Bituminized Waste Products (BWPs) were produced by conditioning the co-precipitation sludge resulting from the industrial reprocessing of spent nuclear fuel in bitumen. A possible solution for the long-term disposal of some intermediate level long-lived (ILW-LL) categorized BWPs in France is their underground geological disposal in a deep repository – CIGEO - which is foreseen to be constructed in the Callovo-Oxfordian clay formation in the area of Meuse Haute Marne. The project is led by ANDRA. One of the challenges with BWPs is their swelling behaviour. Indeed, after several hundred thousand years, the water from the host rock will fully saturate the disposal cells. By an osmotic phenomenon enabled by the semi-permeable capacity of the bituminous matrix, water in direct contact with BWPs could then cause them to swell. This would lead to pressure on the host rock and therefore mechanical consequences on the disposal facility. The BWPs swelling behavior has therefore to be taken into account in safety studies for the disposal facilities.

The objective of this work is to investigate both experimentally and numerically the BWPs’ swelling behavior due to water uptake under confined conditions representative of a situation in a deep geological repository (i.e. under constant volume and constant counter-pressure conditions). Leaching experiments under constant counter-pressure were performed on simplified French BWPs (containing only one or two salts), the swelling was monitored for about 2.5 years. The numerical model detailed in this paper is extended from an existing one that incorporates coupled homogenization of transport terms (diffusion, permeation, osmosis) [2–4]. The extended model takes into account a non-linear poro-viscoelastic model for large strains to better model the BWPs leaching behavior under confined conditions. Taking into account the uncertainties of measurement, the extended numerical model results were fit to reproduce the experimental results of leaching tests under confined conditions. The model is valid for the entire leaching process. The role of the poorly soluble salts BaSO4 within the BWP matrix is also investigated.

References

[1] E. Valcke, A. Marien, S. Smets, X. Li, N. Mokni, S. Olivella, X. Sillen, Osmosis-induced swelling of Eurobitum bituminized radioactive waste in constant total stress conditions, J. Nucl. Mater. 406 (2010) 304–316. https://doi.org/10.1016/j.jnucmat.2010.08.060.

[2] G. Melot, P. Dangla, S. Granet, S. M’Jahad, J.B. Champenois, A. Poulesquen, Chemo-Hydro-Mechanical analysis of Bituminized waste swelling due to water uptake: Experimental and model comparisons, J. Nucl. Mater. 536 (2020) 152165. https://doi.org/10.1016/j.jnucmat.2020.152165.

[3] G. Mélot, Modélisation poromécanique du gonflement d’enrobés bitumineux par reprise d’eau, 2019.

[4] Y. CHEN, Investigation of Bituminized Waste Products swelling behavior due to water uptake under free leaching conditions: experiments and modeling, (2023), under review.