Remediation of $^{137}$Cs-contaminated concrete gravels by supercritical CO$_2$ extraction

International audienceThe removal of cesium contamination is a critical issue for the recycling of concrete gravel in most decommissioning sites . The high solvent strength and diffusivity of supercritical CO$_2$ make it an attractive choice as vector for extractant system in this context. Experimental extraction runs have been carried out in a radioactive environment on gravels contaminated with $^{137}$Cs. The best extraction system was found to be CalixOctyl (25,27-Bis(1-octyloxy)calix(4)arene-crown-6, 1,3-alternate) with pentadecafluorooctanoic acid as a modifier. The effects of various operating parameters were investigated, namely the coarseness of the gravel, the temperature of SC-CO$_2$, the residual water and initial cesium concentrations, and the amounts of extractant and modifier used. The yields from direct extraction were low (< 30percent), because of the virtually irreversible sorption of Cs in concrete. The best extraction yield of $\sim$55percent was achieved by leaching the concrete gravel with nitric acid prior to supercritical CO$_2$ extraction