Diamex solvent behaviour under continuous degradation and regeneration operation

Full text of publication follows. Industrial implementation of partitioning processes based on solvent extraction requires solvent recycling with constant separation and physicochemical properties. In nuclear applications, both radiolysis and acidic hydrolysis leads to degradation products which need to be removed from solvent before recycling. The long-term evolution of DIAMEX process solvent (0.65 M DMDOHEMA - TPH) separation performances and physicochemical properties, under continuous degradation by acidic hydrolysis and -radiolysis, are studied in the laboratory-scale MARCEL γ-irradiation facility, with or without alkaline treatment process (AT). Implemented experimental conditions lead to the same order of magnitude for hydrolysis and radiolysis effect on DMDOHEMA degradation speed. With AT, gas-chromatographic measurements and potentiometric titration of organic phase only show the accumulation of MDOHEMA with a rate of 0.3 mmol/L/h of hydrolysis. Surface tension and refraction index, as well as distribution coefficients of Zr, Pd, Fe and Nd, are constant during the tests. Radiolysis increases viscosity and, slightly, density, if AT is used or not: TPH radiolysis products are probably responsible for this behaviour. In addition, radiolysis increases molybdenum extraction coefficient and decreases its scrubbing, which may be linked with the MDOHEMA accumulation measured. Moreover, without AT, settling-time increases due to surface-active acid compounds accumulation, whereas with AT, settling times remain constant. Furthermore, no catalytic effect of fission and corrosion products is observed. Concluding this study, γ-test results under continuous operation are consolidated by comparative - and α-radiolysis batch tests which show that -radiolysis of DMDOHEMA (without nitric acid phase) is four times slower than γ-radiolysis (with nitric acid phase), leading however to the same break-down products with a slighter accumulation kinetic. Studies are still in progress to observe the effect of the nitric acid aqueous phase during radiolysis of the organic phase (part of hydrolysis or radiolysis of nitric acid). Overall, the different experimental steps validate the DIAMEX solvent degradation study and alkaline treatment performances. Continuous operation with solvent recycling, simulating one to two years industrial operation, has proven the long-term stability of process performances. (authors)