Neutronic simulation of fuel assembly vibrations in a nuclear reactor

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 simulates and analyses the effect in the neutron
field of the oscillation of one single fuel assembly without considering thermal-hydraulic
feedback. The amplitude of the fuel assembly vibration ranges from 0 to 1 millimetres
and this implies the use of fine meshes and accurate numerical solvers. Results show
two different effects in the neutron field caused by the fuel assembly vibration. First, a
global slow variation of the total neutron power due to a change in the criticality of the
system. Second, an in-phase change in the neutron flux with the assembly vibration. This
second effect is more important and has a strong spatial dependence. This paper shows
a comparison between a time domain analysis and a frequency domain analysis of the
phenomena, in order to validate the time domain solution against the frequency domain
solution. Numerical results shows a really close match between these two approaches.