the first time, a device succeeds in studying in-situ the chemical composition of reactor walls. ENEA, in collaboration with a number of European research organizations, has developed an innovative laser system that, for the first time in the world, has made it possible to study the chemical composition of the inner walls of the Joint European Torus (JET), one of the largest experimental nuclear fusion reactors ever built. The activity was funded under the EUROfusion Consortium and involved, in addition to ENEA as a leading partner, prestigious institutions, including: Forschungszentrum Jülich (Germany), VTT Technical Research (Finland), UKAEA - UK Atomic Energy Authority, IPPLM - Polish Institute of Plasma Physics and Laser Microfusion in Warsaw (Poland), as well as a number of European universities such as Comenius University in Bratislava (Slovakia), University of Tartu (Estonia) and the Institute of Solid State Physics at the University of Latvia. Specifically, the research team demonstrated the feasibility of the Laser-Induced-Breakdown Spectroscopy (LIBS) technique for remote, real-time monitoring of plasma-exposed components without removing or manipulating them for analysis. The technique adopted allows qualitative and quantitative analysis of the reactor's internal walls, including detecting the presence of hydrogen and its isotopes, deuterium and tritium, the reactor fuel. Compact, lightweight and versatile, the system built by ENEA and European partners was placed inside JET's vacuum chamber, where it performed the analysis at hundreds of locations, demonstrating that the technique can be used on any type of sample from the first reactor wall and is capable of preserving them, essentially intact, for later analysis. Installed on the JET's remotely controlled robotic arm, the LIBS system strikes the sample-target generating a small explosion that vaporizes a microscopic portion of it. The atoms of the target thus vaporized emit a light that analyzed by a spectroscopic system determines its chemical composition.