Introduction to the physics of molten salt reactors

In the frame of a major re-evaluation of the molten salt reactor (MSR), we have developed a new concept called Thorium Molten Salt Reactor (TMSR), particularly well suited to fulfill the criteria chosen by the Generation IV forum. This reactor may be operated in simplified and safe conditions in the Th/U-233 fuel cycle with fluoride salts. Amongst all TMSR configurations, many studies have highlighted the configurations with no moderator in the core as simple and very promising. Since U-233 does not exist on earth and is not being produced today, we aim at designing a critical MSR able to burn the Plutonium and the Minor Actinides produced in today's reactors, and consequently to convert this Plutonium into U-233. Thus, the current fuel cycle can be closed thanks to TMSRs started with transuranic elements on a Thorium base, i.e. started in the Th/Pu fuel cycle, similarly to fast neutron reactors operated in the U/Pu fuel cycle. We analyze the characteristics of these reactor configurations, in terms of fissile matter inventory, salt reprocessing, waste production and burning, and finally deployment capabilities. Using a simple kinetic-point model, we analyze the reactor's behaviour as the total reactivity margins are introduced in the core. We thus confirm, beyond the classical advantages of molten salt reactors, the satisfactory behaviour of the TMSR and the excellent level of deterministic safety which can be achieved in such reactors. We then illustrate how the reactor can be driven with no control rod, either by controlling the extracted power or by monitoring the operating temperature. Finally we stress the hardiness and the flexibility of this TMSR concept, allowing it to be adjustable without loosing its advantages in the event of any technological stumbling block.