Structural Role of U and Th in Different Glass-Forming Systems

Vitrification is a critical technology for long-term immobilization of nuclear waste. While most U from used nuclear fuel is not planned for immobilization in glass, there are certain situations where U and/or Th-rich waste streams may be vitrified. One example is the 56 million gallons of nuclear waste at the Hanford Site in Washington State, where some actinides remain in certain tanks. When planning for vitrification efforts, understanding the structural impact of different waste constituents is essential for efficient and successful deployment. This review explores existing literature on U and Th incorporation into silicate, aluminosilicate, borosilicate, and phosphate glasses. Additionally, novel insights into less-studied glass systems (for example, titanate, aluminate, borate, and sulfate) as well as waste relevant complex systems are discussed based on structural characterization techniques including Raman spectroscopy. In addition, the potential of complementary X-ray absorption spectroscopy and X-ray total scattering measurements for probing U/Th incorporation are introduced. Crystalline products resulting from devitrification processes are probed with X-ray diffraction. These findings not only advance the fundamental understanding of U and Th incorporation into non-crystalline matrices but also provide guidance for future vitrification endeavors.