Calculation of the Flux Variation and the Reactivity Change Induced by a Central Perturbation in a Fast Critical Assembly The Reduced Reactor Model

A method is described for the analysis of experiments involving a central perturbation in a critical reactor. This method is particularly applicable to measurements dealing with reactivity changes or, as in some cases, with the variation of a fission chamber activation, in the vicinity of the perturbing sample (“local” signal). It is shown that the flux perturbation, induced by introducing the sample, can be calculated directly by solving a transport equation with a given source in the sample. This treatment, linked with the reduced reactor model, considerably shortens the required calculations. This method is applied to experiments performed in the ERMINE fast-thermal coupled critical facility at Fontenay-aux-Roses.