Hydrothermal synthesis and growth of stable strontium apatite for radioactive salt waste immobilization

Safe management of chloride-rich radioactive salt waste produced during molten-salt dry reprocessing of spent nuclear fuel is essential for the sustainable deployment of advanced nuclear energy systems. Although ⁹⁰SrCl₂ represents only a minor mass fraction, its high specific activity and decay heat dominate short-term thermal management. In this work, a low temperature hydrothermal synthesis of Sr₅(PO₄)₃Cl effectively immobilized Sr and about 10 % of Cl, while the remaining Cl was released as NaCl. Over a 10-h hydrothermal process, the step-like growth of crystalline and partially disordered domains promotes enhanced crystallinity and increased particle size of the Sr₅(PO₄)₃Cl solidified form, thereby improving its chemical durability. The well-crystallized product exhibits a mass loss of less than 1.8 wt.% at 1000 °C, and normalized leach rate of Sr²⁺ is 3.5 × 10⁻⁶ gm⁻²d⁻¹ after 28 d in deionized water at 90 °C. Reliable thermal and chemical stability confirm that low-temperature hydrothermal processing is an energy-efficient strategy for conditioning ⁹⁰SrCl₂ chloride wastes prior to geological disposal.