[en] In this report we compare various combinations of the fissile materials ²³⁵U, ²³³U, plutonium and the two fertile materials ²³⁸U and ²³²Th in thermal reactors having a neutron economy similar to that of CANDU. We assume that fuel burn-up is limited only by reactivity. The basis for this comparison is the fuel cost so that attention must be paid to factors affecting this cost such as the manufacturing cost, interest on fuel inventory and processing costs. The operating fuel cost depends on manufacturing cost and the burn-up. The inventory charges depend on the manufacturing cost and the fuel rating. The processing contribution to power cost depends on the concentration of fissile material in the fuel at the end of the irradiation and its value which is determined to some extent by its radioactivity and the consequent handling difficulties. A single once-through fuel cycle followed by processing has been considered. The effect of further recycling is taken into account by ascribing a value to the irradiated fuel. 7 refs, 17 figs, 3 tabs