CANDU reactors and AGRs generally have fuel rod design specifications similar to those of LWRs. The CANDU reactors use natural uranium oxide or slightly enriched uranium oxide contained within a thin Zircaloy clad, and design burn-up is lower than that of LWRs. In AGR fuel rods, uranium dioxide pellets, enriched to about 3%, are encased in a stainless steel clad. Fuel bundles of both the reactors have circular, cylindrical shapes to fit in the pressure tube ofCANDU reactors or in the graphite sleeve ofAGRs. The fuel rod diameter differs according to the number of fuel rods per bundle. Typical CANDU fuel rod design specifica­tions for a 28-rod bundle are presented in Table 2.3 The overall fuel rod lengths of both the reactor types are much shorter than those of LWRs in order to fit their fuel assembly design which enables on-load refueling.

2.15.3.1.3 Fuel rods for FBRs

FBR fuel rods contain MOX pellets having high plutonium content, with the exception of Russian FBRs, BN-350, and BN-600 in which high enrich­ment UO2 fuel pellets have been mostly used. Fuel pellets of less than 8 mm diameter are encased in a stainless steel cladding; they operate at a high linear heat rate with centerline temperatures of around 2000 °C or higher. Under these conditions, fission gas release is typically high (>80%) and a very large plenum is included to limit gas pressure. The gas plenum is located at the bottom of the rod in some fuel designs, aimed at minimizing plenum length, thanks to the lower gas temperature at the bottom of the rod. Upper and lower sections of the depleted UO2 pellets are included for breeding. Pellet-smeared density is set not to exceed a crite­rion that is formulated as a function of burn-up to avoid fuel-cladding mechanical interaction at high burn-up; high-density annular pellets or low- density solid pellets are used; the former lower the fuel centerline temperature allowing a higher linear heat rate.3