Understanding the neutron noise induced by fuel assembly vibrations in linear theory

This paper investigates the effects of stationary vibrations of fuel assemblies in nuclear reactors. The incentive of this work is to understand the spatial pattern of the induced stationary fluctuations of the neutron flux. For that purpose, an as simple as possible system was considered: a one-dimensional three-region system, for which the boundaries between the regions are vibrating. Using one-group diffusion theory, it is demonstrated that in the frequency domain and in linear theory the amplitude and phase of the fluctuations in neutron flux exhibit different patterns, stemming from competing contributions between the point-kinetic and the space-dependent responses associated to each moving boundary. The results of this work can be used for interpreting the results of more sophisticated tools used for modelling the effect of fuel assembly vibrations onto the neutron flux, as well as measurements in reactors where vibrations of fuel assemblies occur. (authors)