Coupled study of the Molten Salt Fast Reactor core physics and its associated reprocessing unit

• The limit on the reprocessing is due to the redox potential control.
• Alkali and Earth-alkaline elements do not have to be extracted.
• Criticality risks have to be studied in the reprocessing unit.
• The neutronics properties are not sensitive to chemical data.
• The reprocessing chemistry, from a pure numerical point of view, is an issue.

Molten Salt Reactors (MSRs) are liquid-fuel reactors, in which the fuel is also the coolant and flows through the core. A particular configuration presented in this paper called the Molten Salt Fast Reactor consists in a Molten Salt Reactor with no moderator inside the core and a salt composition that leads to a fast neutron spectrum. Previous studies showed that this concept (previously called Thorium Molten Salt Reactor – Nonmoderated) has very promising characteristics. The liquid fuel implies a special reprocessing. Each day a small amount of the fuel salt is extracted from the core for on-site reprocessing.To study such a reactor, the materials evolution within the core has to be coupled to the reprocessing unit, since the latter cleans the salt quasi continuously and feeds the reactor. This paper details the issues associated to the numerical coupling of the core and the reprocessing. It presents how the chemistry is introduced inside the classical Bateman equation (evolution of nuclei within a neutron flux) in order to carry a numerical coupled study. To achieve this goal, the chemistry has to be modeled numerically and integrated to the equations of evolution. This paper presents how is it possible to describe the whole concept (reactor + reprocessing unit) by a system of equations that can be numerically solved.Our program is a connection between MCNP and a homemade evolution code called REM. Thanks to this tool; constraints on the fuel reprocessing were identified. Limits are specified to preserve the good neutronics properties of the MSFR. In this paper, we show that the limit rate for the reprocessing is 2.5 l of fuel salt a day, which means that the fuel should be reprocessed within 7000 days approximately if there is a specific way to control the redox potential of the salt. Finally, a last part of this paper analyzes the impact of chemical parameter uncertainties on the reprocessing performance.