Applicability of adiabatic approximation on neutron noise analysis in molten salt reactor

• The interference occurs at higher frequencies as velocity increases.
• Adiabatic approximation is an effective tool to estimate neutron noise in MSR.
• Characteristic ripples induced by fuel recirculation can be accurately reconstructed.

In this paper, the applicability of the adiabatic approximation on the neutron noise analysis in the molten salt reactor (MSR) is investigated. The neutronic model considering the fuel recirculation is established based on one-group neutron diffusion theory. In linear perturbation theory, the fluctuation equations are derived due to the smallness of the perturbation. Following the standard procedure, the kinetic equations for the fluctuations of amplitude factors, which are dependent on the unknown fluctuations of shape functions, are obtained. If the fluctuations of shape functions are neglected, the solutions of both the total neutron noise and its point kinetic term are inaccurate. In order to estimate the effects of the fluctuations of shape functions and the interference between the point kinetic and space-dependent terms, the adiabatic approximation is applied. The solutions are compared to the exact ones and those obtained in the point kinetic approximation to figure out the applicability of the adiabatic approximation. The results show that the adiabatic approximation is accurate enough for low velocities. With higher velocities, the solutions obtained in both the point kinetic and adiabatic approximations gradually deviate from the exact ones, whereas the adiabatic approximation can still provide good estimation of the frequency- and space-dependent behavior of neutron noise in the plateau region. In particular, the spatial oscillation of the amplitude of neutron noise can be well reconstructed. Moreover, the characteristic peak, which is related to the inverse of fuel recirculation, is absent in the point kinetic approximation but can be observed in the adiabatic approximation. Therefore, the adiabatic approximation can be regarded as an effective tool for reconstructing and interpreting the neutron noise in MSR.