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Introduction Radionuclides and radiation are constituents of the Earth and play important roles in natural processes. Ever since Becquerel discovered natural radioactivity in 1896, radioactive materials have been utilized for society’s benefit in science, medicine, industry, and defense. However, using naturally occurring and artificially created radioactive materials leads to waste products, many of which contain significant levels of radionuclides. In particular, the radioactive wastes generated during production of nuclear weapons have high chemical complexity, as they are the result of multiple chemical processes and contain a range of fission products. These so-called “legacy” wastes, as well as those created during used nuclear-fuel reprocessing, constitute the bulk of “high-level waste” (HLW) as defined by the International Atomic Energy Agency (IAEA).1 Other nuclear-waste sources include medical treatments, oil and gas exploration, research reactors, nuclear facility decommissioning activities, and accidental releases requiring cleanup activities (e.g., the Fukushima Daiichi disaster in recent years).2 HLW is typically heat generating, due to extensive radioactive decay, and is the most hazardous, but the lowest volume of nuclear waste. Lower radioactivity classes such as low-level/ low-activity wastes (LLWs/LAWs) and intermediate-level wastes (ILWs) generally have higher volumes and different disposal regulations. Classification of these wastes and regulations about their disposition depend on many factors,

including the quantities and half-lives (t1/2) of the various radioisotopes, types of radiation generated, and origination of the material.1 The intent of radioactive waste management is to immobilize wastes in suitable matrices (known as waste forms) such that any release is controlled below thresholds set by local and international regulations protecting humankind and the environment. Depending on the applicable regulations, immobilized wastes are intended to be stored in various ways, including landfill disposal, shallow burial, or deep geologic repository, with risk and economics being the primary driving requirements. Since long-term stability of the waste form is desirable, chemically durable materials with high resistance to radiation damage are considered for waste forms. Various material types have been considered as matrices for immobilization of specific radioactive wastes, including glasses, ceramics, cements, bitumen (hydrocarbon, also known as asphalt), and metals. However, the use of glasses, crystalline ceramics, and glass-ceramics for immobilization of HLW and for many other wastes has garnered the maximum support among scientists.3–5 Glass is widely considered to be the benchmark material for long-term immobilization of complex mixed radioactive wastes, while single-phase ceramics may have advantages when considering highly uniform separated wastes such as actinides (An).6 Glass-ceramics used for immobilization of nuclear waste were first proposed in 1976 by researchers at the

John S. McCloy,