After decades of scientific research, nuclear fusion is on the verge of maturing into a sustainable and safe power generating technology. The most consolidated designs of power plants envisioned have in common the deuterium-tritium fusion reaction, which implies the production of the latter from Li, in a breeding blanket (BB), the technological cornerstone for a self-sustaining power plant. Whatever the specific design of the BB, two issues are omnipresent: T permeation from the breeding zone and corrosion of the structural materials by the Li compounds used.
Here we present the summary of our work on ductile amorphous ceramic coatings (DACs) deposited by Pulsed Laser Deposition, to overcome these challenges, aiming at simplifying the BB and the T2 management subsystems. This new class of ceramic coatings have unique properties: high strength and ductility and a high density amorphous structure. The mechanical properties assure the reliability of coated steel parts under the most extreme thermal cycles and the isotropic, homogeneous structure impermeability to gases (H2, D2, O2) even at high temperature. The key engineering performances are: a permeation reduction factor (PRF) higher than 105 (T>450 °C) for coated EUROFER with respect to an uncoated reference; corrosion protection of the EUROFER in contact with Pb-16Li both in static and dynamic conditions; a high resistivity capable to drastically reduce Magneto Hydrodynamic Effects. In recognition of the key enabling potential of DAC in fusion, the European Commission awarded us the 2020 SOFT Innovation Prize. Even if more validation is needed and scale-up of the coating technologies used to produce DACs are in progress, they hold the promise to minimize the time and the cost to transition fusion from a plasma physics experiment to a safe and unlimited power generating technology.
Nuclear fusion energy is the holy grail of scientists and engineers working to find a sustainable way to produce electricity abundantly and at low cost. The International Thermonuclear Experimental Reactor (ITER) will be the last multinational science-driven experiment to study plasma physics and fusion ignition. After, fusion will enter the pre-commercialization stage, with the development of demonstrative fusion power plants designed to harvest the fusion energy and generate net power. The European DEMOnstration power plant (DEMO) is the archetypal of these demonstrative plants. Lately, this transition is speeding up, with the "nationalization" of the programs and the emergence of start-ups (among others General Fusion (CA), Commonwealth Fusion Systems (USA), Tokamak Energy (UK)). Most of these demonstrative power plants have in common the deuterium-tritium fusion reaction, which implies the production of the latter from Li (while D is extracted from the sea), in a breeding device, which in DEMO is called breeding blanket (BB). In the case of DEMO, it consists of a set of D-shaped segments surrounding the reactor's main vessel having a threefold purpose: host the heat extraction system, shield from the radiation field incoming from the main reaction core and finally produce tritium in situ, thus guaranteeing the self- sufficiency of the power plant. Currently, two different breeding blanket concepts are investigated for DEMO: the water-cooled lithium-lead (WCLL) and the helium-cooled pebble bed (HCPB). Also to mention two alternative solutions investigated for the future: the dual coolant lithium-lead (DCLL) andthe helium-cooled lithium-lead (HCLL). The WCLL, HCLL and DCLL foresee a liquid eutectic alloy of lithium and lead (Pb-16Li) as the breeding media [1-5], while the HCPB solid LiSiO4 pebbles. For all designs, the major issue is T permeation. In fact, whilst the BB is designed to produce a large amount of tritium, EUROFER, a reduced activation ferritic-martensitic steel chosen as structural material for DEMO, is highly permeable to all hydrogen isotopes, and hence to T, at the working temperature of the BB, 450 °C. This phenomenon is of particular concern since the Pressurised Heat Transfer System (PHTS) is directly interfaced with the BB and hence T could escape the nuclear island of the plant, with a possible radioactive hazard for the workers and the public, being tritium a radioactive gas. To face this issue, a tritium recovery system from the PHTS would be then necessary, thus complicating the overall engineering. Two secondary issues are instead strictly linked to Pb-16Li: corrosion and magnetohydrodynamic (MHD) effects. Even if corrosion is not a major issue in WCLL due to the low speed of the Pb-16Li, it could cause the dispersion of activated corrosion products in the breeding medium, which could eventually accumulate in the cold segments of the circuit. MHD originates from the interaction between the flowing Pb-16Li and the steel structures under the intense magnetic field of the plasma confinement, causing high pressure drops and unsafe loadings on the structures [6-14]. Since traditional structural materials employed in the nuclear industry are not able to withstand these strict conditions, innovative solutions have been actively investigated during the years. Towards this goal, IIT and ENEA, in the framework of EUROFUSION, developed multifunctional DAC coatings designed to solve all the three issues above mentioned at once. In the present paper, we summarize the main results obtained so far in the development of multifunctional DAC coatings produced by pulsed laser deposition (PLD) and their testing in fusion relevant experiments.
