Production of denaturated fissile fuel in a thorium fusion breeder has been investigated by mixing the fertile fuel with natural-UO2 and LWR (light water reactors) spent nuclear fuel. Four different coolants (pressurised helium, Flibe 'Li2BeF4', natural lithium and Li17Pb83 eutectic) are selected for the nuclear heat transfer. In order to obtain a power flattening in the fissile-fertile zone, the UO2- or the spent fuel-fraction in the mixed oxide (MOX) fuel has been gradually increased in radial direction. Power plant operation periods between 13 and 29 months are evaluated to achieve a fissile fuel enrichment similar to4%, under a first-wall fusion neutron energy current of 5 MW m(-2) (plant factor of 100%). For a plant operation over 4 years, enrichment grades between 6.0 and 11.5% are calculated for the investigated MOX fuel and coolant compositions. The U-238 component of natural-UO2 can provide a limited proliferation hardening only for the U-233 component, whereas, the homogenous mixture of ThO2 with a small quantity of (>4%) LWR spent nuclear fuel remains all-over non-prolific for both, uranium and plutonium components. (C) 2001 Elsevier Science B.V. All rights reserved.