Reactivity monitoring for Accelerator-Driven Systems (ADS) using beam interruption experiments

Radioactive waste originating from nuclear power plants are a major societal issue. In order to reduce their radio-toxic impact on the environment, it is envisaged to incinerate minor actinides using neutron-induced nuclear reactions. The potential of Accelerator-Driven Systems (or ADS) to burn minor actinides is being investigated. Many studies, both experimental and theoretical, are being carried out to evaluate the feasibility of these hybrid systems which couple a subcritical reactor to an external neutron source. One of the points investigated concerns the reactivity of the reactor, an essential parameter for the safety of the facility, and its online monitoring that requires the evaluation of appropriate methods. This thesis deals with the analysis of the neutron population evolution over time, measured by several fission chambers located in the reactor, during short interruptions of the beam delivered by the accelerator that provides the external neutron source. Data from experiments carried out at the GUINEVERE facility located at the SCK.CEN center (Mol, Belgium) in the framework of the FREYA project (FP7 Euratom), are analyzed, then interpreted and successfully corrected using neutron transport codes. The results are discussed and used to provide recommendations regarding the use of such experiments to measure the reactivity. Finally, an interpretation of the experiments based on a modal decomposition of the neutron flux is proposed. (author)