The High Flux Reactor at the Laue Langevin Institute produces the most intense neutron flux in the world, with a thermal power of 58 MW. As part of the global non-proliferation initiative, the Laue Langevin Institute aims to convert highly enriched fuel into low enriched uranium. This conversion is a challenging process, requiring strict compliance with safety standards and the preservation of the same performance quality for the new fuel element. During an irradiation cycle of the fuel element, multiple microstructural and physicochemical transformations occur, depending on the specific irradiation history of the fuel element, leading to a swelling phenomenon of the fuel plates and a decrease in the inter-plate distance. The Perseus project therefore aims to develop new evaluation tools, particularly a measurement of the inter-plate distance using an ultrasonic pulse-echo system with a frequency of 100 MHz. In this article, we will present the system developed over the course of three theses, the different solutions implemented to measure the inter-plate distance on an irradiated fuel element. The radiation-resistant transducer, custom-built along with its instrumentation, and the associated signal processing will be detailed to present measurements on irradiated fuel elements.