Garnet nuclear waste forms – Solubility at repository conditions

• Rare-earth elements are a significant waste stream produced by nuclear fuel cycles.
• Suitability of garnets as potential waste forms.
• Single-crystal X-ray structural refinements for grossular, LuAG and YAG.
• Garnets have low solubility, flexible crystal structure to take on large cations.
• Demonstrate garnets are potentially robust waste forms for radioactive REE.

Radioactive rare-earth elements (REEs) constitute a significant waste stream produced from modified open and full nuclear fuel cycles. Immobilization of these REE radionuclides is thus important for sustainable nuclear energy growth. In this work, we investigated the suitability of garnets as potential waste forms for REEs by measuring their aqueous stability at repository conditions. Three garnet samples, including one natural grossular (Ca3Al2Si3O12) and two synthetic phases (LuAG – Lu3Al5O12 and YAG – Y3Al5O12), were studied. Single-crystal X-ray structural refinements show that the unit-cell volumes increase from 1657.19 Å3 for grossular to 1679.8 Å3 for LuAG and to 1721.7 Å3 for YAG. This trend is due to increases in ionic radii in both the 8-coordinated X (from Ca to Lu to Y) and 4-coordinated Z (from Si to Al) cations. Hydrothermal experiments of the three samples were performed at 200 °C and 150 bar for 4 weeks using water and brine solutions to evaluate their solubility. The natural grossular sample exhibited Al leach rates ranging from 2.5 × 10−4 to 6.43 × 10−5 g/L·day and Ca leach rates from 1.39 × 10−3 to 4.57 × 10−3 g/L·day, indicating incongruent nature of the cation dissolution. The LuAG sample exhibited Lu leach rates of 3.73 × 10−4 to 2.19 × 10−4 g/L·day, and the YAG sample had Y leach rates of 1.29 × 10−4 to 5.64 × 10−5 g/L·day. Although these samples are generally more soluble in brine (which is more representative of repository conditions) than in water, as evidenced by both water chemistry and microstructural features, all the dissolution rates are relatively small. Thus, the high aqueous stability, together with the flexibility of the garnet structure to incorporate large cations, demonstrate that garnets are potentially robust waste forms for storing radioactive REEs.