Investigations of neutron noise induced by transient cross flow in a PWR reactor core

Classical neutron noise simulations in pressurized water reactors (PWRs) assume that the moderator flows straight up through an axial fuel assembly channel without mixing with neighboring channels. This approach does not fully reflect operating conditions because fuel assemblies in PWR type of reactors usually are not bounded by channel boxes. Cross flow between assemblies exists because the inlet mass flow distribution typically has higher values in the center of the core than at the periphery. The homogenization of the fluid velocity leads to lateral cross flow in the lower part of the core. If this cross flow is not stationary but has also a transient component it will contribute to neutron noise generation by creating a time dependent moderator temperature fluctuation. The transient component can be introduced, for example, by small variations in primary pump head. All these effects have never been investigated as a potential source of neutron noise and this paper is a first attempt to quantitatively estimate their impact. Both a simple node-per-node numerical model and two more sophisticated models (COBRA-TF and CFX) are used to analyze the effects of non-stationary cross flow on local moderator temperature variations. The impact on neutron noise amplitudes is estimated with the help of a simple 1D point reactor model. In the future a fully coupled calculation connecting the 3D thermal-hydraulic variations with changes in the neutron population will be developed.