Modelisation of a postulated reactivity insertion in a generation IV molten salt reactor

The reference MSFR (Molten Salt Fast Reactor) is a 3000 MWth molten salt reactor. In most molten salt
reactors, such as the MSFR, the fissile and fertile materials are dissolved in a circulating salt that acts both as fuel
and as coolant. The physical state of the fuel permits to consider draining as a way to mitigate hypothetical
accidents. Moreover, a large-scale compaction cannot occur in such a core because, contrary to the solid fuel in a
Fast Neutron Reactor, the fuel is nearly in its most compact geometry. This concept can be operated in the Th/U
cycle and a fluoride salt or in the U/Pu cycle and a chloride salt. The goal of this work is to study the MSFR
behaviour in case of a postulated reactivity insertion. In order to study the consequences of extreme reactivity
insertions, the first objective is to study slow reactivity insertions to verify the efficiency of the draining of the
liquid core. Then, in the case of extreme reactivity insertion, at the beginning of the transient, the salt cannot freely
expand and the neutronic feedback is reduced. When the temperature of the salt rises, some vapor could be formed
in the fuel, the vaporization of the salt could then lead to a quick expansion of the vaporized fluid. To perform
these studies, we developed two independent codes. The first one, described in the present paper together with
some associated studies and results, is being developed to study slow reactivity insertion. The second code, still
under development, aims at calculating pressure and vaporization transient. The calculation tool seems to correctly
represent the evolution of the calculated physical quantities and the mesh convergence is easily reached. This is
encouraging for the continuation of this work to achieve the chaining with the fast phase calculation tool.