Research on the Numerical Simulation of Neutron Noise based on the Unstructured-Grid Variational Nodal Method

Neutron noise is the stochastic fluctuation of the neutron field under the influence of various weak perturbations, attracting attention as an indirect monitoring tool for nuclear reactors. This paper establishes a frequency-domain numerical simulation method for neutron noise based on the unstructured-grid variational nodal method using the first-order perturbation theory and Fourier transform techniques. This work extends the functionality of the general-purpose neutron transport code VITAS. The accuracy of the frequency-domain method is verified by comparing its results with those from time-domain methods in the MOX case and the C4V benchmark. Numerical results indicate that the frequency-domain method can achieve a precision comparable to the existing time-domain method under weak perturbations. The relative error in the amplitude of neutron noise is less than 2%, and the relative error in the phase of neutron noise is less than 1%.