Transfer Function Modeling of Zero-Power Dynamics of Circulating Fuel Reactors

In this paper, the zero-power behavior of circulating fuel reactors (CFRs) has been investigated by means of a zero-dimensional neutron kinetics model that provides a simplified but useful approach to the simulation of the dynamics of this class of nuclear reactors. Among CFRs, the most promising is the molten salt reactor (MSR), which is one of the six innovative concepts of reactor proposed by the “Generation IV International Forum” for future nuclear energy supply. One of the key features of CFRs is represented by the fission material, which is dissolved in a liquid mixture that serves both as fuel and coolant. This causes a relevant coupling between neutronics and thermo-hydrodynamics, so that fuel velocity plays a relevant role in determining the dynamic performance of such systems. In the present study, a preliminary model has been developed that is based on the zero-power kinetics equations (i.e., reactivity feedbacks due to temperature change are neglected), modified in order to take into account the effects of the molten salt circulation on the drift of delayed neutron precursors. The system dynamic behavior has been analyzed using the theory of linear systems, and the transfer functions of the neutron density with respect to both reactivity and fuel velocity have been calculated. The developed model has been assessed on the basis of the available experimental data from the molten salt reactor experiment (MSRE) provided by the Oak Ridge National Laboratory. The results of the present work show that the developed simplified theoretical model is well descriptive of the MSRE zero-power dynamics, allowing a preliminary evaluation of the effects due to the circulation of the fuel salt on the neutronics of the system. Moreover, the model is of general validity for any kind of CFRs, and hence is applicable to study other MSR concepts in order to have some indications on the control strategy to be adopted in the MSR development envisaged by Generation IV.