Investigation of the neutron noise induced by shell-mode core-barrel vibrations

Paper in proceeding, 2005

The subject of this work is the calculation of the in-core neutron noise, induced by the shellmode vibrations of the core-barrel. The original motivation was to investigate whether an out-ofphase behaviour can exist between the in-core and ex-core (ex-vessel) detectors lying at the same azimuthal position. To this order a two-region two-group diffusion model was used in one dimension. The noise was calculated with three different methods: with a full space-frequencydependent solution, with a numerical code (“noise simulator”), and in the adiabatic approximation. It was found that such an out-of-phase behaviour indeed exists for in-core detector positions very close to the core boundary, due to the local component of the noise. In addition to its effect on the phase, the local component also manifests itself by a large amplitude of the noise around the vibrating core boundary, i.e. both in the core and the reflector. On the other hand if the movement of the detector together with the core is accounted for, which is possible in the adiabatic approximation, the effects of the local component disappear. Recently performed measurements in a power reactor were unsuccessful to detect the predicted local component, presumably because in-core detectors are not close enough to the core boundary.