Abstract
In the framework of minimizing nuclear proliferation risks, most of the High-Performance Research Reactors (HPRR) have joined a program to reduce the enrichment of their fuel elements. They are currently using a type of plate where the fuel is dispersed in a pure Aluminum matrix. The conversion program requires the use of a Low-Enriched Uranium (LEU) with minimal losses and to achieve similar performance as the High-Enriched Uranium (HEU).The High Flux Reactor (RHF) of the Institute Laue Langevin (ILL) is involved in this program and has launched a collaboration with the Institute of Electronics and Systems (IES) of Montpellier University (UM) to investigate in a non-destructive way the fuel behavior and the history of irradiation to improve its qualification.The ILL operates a High Flux Reactor (RHF) dedicated only to fundamental research. It has a thermal power of 58 MW and produces the most intense continuous flux of neutrons in the world, with a flux density of approximately 1.5* 10^{15} /cm^{2}/ s.The core of the RHF is mainly composed of 280 curved fuel plates, which are machined in an involute shape to ensure the nominal inter-plate distance of 1.8 mm.To maintain the reactor at a safe operating temperature after the cycle of the radiation, the fuel element is immersed in a cooling pool at a depth of 8 meters, that also helps to ensure safety against gamma and neutron radiation which are produced during the cycle of the reactor.The main objective of this research project is to develop a high frequency ultrasonic device that combines mechanics, electronics and acoustics for a non-destructive in-situ measurement of the fuel plate with a micrometric resolution. The specific device integrates two ultrasonic transducers for measuring the water channel width between two fuel plates of the HPRR.In previous studies, two specific ultrasonic devices have been designed for measuring the inter-plate distance of the HPRR spent fuel element with a microscopic resolution. They are intended to be introduced into the gap of 1.8 mm between two fuel plates. The feasibility of this measurement has already been demonstrated during a series of experiments conducted in December 2013 and July 2015, where the different components of the ultrasonic device have demonstrated a good resistance to radiation and the experimental difficulties and constraints associated with the measurement were identified. The aim of this thesis is to improve the measurement accuracy by optimizing the ultrasonic device considering the limitations associated with the measurement. The objective is to obtain the radiation history and feedback on the whole set of plates of the ILL spent fuel elements.