Session: 05-01: Waste Conditioning, Stabilization and Encapsulation Studies

Paper Number: 109925

109925 - Overcoming Conditioning Issues of Radioactive Organic Waste Treated by Molten Salt Oxidation 

Molten salt oxidation (MSO) is an equitable process for treating radioactive solid organic waste (RSOW), providing complete degradation of its carbonaceous matrix—which is responsible for short- and long-term instabilities—and significant volume reductions. However, conditioning the obtained residues in an inert matrix may be difficult due to their chemical nature: the high content of carbonates causes substantial volume expansions during hydration, which induce internal mechanical stresses and, eventually, cracking of the conditioned waste forms [1]. In this work, an alternative encapsulation strategy for MSO residues is proposed and preliminarily characterised to verify compliance with generally agreed waste acceptance criteria.

The strategy involves the MSO residues being conditioned in a geopolymeric matrix. To do this, the setting of the mortar is delayed so that carbonates undergo complete hydration before grouts set, allowing for volumetric stabilisation of the forms during curing and exposure to moist environments. Delay of the setting is achieved by adding inorganic ash produced in the pilot Installation for Research on Incineration of Solids (IRIS) operated by French Alternative Energies and Atomic Energy Commission (CEA), which treats surrogate RSOW by means of incineration processes [2]. Accordingly, the strategy aims to exploit the retarding effect of one radioactive waste to successfully stabilise the other. This approach would concurrently lower the overall disposal costs by reducing the volume of the final waste and promoting the circular use of resources. Furthermore, the conditioning matrix is a metakaolin-free geopolymer synthesised from natural precursors and recycled industrial by-products, to reduce environmental impact [3].

Different MSO-residue/IRIS-ash ratios were tested and loading factors of MSO residues up to 35 wt.% were achieved using 15 wt.% IRIS ash (total waste loading of 50 wt.%). Preliminary compliance with waste acceptance criteria was demonstrated by means of compressive strength measurements and immersion tests in synthetic water, to mimic accidental scenarios at low- and intermediate-level waste repositories. Specimens were doped with stable isotopes representative of typical fission and activation products, and their release, as well as of matrix and waste constituents, was monitored via inductively coupled plasma-mass spectrometry to calculate diffusion coefficients and leachability indices. Phase analysis and microstructural characterisation of the hardened waste forms was performed via X-ray powder diffraction to investigate waste-matrix interactions.

The promising results obtained so far, i.e., compressive strengths above 10 MPa and leachability indices above 6, foster further study to investigate compliance with additional acceptance criteria and the scale-up of the conditioning strategy. Moreover, other retardants could be investigated to replace the IRIS ash with more available materials and accessible to the MSO processing facilities.

This project has received funding from the Euratom research and training programme 2019-2020 under grant agreement No 945098.

[1] Z. Yao et al. «Molten salt oxidation: A versatile and promising technology for the destruction of organic-containing wastes». In: Chemosphere, vol. 84, pp. 1167-1174 (2011).
[2] F. Lemont «Management of metal chlorides in high temperature processes—Application to the nuclear wastes treatment». In: Journal of Hazardous Materials, vol. 213, pp. 38-45 (2012).
[3] A. Santi et al. «Design of sustainable geopolymeric matrices for encapsulation of treated radioactive solid organic waste». In: Frontiers in Materials, vol. 9, pp. 1-17 (2022).

Presenting Author: Andrea Santi Politecnico di Milano

Presenting Author Biography: Enthusiast, determined, meticulous. I am a nuclear engineer focused on the chemistry and the physics of condensate matter, with regard to the management of radioactive waste.