ASSESSMENT OF THE NEUTRON NOISE INDUCED BY STATIONARY FUEL ASSEMBLY VIBRATIONS IN A LIGHT WATER REACTOR

A systematic increase of the neutron noise levels over time has been observed in some
of the pre-KONVOI PWRs operating in Europe. A possible reason for this anomaly
was identified as increased mechanical vibrations of reactor internals, specifically of fuel
assemblies. To verify this conjecture, the modeling of stationary vibrations of fuel assemblies
and of the corresponding neutron noise is essential. In this paper, using the =d
model basis, we illustrate the modeling of the neutron noise sources for fuel assembly vibrations
and study the effect of homogenization of cross sections on such stationary perturbations.
A comparative analysis between the classical nodal approach (both localized
at the boundaries of the vibrating fuel assembly or involving the entire neighboring fuel
assemblies) and a pin-wise approach shows that the ‘boundary-localized’ nodal approach
seems to capture local noise information, as a pin-wise approach would do, without the
need of a complex pin-by-pin model when the detectors are placed close to the perturbation.
However, when considering region of the active core away from the perturbation, all
three approaches lead to comparable results.