Radionuclide Immobilization in Cementitious Materials

The immobilization of radionuclides within cementitious matrices lies at the heart of strategies for the long-term containment of radioactive waste. Cement-based systems, chiefly composed of calcium–silicate–hydrate (C–S–H) phases, calcium hydroxide and various ettringite or monosulphoaluminate hydrates, provide both chemical and physical barriers to radionuclide migration. Radionuclides may be retained by surface sorption, incorporation into evolving hydrates or precipitation of secondary solid phases. The high pH environment generated by cement hydration promotes formation of low-solubility hydroxides and silicates for many actinides and fission products, while the microstructure of hardened cement paste limits advective transport through low-permeability pore networks. Recent advances have focused on the influence of binder composition, the role of competing ions and ligands, and the long-term stability of sequestered species under repository conditions. Practical applications include cement grouts for engineered barrier systems, backfills for repository vaults and encapsulation of low- to intermediate-level wastes. The global significance of this research spans national disposal programmes, decommissioning of nuclear facilities and in situ immobilization of contaminated soils, reinforcing the need for robust mechanistic models that integrate geochemical, spectroscopic and transport data to predict radionuclide behaviour over geological timescales.

Research from Nature Portfolio

Recent studies have employed advanced spectroscopic and diffraction methods to unravel the speciation of uranium in contact with C–S–H phases of varying calcium to silicon ratios. At elevated Ca/Si ratios, formation of uranophane-type uranyl silicates and calcium-uranate precipitates has been identified, whereas lower Ca/Si systems favour Ca-bearing uranyl silicate minerals akin to haiweeite or weeksite. These findings demonstrate that subtle variations in binder chemistry can dictate the nature of secondary phases that control uranium retention. Such insights inform safety models for post-closure performance of cementitious barriers in radioactive waste disposal facilities.

Radionuclide Immobilization in Cementitious Materials publication trend

The graph below shows the total number of articles in radionuclide immobilization in cementitious materials across all publications each year (not limited to Nature Index journals).

Technical terms

Calcium–silicate–hydrate (C–S–H): The primary gel phase formed during cement hydration that provides binding strength and sorption sites for radionuclides.

Distribution ratio (Rd): A measure of the extent of radionuclide retention, defined as the concentration of radionuclide on the solid phase divided by its concentration in solution.

Backfill: Cementitious or mineral material placed around waste packages in a repository to act as a secondary barrier to radionuclide migration.

Sorption: General term encompassing adsorption and absorption processes by which radionuclides are held at or within solid surfaces.

Reactive zone: Localised region within a cementitious matrix where chemical interactions lead to retention or transformation of migrating species.

References

1. Spectroscopic identification of Ca-bearing uranyl silicates formed in C–S–H systems. Scientific Reports (2023).
2. Sorption experiments as efficient as possible: Mini-column experiments (MCE) using HPLC-ICP-MS coupling with a new data analysis approach to determine sorption parameters. Microchemical Journal (2024).
3. Retardation of Chlorine-36 by Cementitious Materials Relevant to the Disposal of Radioactive Wastes. Minerals (2024).
4. Effects of organic degradation products on the migration behaviour of radionuclides in cementitious materials. MRS Advances (2024).