Abstract
The appearance of perturbations characterized by a periodic time behavior in fluid fuel reactors is connected
to the possible precipitation of fissile compounds which are moved within the primary circuit
by the fuel motion. In this paper the time-dependent response of a critical fluid fuel system to periodic
perturbations is analyzed, solving the full neutronic model and comparing the results with approximate
methods, such as point kinetics. A fundamental eigenvalue of the problem is defined, characterizing the
trend of divergence of the power. Parametric studies on the reactivity insertion, the fuel velocity and the
recirculation time are performed, evidencing the sensitivity of the eigenvalue on typical design parameters.
Non-linear calculations in the presence of a negative feedback term are then performed, in order to
assess the possibility to control a fluid fuel system when periodic reactivity perturbations are involved.