Investigation on fluctuations in full-size Molten Salt Reactor with coupled neutronic/thermal-hydraulic model

• A coupled model is developed for calculating first-order fluctuations in MSR.
• Thermal feedback effect is prominent at low frequencies.
• Fuel recirculation has a small effect on neutron noise.
• At high frequencies, the perturbation has more prominent effect on thermal noise.

In this paper, the fluctuations in neutron fluxes, delayed neutron precursors, fuel salt temperature and velocity in a full-size MSR are investigated by coupling the neutronic and thermal-hydraulic models. The neutronic model is established based on two-group diffusion theory with one averaged group of delayed neutron precursors. The balance equations for mass and energy are chosen to build the thermal-hydraulic model, while the momentum equation is neglected in the assumption of constant pressure. The group constants, including the cross sections and diffusion coefficients, are generated by means of the HELIOS code and fitted as functions of temperature to build the thermal feedback. The equations for fluctuations are derived based on linear perturbation theory, assuming the perturbation is small enough. All the equations are discretized and numerically solved with the help of a developed code. The static results show that the neutron flux distributions are almost independent of the fuel recirculation, and the distribution of precursors is not that sensitive to the fuel velocity change, due to the small delayed neutron fraction and large decay constant of the fuel material. In the dynamic case, it is found that the thermal feedback effect is important at low frequencies, i.e. when the point kinetic effect is prominent and the neutronic power is significantly perturbed. Similar to the static case, the fuel recirculation has apparent influence on the precursor noise, whereas its effect on the neutron noise is very small. Moreover, due to the interference among the effects of fluctuations in various group constants, the fast and thermal neutron noise are in opposite phases. Such out-phase behavior has been first observed in the case of traditional reactors and reconfirmed in the present study for a closed-loop MSR system.