The proliferation hardening of the U-233 fuel in a thorium fusion breeder has been realised successfully with a homogenous mixture of ThO2, natural-UO2 and CANDU spent nuclear fuel in the form of a triple mixed oxide (TMOX) fuel. The new U-233 component will be successfully hardened against proliferation with the help of the U-238 component in the natural-UO2 and spent fuel. The plutonium component remains non-prolific through the presence of the Pu-240 isotope in the spent CANDU fuel due to its high spontaneous fission rate. A (D,T) fusion reactor acts as an external high energetic (14.1 MeV) neutron source. The fissile fuel zone, containing 10 fuel rod rows in the radial direction, covers the cylindrical fusion plasma chamber. A quasi-constant power density in the fissile zone has been achieved by reducing the ThO2 component in the rods continuously in the radial direction (from 91 down to 64%). Three different coolants (pressurised helium, natural lithium and Li17Pb83 eutectic) are selected for the nuclear heat transfer out of the fissile fuel breeding zone with a volume ratio of V-coolant/V-fuel = 1 in the fissile zone. The fissile fuel breeding occurs through the neutron capture reaction in the Th-232 (ThO2), in the U-238 (natural-UO2 and CANDU spent fuel) isotopes. The fusion breeder increases the nuclear quality of the spent fuel, which can be defined with the help of the cumulative fissile fuel enrichment (CFFE) grade of the nuclear fuel calculated as the sum of the isotopic ratios of all fissile materials (U-233+U-235+Pu-239+Pu-241) in the TMOX fuel. Under a first-wall fusion neutron current load of 10(14) (14.1 MeVn/cm(2) s), corresponding to 2.25 MW/m(2) and by a plant factor of 100%, the TMOX fuel can achieve an enrichment degree of similar to1% after similar to 12-15 months. A longer irradiation period (similar to 30 months) increases the fissile fuel enrichment levels of the TMOX towards much higher degrees (similar to2%), opening new possibilities for utilisation in advanced CANDU thorium breeders. The selected TMOX fuel remains non-prolific over the entire period for both uranium and plutonium components. This is an important factor with regard to international safeguarding. (C) 2000 Elsevier Science Ltd. All rights reserved.