Symbiotic Pu/Th fuel cycle for HTGR

HTGRs, loaded with uranium or plutonium fuel do not authorize high breeding ratios. Consequently, the Gen IV requirement of sustainability with respect to the use of fissile resources may be difficult to meet. Relying on the thorium conversion into fissile uranium 233, the use of thorium is promising regarding the breeding ratio. Accordingly, the symbiotic Pu/Th fuel cycle is an interesting way to optimize the energetic use of plutonium. In the first part of this paper, we compare the performances of HTRs with different types of fuel (15% enriched U, Pu alone, Th-Pu) on the resource consumption. Relying on HTGRs, a good conversion ratio (typically 0.8) is mandatory to satisfy high temperature needs in the long term. In the second part, we discuss the neutronic calculation scheme for a prismatic HTGR annular core (GTMHR like, with an internal graphite reflector), and for a large HTGR (modeled with an assembly calculation). In the last part of the paper, Pu/Th, U233/Th and Pu-U233/Th fuel cycle are studied for the two types of HTGRs. The performances in terms of conversion ratios are compared. For a given power of 300 Mwe, several compositions for the fuel are studied. The fraction of thorium in the fuel composition is varying from 0% to 99% and the total fuel mass charged in the core ranges from 1200 kg to 15,000 kg. We show that good conversion ratios are very difficult to obtain for annular HTGRs. New designs must be considered, with a larger core, and a smaller power density (to reach the passive evacuation of residual power). Finally, a preliminary strategy for the deployment of these fuel cycles is presented. (authors)