Abstract : In the event of a significant nuclear contribution to world energy needs, the problem of present nuclear waste management throws the durability of the PWR fuel cycle back into question. Studies on storage and incineration of these waste should therefore go hand in hand with studies on innovative systems dedicated to a durable nuclear energy production, as sober, clean and safe as possible. We are here interested in the concept of molten salt reactor, whose fuel is liquid. This particularity allows an online pyrochemical reprocessing which gives the possibility to overcome some neutronic limits. In the late sixties, the MSBR (Molten Salt Breeder Reactor) project of a graphite-moderated fluoride molten salt reactor proved thus that breeding is attainable with thorium in a thermal spectrum, provided that the online reprocessing is appropriate.
By means of simulation tools developed around the Monte Carlo code MCNP, we first re-evaluate the performance of a reference system, which is inspired by the MSBR project. The complete study of the pre-equilibrium transient of this 2500 MWth reactor, started with 232Th/233U fuel, allows us to validate our reference choices. The obtained equilibrium shows an important reduction of inventories and induced radiotoxicities in comparison with the other possible fuel cycles. The online reprocessing is efficient enough to make the system breed, with a doubling time of about thirty years at equilibrium.
From the reference system, we then test different options in terms of neutron economy, transmutation and control of reactivity. We find that the online reprocessing brings most of its exibility to this system, which is particularly well adapted to energy production with thorium. The study of transition scenarios to this fuel cycle quantifies the limits of a possible deployment from the present french park, and finally shows that a reasoned management of the available plutonium would be necessary in any case.