The present paper is devoted to the analysis of the coupled thermo-fluid and neutronic dynamics of fast fluid-fuel multiplying nuclear systems. A completely coupled model is needed since in some fast reactors designs, the velocity pattern could be very complicated and strongly affected by the neutron dynamics via the heat source from fission reactions. Furthermore, the neutron dynamics is strongly affected by the thermohydrodynamics via the motion of precursors and by feedback effects. The methods typical of solid fuel reactors of previous generations are not sufficient to handle these more highly coupled concepts. In the preset paper, we consider the coupling between neutronics and thermohydrodynamics with simple but realistic hypotheses assumed to model the evolution of all the variables involved in the calculation. The numerical scheme used represents the current state of the art in the solution of non-linear systems: the Newton-Krylov algorithm. Several calculations are presented to demonstrate the ability of the methods described here to study the behavior of molten salt reactors in both steady state and transient situations.
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