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Although the vitrification of nuclear waste has a decades‐long history, numerous opportunities still exist to improve its efficiency and to increase the waste loading in glass. This is especially true for the vitrification of low‐activity waste (LAW), which has been historically treated by other immobilization technologies and is less mature than high‐level waste (HLW) vitrification. In this work, we address one of the least understood phenomena during the conversion of nuclear waste feeds to glass—the formation of molten salt and transient glass‐forming melt. Using high‐temperature environmental scanning electron microscopy (HT‐ESEM) in combination with X‐ray diffraction, thermogravimetry, and evolved gas analysis, we have analyzed the complex chemical reactions and phase transitions as they occur during melting of representative HLW and LAW melter feeds. We evaluated the compositions of amorphous phases and the fractions of salt components, and estimated the fractions of molten salt phases present in the feeds as a function of temperature. We show that the maximum fraction of molten salts is ∼4 % and ∼28 % during HLW and LAW feed melting, respectively, and discuss the possibility of molten salt migration in LAW feeds. We also argue that the presence of significant fractions of molten salt phase can hinder the retention of rhenium (and, hence, radioactive technetium), and discuss how the properties of molten salt phase and transient glass‐forming melt are related to primary foam formation and behavior. Finally, we summarize key unanswered questions requiring further research to increase the understanding of the conversion process and enhance the nuclear waste vitrification efficiency. [ABSTRACT FROM AUTHOR]

In France, high-level nuclear wastes are confined in glass using a calcination-vitrification process. The waste can also be vitrified by liquid feeding, which imply to add directly the liquid waste. In this study, we focus on nuclear waste vitrification process by direct liquid feeding in order to decipher the dissolution kinetics of the waste in the molten glass. We propose an experimental and analytical protocol to study the dissolution in the solid glass frit of the liquid waste dried beforehand during the glass melting process. This methodology allows to: i) identify the intermediate reaction phases which formed and dissolved during the process; ii) characterizes the evolution of waste element concentrations in the dried waste as a function of time and temperature on glass obtained after cooling; iii) define kinetic parameters relative to the process of intermediate reaction phases dissolution in the molten glass. The three main intermediate reaction phases formed and dissolved in function of temperature are Ca-REE (Rare Earth Elements)- silicate, Ce- oxide and Zr- oxide. At each temperature step (800 to 1200 °C) the evolution of REE (La, Ce, Nd and Pr) and Zr concentrations in the glass samples obtained after cooling shows a fast increase followed by a stabilization in time, which reflects the fast dissolution of the dried waste in the molten glass, i.e. the fast dissolution of intermediate reaction phases. The molten glass is completely homogeneous at classical synthesis temperature (1200 °C). A parameterization methodology of element concentration evolution as a function of time and temperature is proposed here in order to define the kinetic parameters corresponding to the dried waste dissolution (i.e. concentration at a given time t, maximal concentration at a given temperature, characteristic time and activation energy). The results show that the activation related to the formation and dissolution processes of the intermediate reaction phases, i.e. whether they constitute intermediate compounds formed in temperature by the interaction between the dried waste and the glass frit, and the phases formed without reacting with the glass frit, directly in the dried waste during heat treatment. [ABSTRACT FROM AUTHOR]

• Mo-bearing borosilicate melt can undergo unmixing. • Alkali- and Mo-rich droplets form at high temperature. • In-situ Raman spectroscopy can be used to measure melt polymerization in response to unmixing. • This droplet formation polymerizes the silicate component on the network. • This droplet formation polymerizes the borate component on the network. Glass network polymerization critically influences rheological behavior and with it the kinetics and dynamics of nuclear waste immobilization. Molybdenum-bearing borosilicate melts may undergo unmixing and rheological changes, which is dominantly controlled by the associated chemical modifications of the melt network. Here, we obtain in-situ (500–940 °C) Raman spectra to probe structural changes of a sodium-molybdenum borosilicate melt undergoing unmixing. The extraction of alkali and molybdenum to form droplets induces polymerization of the residual borosilicate network. Conversely, the opposite phenomenon is observed during droplet re-dissolution. This work provides new insights into the polymerization of a molybdenum-bearing borosilicate composed of two composition sets due to a miscibility gap and has direct contributions for the immobilization of nuclear wastes. [ABSTRACT FROM AUTHOR]

A series of calcium borosilicate glasses with varying [B2O3], [MoO3], and [CaO] were prepared and subjected to 92 MeV Xe ions used to simulate the damage from long-term α-decay in nuclear waste glasses. Modifications to the solubility of molybdenum, the microstructure of separated phases, and the Si–O–B network topology were investigated following five irradiation experiments that achieved doses between 5 × 1012 and 1.8 × 1014 Xe ions/cm2 in order to test the hypotheses of whether irradiation would induce, propagate, or anneal phase separation. Using electron microscopy, EDS analysis, Raman spectroscopy, and XRD, irradiation was observed to increase the integration of MoO42− by increasing the structural disorder within and between heterogeneous amorphous phases. This occurred through Si/B-O-Si/B bond breakage and reformation of boroxyl and 3/4-membered SiO4 rings. De-mixing of the Si–O–B network concurrently enabled cross directional Ca and Mo diffusion along defect created pathways, which were prevalent along the interface between phases. The initiation and extent of these changes was dependent primarily on the [SiO2]/[B2O3] ratio, with [MoO3] having a secondary effect on influencing the defect population with increasing dose. Microstructurally, these changes to bonding caused a reduction in heterogeneities between amorphous phases by reducing the size and increasing the spatial distribution of immiscible droplets. This general increase in structural disorder prevented crystallization in most cases, but where precipitation was initiated by radiation, it was re-amorphized with increasing dose. These outcomes suggest that internal radiation can alter phase separation tie lines, and can therefore be used as a tool to design certain structural environments for long-term encapsulation of radioisotopes. [ABSTRACT FROM AUTHOR]