Hydrothermal Synthesis for Radioactive Waste Immobilization

Hydrothermal synthesis harnesses high-temperature, high-pressure aqueous environments to produce robust crystalline matrices that sequester radioactive isotopes within their lattices. By dissolving sources of silicon, aluminium and, where appropriate, clays or industrial by-products in a closed-system reactor, a variety of host phases—such as pollucite, hollandite and apatite—can be crystallised under comparatively mild conditions. This approach offers advantages over traditional vitrification or solid-state sintering, including lower energy demand, tunable chemical composition, and the capacity to immobilise a broad spectrum of radionuclides (for instance caesium-137, strontium-90 and minor actinides). The resulting materials display high compressive strength, excellent leaching resistance and structural stability under repository conditions. Recent advances have refined precursor chemistries, optimised reaction parameters and elucidated crystal-growth mechanisms, advancing the field towards scalable treatments for low- and intermediate-level wastes as well as contaminated soils and effluents.

Research from Nature Portfolio

Recent studies have demonstrated the efficient conversion of caesium-contaminated media into crystalline pollucite by substituting water with ethylene glycol in hydrothermal reactions. Operating at around 350 °C and pressures near 15 MPa, these methods produce highly crystalline CsAlSi₂O₆ within 20 hours, markedly reducing free water content to accelerate nucleation. The resulting pollucite exhibits exceptional confinement of radioactive caesium, even in the presence of seawater, and delivers a reproducible, cost-effective route to a durable waste form suitable for final disposal.

Hydrothermal Synthesis for Radioactive Waste Immobilization publication trend

The graph below shows the total number of articles in hydrothermal synthesis for radioactive waste immobilization across all publications each year (not limited to Nature Index journals).

Technical terms

Hydrothermal Synthesis: Preparation of crystalline materials by reacting aqueous solutions under elevated temperature and pressure in a sealed vessel.

Pollucite: A zeolitic aluminosilicate (CsAlSi₂O₆) valued for its capacity to immobilise radioactive caesium within a stable framework.

Analcime-Pollucite Solid Solution: A mixed aluminosilicate phase in which sodium and caesium share sites in an analcime-derived framework, enhancing in situ immobilisation of Cs⁺.

Leaching Resistance: The ability of a solid waste form to retain radionuclides when exposed to aqueous environments over extended periods.

Autoclave: A pressure vessel designed to conduct chemical reactions at temperatures above the boiling point of water, essential for hydrothermal processes.

References

1. Versatile chemical handling to confine radioactive cesium as stable inorganic crystal. Scientific Reports (2018).
2. Local Structure and Protons in Non-Stoichiometric Pseudo-Cubic Pollucite Mineral by Multinuclear NMR. Minerals (2022).
3. Hydrothermal conversion of analcime-pollucite solid solution from soil for immobilization of Cs in situand its characterization. Materials Research Express (2021).