A time and frequency domain analysis of the effect of vibrating fuel assemblies on the neutron noise

The mechanical vibrations of fuel assemblies have been shown to give rise to high levels of neutron
noise, triggering in some circumstances the necessity to operate nuclear reactors at a reduced
power level. This work analyses the eect in the neutron field of the oscillation of one single
fuel assembly. Results show two dierent eects in the neutron field caused by the fuel assembly
vibration. First, a global slow variation of the total reactor power due to a change in the criticality
of the system. Second, an oscillation in the neutron flux in-phase with the assembly vibration. This
second eect has a strong spatial dependence that can be used to localize the oscillating assembly.
This paper shows a comparison between a time-domain and a frequency-domain analysis of the
phenomena to calculate the spatial response of the neutron noise. Numerical results show a really
close agreement between these two approaches.