Mechanical model for the motion of RPV internals affecting neutron flux noise

Abstract

In the decade after 2000, the amplitude of the neutron flux fluctuations in German 1300 MWe class PWR built by Kraftwerk Union (KWU) increased and later decreased significantly. To further investigate the hypothesis that changes in the mechanical properties of the fuel assemblies (FAs) and subsequent stronger mechanical oscillations of fuel assemblies and other RPV internals are responsible for the observations and to find possible excitation mechanisms, a simple mechanical model is developed. It describes the dynamic motion response of the mechanically coupled system of Reactor Pressure Vessel (RPV), core barrel and a row of fuel assemblies and takes reactive fluidic forces into account. To determine the components to be considered in this model, RPV internals with an effect on neutron flux are identified in a prior step.The dynamic answers of the model to generic excitation scenarios and parametric studies reveal distinct properties of the system. They indicate 1) that fluidic near-field coupling can equalize the fuel assemblies’ reaction amplitudes within a certain region regardless of their individual stiffnesses, 2) that the fluid must be part of the oscillation, 3) that a non-periodicity might stem from a superposition and interaction of several oscillators, 4) that a local excitation source does not spread or synchronize over the whole core, 5) that a neglection or separate consideration of the RPV and core barrel may be justified in some cases. The findings altogether affirm that core-wide oscillations of the fluid flow leading to a simultaneous oscillation of individual fuel assembly groups, possibly including bidirectional effects between fluid and structure, in combination with the changes of the fuel assemblies’ mechanical properties, might be responsible for the temporary increase of neutron flux fluctuations observed in PWR built by KWU.A variant of the mechanical model was studied in a full core investigation with coupled neutron kinetics simulations. These simulations qualitatively reproduce important features of measurement data, while being overall too small in amplitude to explain the observations quantitatively, which confirmed the necessity of considering further effects.