Investigation of spatial effects on neutron noise measurements in the zero power reactor CROCUS

This article presents the results of an experimental campaign in the CROCUS zero power reactor aimed at investigating the spatial effects of neutron noise measurements. Neutron noise measurements offer a non-invasive method to determine kinetic reactor parameters – such as the prompt decay constant at criticality α = βeff / Λ, the effective delayed neutron fraction βeff, and the mean generation time Λ – for code validation and reactor characterization efforts. Measurements are usually interpreted assuming point kinetic behavior of the reactor. In a point kinetic formulation, the reactor noise and resulting kinetic parameters are the same in all measurement locations. Future experimental programs in CROCUS require measuring the reactor noise further away from the core, deep in the water reflector of the reactor, where the point kinetic assumption may not be valid anymore due to loss of temporal correlation between neutrons. In order to test this hypothesis, we conducted a set of noise experiments, using the CROCUS reactor at criticality, in which two 1g 235U fission chambers were placed at varying distances from the fuel in the reflector. The measured auto power spectrum shape as a function of distance was found to agree within 2σ with point kinetic predictions. At the furthest measured distance, the efficiency was found to be too low for kinetic parameter determination. A possible accelerated decrease of the shape at large distances due to diffusion randomness is suggested, as a decreasing trend is visible at 1σ. Within the point kinetic assumption, we conclude that the efficiency is the dominant parameter with regards to successful noise measurements. This data will serve as a basis for further experiments aiding noise simulation code development efforts.