  Citation: Ojovan, M.I. Challenges in the Long-Term Behaviour of Highly Radioactive Materials. Sustainability 2022, 14, 2445. https://doi.org/ 10.3390/su14042445 Received: 14 February 2022 Accepted: 18 February 2022 Published: 21 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). sustainability Editorial Challenges in the Long-Term Behaviour of Highly Radioactive Materials Michael I. Ojovan 1,2 1 Department of Materials, South Kensington Campus, Imperial College London, Exhibition Road, London SW7 2AZ, UK; m.ojovan@imperial.ac.uk 2 Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry of Russian Academy of Sciences (IGEM RAS), 119017 Moscow, Russia Highly radioactive materials are at the core in many useful applications ranging from operating nuclear reactors (including fast breeder reactors) to vitrified high-level radioactive waste, which is currently stored and awaiting final disposal into dedicated facilities within deep geological formations. The stability and durability of highly radioactive materials are greatly affected by both continuous irradiation and adverse action of the environment. The crucial question in all useful applications stands, therefore, with the behaviour of materials in the conditions of intense irradiation combined with adverse and often highly corrosive environments. The effect of self-irradiation has especially emerged in nuclear waste immobilisation with the importance of predictable long-term behaviour of materials which extends for time periods, exceeding many hundred and thousand years. Indeed, the nuclear waste shall withstand immobilised in a geological repository for many millennia if not much longer, depending on the content of long-lived radionuclides [1,2]. Even small changes in material performance, which are negligible from a short-term standpoint, can gradually lead to structural and functional changes and consequently cause materials failure in the long-term perspective. The accidents in nuclear installations, including that at Three Mile Island, Chernobyl, and Fukushima, have focused the attention of researchers on the highly radioactive materials in the form of nuclear fuel debris and hot particles generated within or after the accident [3,4]. Moreover, the analysis of these materials can indicate the nature of processes that have caused the accident [4]. Irreversible transformations, such as swelling and phase separation, acceleration of material ageing, and corrosion, have already been reported for highly radioactive crystalline and vitreous materials, which are of practical importance, e.g., within the safety assessment [5–8]. One of the aspects herewith is the upper limit of wasteform loading with radioactive species, where an incentive is to increase it without, however, compromising the performance during the storage period and within the disposal environment. The IAEA has recently launched a dedicated coordinated research project INWARD to combine efforts of researchers to utilise accelerators aiming to simulate and analyse the effect of the radiation of materials [9]. Although radiation effects have been comprehensively overviewed in two fundamen- tal publications, both for crystalline [5] and glassy [6] materials, some updates published more recently indicate additional unexpected effects, i.e., the so-called “unknown un- knowns” within the science of highly radioactive materials, see, e.g., [8,10,11]. A typical limitation of the content of radionuclides in a durable matrix material is related to the content of the fissile element in the wasteform aiming to avoid any potential criticality in the nuclear waste facility both currently and in the future, accounting for any potential scenario of events in a repository or a disposal facility. The content of both fissile and non-fissile radionuclides is also limited by the detrimental effects caused by radiation damage. For crystalline materials, this is typically related to the amorphisation of materials which results in material swelling, mechanical damage, and, overall, leads to a loss of radionuclide retention performance. However, long-term experiments with materials that Sustainability 2022, 14, 2445. https://doi.org/10.3390/su14042445 https://www.mdpi.com/journal/sustainability