Abstract : Conversion performance of thorium-fueled CANDU and PWR reactors has been studied to evaluate their potential use as the third and last tier of symbiotic scenarios. For instance, plutonium extracted from the spent fuel of UOX PWRs could be used in Th/Pu CANDUs to produce uranium (mainly 233U), which could finally feed a high conversion water-cooled third reactor fleet component. This option could help to replace likely delayed Generation IV in the case of a strong increase of uranium-based energy production. In order to assess the competitiveness of such scenarios, detailed cycle data have been obtained by means of a core-equivalent simulation methodology developed for CANDU-6 and adapted to N4-type PWR. Breeding in Th/233U CANDU is achieved for a 1.30 wt% homogeneous enrichment and a short burnup of 7 GWd/t. Small increase of enrichment (to 1.35 wt%) considerably extends cycle length (to 14 GWd/t), at the cost of sub-breeding. Multirecycling induces conversion loss too, which can yet be compensated by fissile heterogeneity. In Th/233U PWR at standard power, the conversion is lower than in CANDU (with about half of initial fissile load after 50 GWd/t) but can be improved by sub-moderation. Neutronic comparative analysis shows that main part of the CANDU-PWR conversion gap comes from neutron-economical CANDU operation conditions. Scenarios have been compared as regards uranium savings and fuel back-end in both cases, and have confi rmed great interest in CANDU. Two research tracks have been identi ed and preliminarily explored : the safety assessment of thorium-fueled CANDUs by kinetics with thermal feedback, and the study of strongly sub-moderated cores in a standard PWR envelope.