm).

On-power refuelling increases the energy extracted from the fuel by about 25% compared to batch refuelling. This is because it improves neutron economy by avoiding the need for burnable neutron absorbers or control rods that, with batch refuelling, would be needed to suppress the excess reactivity. During on-power refuelling, a pair of refuelling machines attaches to each end of a fuel channel: new fuel bundles are inserted into one end of the channel and an equal number of old fuel bundles are discharged from the opposite end (Fig. 11.5). The CANDU 6 fuel reactor is refuelled in the direction of coolant flow, with the coolant drag pushing the fuel string down the channel. So both coolant flow and refuelling are bi-directional (coolant and refuelling direction being opposite in adjacent channels). This helps to flatten the axial power distribution, since old fuel at the end of one channel is next to new fuel in the adjacent four channels.

The refuelling rate matches the reactivity decay rate. In a CANDU 6 reactor with natural uranium fuel, about 15 bundles are replaced each day, using 8-bundle fuel shifts. Hence, two or three channels are refuelled on average each day. The number of bundles inserted at each visit of the refuelling machines to a channel is a balance between the reactivity perturbation (and resultant local power peaking) and the achievable refuelling frequency.

On-power refuelling provides the CANDU reactor with a great deal of flexibility, allowing it to accommodate different fuel types and fuel cycles. The number of bundles added during each visit of the refuelling machines can be reduced to accommodate higher enrichment fuel. It is also feasible to shuffle the bundles axially during refuelling, to shape the axial power distribution along the channel. (See Younis and Boczar (1989b) for an example of axial shuffling with LEU fuel.)

After discharge from the reactor, the used fuel bundles are stored in a water — filled bay at the station. After about six years, the decay heat from the used natural uranium fuel drops to a level which allows the fuel to be air cooled, and transferred to a dry-storage facility, if so desired. A number of such dry-storage designs are in use at CANDU power reactor sites (see for instance, several papers in CNS, 2005). The reference plan in Canada for the long-term management of used fuel is based on adaptive phased management. The ultimate endpoint of emplacement of the used fuel in a deep repository would be preceded by a long period of interim used-fuel storage, in which technical advances or changes in societal values or public policy could be accommodated (NWMO, 2005). The technical aspects of deep geological disposal have been well established. Funding is provided through a levy on the cost of electricity. The Canadian programme is managed by the Nuclear Waste Management Organization (NWMO), which includes participation from the nuclear utilities. This approach for the ultimate disposition of the used fuel has been established through extensive public consultation and includes site selection in a willing host community with appropriate geology.

77.5 Schematic showing CANDU on-power refueling (figure is copyright Atomic Energy of Canada Limited and is used with permission).

A number of intrinsic and extrinsic measures provide a high degree of proliferation resistance for all stages of CANDU reactor technology, from fuel fabrication to the handling and use of the fuel at the station, including fresh fuel receipt and storage, refuelling the reactor and used fuel management. Extrinsic measures ensure rigorous compliance with IAEA safeguards (Whitlock and Lee, 2009).

The achievable burnup in a CANDU reactor is determined by a number of factors: the fuel type (natural uranium, LEU, MOX, thorium); the core size (which determines the fraction of neutrons lost through leakage — so larger CANDU plants such as Bruce and Darlington have higher burnups than the smaller CANDU 6 reactors); and the number and reactivity worth of adjuster rods (reactivity control devices, which are discussed in Section 11.5). The burnup in a CANDU 6 reactor with natural uranium fuel is nominally 7.5 MWd/kg heavy element (HE); in the Bruce A reactor (which is larger and which has no adjuster rods) the burnup is about 8.9 MWd/kg HE.