Michael Stueber

Protective coatings on zirconium-based alloys as accident-Tolerant fuel (ATF) claddings

Abstract Surface-modified zirconium (Zr)-based alloys, mainly by fabricating protective coatings, are being developed and evaluated as accident-tolerant fuel (ATF) claddings, aiming to improve fuel reliability and safety during normal operations, anticipated operational occurrences, and accident scenarios in water-cooled reactors. In this overview, the performance of Zr alloy claddings under normal and accident conditions is first briefly summarized. In evaluating previous studies, various coating concepts are highlighted based on coating materials, focusing on their performance in autoclave hydrothermal corrosion tests and high-temperature steam oxidation tests. The challenges for the utilization of coatings, including materials selection, deposition technology, and stability under various situations, are discussed to provide some valuable guidance to future research activities.

WEAR STUDIES AND CUTTING TESTS OF Ti–Al–N–C NANOCOMPOSITE COATINGS IN MILLING OPERATIONS – TECHNICAL COMMUNICATION

New carbon-based nanostructured composite coatings of the material system Ti–Al–N–C were deposited on cemented carbide milling tools, and their feasibility for selected dry high-speed machining operations was assessed. For this purpose, wear studies with instrumented cutting machines in dry cutting tests were carried out in milling operations on Uddeholm hardened steels (AISI H13, and, AISI A2, respectively). The tools were coated in an industrial sputtering machine of the Hauzer HTC 625 type. The new nanocomposite coatings showed a good performance in the dry cutting operations and were benchmarked versus industrial state-of-the-art coatings.

High-temperature oxidation resistance and self-healing behavior of Cr2AlC MAX phase coating on Zircaloy-4

Zirconium-based alloys are currently utilized as fuel cladding and structural components in commercial light water reactors due to their low thermal neutron absorption cross section, good mechanical properties and reasonable corrosion resistance during operation conditions. One undesirable feature of zirconium-based alloy cladding is their extremely fast oxidation kinetics with high-temperature steam during loss of cooling accidents (LOCA). A considerable amount of heat and hydrogen gas is produced by the reaction of zirconium and steam. The claddings undergo severe degradation and hydrogen explosion can occur, followed by subsequent release of highly-radioactive fission products to the environment like during the nuclear accidents at the Fukushima Daiichi Nuclear Power Plant in 2011. One strategy to improve the accident tolerance of the state-of-the-art zirconium-based alloy fuel claddings is to coat the outer surface with an oxidation resistant coating. This solution promises the elimination of corrosion degradation during normal operation, as well as significant reduced oxidation kinetics with steam during off-normal conditions. Mn+1AXn(MAX) phases represent a family of ternary layered carbides or nitrides which possess a unique combination of the merits of both metals and ceramics. Alumina-forming MAX phase materials, like Ti2AlC and Cr2AlC, are being considered as protective coatings with respect to their excellent oxidation resistance up to 1400°C. In this study, Cr2AlC coatings have been deposited on Zircaloy-4 substrates by magnetron sputtering using elemental nano-multilayer thin films, and subsequent thermal annealing in argon. The total thickness of the coatings is around 6.5 μm and both coatings have a 500 nm Cr layer as bonding layer and diffusion barrier. One design of coatings also deposited a 1.5μm thick Cr capping layer to migrate potential fast dissolve of Al during normal operation. Crystallization of Cr2AlC MAX phase starts from 480°C by annealing in Ar and formation of phase-pure Cr2AlC MAX phase but with surface microcracks at 550°C is confirmed. Both coatings demonstrated high adherence, excellent oxidation resistance up to at least 1200°C and self-healing capability with growth of protective Al2O3 scale or of protective Al2O3 scale beneath Cr2O3 during high-temperature oxidation.