Minor actinoids from the reprocessing of MOX spent fuels are expected to be greatly increased compared with the amount from the present spent fuels of UO2. Their high-decay heat prevents high loading to the glass waste form and so they should be separated from other waste elements and encapsulated for immobilization and disposal, because glass matrices for this application have a limit against the temperature increase due to decay heat by “glass transition temperature” at maximum. On the other hand, glass ceramics do not have this kind of limitation if the constituent crystalline phases have a high melting point and thermal stability. For future reprocessing and disposal after MOX fuel utilization, we need to prepare suitable materials which have high capability and ability to encapsulate MA for geological periods of time. Separation of this kind of nuclear wastes and encapsulation would promise the reduction of space for geological disposal.

Recently, our group has developed a new glass forming system, RO-Al2O3-La2O3-ZrO2, by an In-Flight Melting technique, and also found that this system has good potential to form glass ceramics suitable for the encapsulation of MA; consisting of crystalline phases with very high melting point, very small residual glass phase, very low porosity, high mechanical properties etc.

In-Flight Melting experiment using oxy-hydrogen combustion flame was performed for calcined oxide mixtures of RO-Al2O3-La2O3-ZrO2, (R=Ca and Sr) as starting materials. Introduced oxide powders were heated in the combustion flame and melted to form liquid droplets in flight. After departing from the combustion flame, the droplets were cooled rapidly in air and collected. Figure 1 shows a photograph of the prepared glass particles from the CaO-SrO-Al2O3-La2O3-ZrO2 system. High transparency and spherical shape indicate the formation of a melt and quenching in air. From thermal analyses, the obtained glasses were found to have glass transition temperatures in the range 750-800 oC; these are “glasses”.

Crucible melting has also been conducted using a Radio-Frequency (RF) Induction heating system and metal crucible on the same calcined oxide mixture as in the In-Flight melting experiment. Availability of crucible melting is very important to confirm the potential of practical development at large scale. At present, small scale melting of about 4.5 g glass has been conducted. However, even after 10 min melting, formation of a melt in the crucible, and no corrosion of the metal crucible were confirmed, and pore-free glass ceramics were formed, as shown in Figure 2. Powder X-ray diffraction measurements showed the main crystalline phases of perovskite LaAlO3 (melting point=2100 oC) and alkaline earth oxide containing ZrO2-based crystals (melting point >2325 oC) without glass/amorphous phase. MA would be incorporated in LaAlO3 by replacing La. The microstructure is shown in Figure 3. Micrometer-size crystal growth with fine “vein-of-leaf” structure was found in all samples. Vicker’s indentation test revealed Hv=9-10 GPa and KIC>3 MP/m1/2, corresponding to Cordierite (Mg2Al3(AlSi5O18)) or alumina (Al2O3). These high mechanical properties were due to the components and microstructure after crystallization from the melt. Thermal and mechanical stabilities and resistance of these glass ceramics are the factors to be given to the matrix for MA encapsulation. In the presentation, more details of functionalities and nature of these glass ceramics are shown.

This work was carried out using the grant of research program of Chubu Electric Power Inc., Japan, 2021-2022.