The fissile breeding capability of a (D,T) fusion-fission (hybrid) reactor fueled with thorium is analyzed to provide nuclear fuel for light water reactors (LWRs). Three different fertile material compositions are investigated for fissile fuel breeding: (1) ThO2; (2) ThO2 denaturated with 10% natural-UO2 and (3) ThO2 denaturated with 10% LWR spent fuel. Two different coolants (pressurized helium and Flibe 'Li2BeF4') are selected for the nuclear heat transfer out of the fissile fuel breeding zone. Depending on the type of the coolant in the fission zone, fusion power plant operation periods between 30 and 48 months are evaluated to achieve a fissile fuel enrichment quality between 3 and 4%, under a first-wall fusion neutron energy load of 5 MW/m(2) and a plant factor of 75%. Flibe coolant is superior to helium with regard to fissile fuel breeding. During a plant operation over four years, enrichment grades between 3.0 and 5.8% are calculated for different fertile fuel and coolant compositions. Fusion breeder with ThO2 produces weapon grade U-233. Th, denaturation of the U-233 fuel is realized with a homogenous mixture of 90% ThO2 with 10% natural-UO2 as well as with 10% LWR spent nuclear fuel. The homogenous mixture of 90% ThO2 with 10% natural-UO2 can successfully denaturate U-233 With U-238. The uranium component of the mixture remains denaturated over the entire plant operation period of 48 months. However, at the early stages of plant operation, the generated plutonium component is of weapon grade quality. The plutonium component can be denaturated after a plant operation period of 24 and 30 months in Flibe cooled and helium cooled blankets, respectively. On the other hand, the homogenous mixture of 90% ThO2 with 10% LWR spent nuclear 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) 1998 Elsevier Science Ltd. All rights reserved.