On the influence of dynamical fuel assembly deflections on the neutron noise level

In German KWU pressurized water reactors, the neutron flux is measured with in-core and the ex-core detectors. Investigating the signal fluctuations of these detectors at fixed burnup, periods of growing and periods of decreasing fluctuation amplitudes can be found, a phenomenon that is not entirely explained. Respecting the phenomenology of measured neutron noise data as well as of simulation results from the reactor-dynamics code DYN3D, the model hypothesis formulated here is a first attempt to couple low-frequency, collective fuel assembly deflections to power reactor noise: pressure fluctuations dynamically drive fuel assembly deflections, which alter the position of the common center of mass of all fuel assemblies; the center of mass fluctuations drive fluctuating changes of the reflector thickness and, thus, induce long-range dynamical effects on the neutron flux. As one detail, the proposed model states that a lowered lateral fuel assembly stiffness results in an increase of the neutron flux fluctuation amplitudes and vice versa. For testing this hypothesis, the model has been coupled to DYN3D. The signal analysis of the results obtained with the modified DYN3D indicates that the proposed model may explain the phenomenon of increasing and decreasing neutron flux fluctuation amplitudes. Steps for further modelling are proposed.