Most of the batch experiments relevant to caesium selective extraction studies, involving calix[4]arenes-(mono/bis)crown-6, have been carried out in polar aromatic organic diluents, such as o-nitro-phenyl-alkylethers. Although they greatly enhance the extraction properties of calix[4]arenes- (mono/bis)crown-6 compared with alkanes conventionally used in the nuclear industry, these organic diluents are nonetheless incompatible with the implementation of counter-current process flowsheets: their viscosity and density are too high, and the kinetics of mass transfer too slow. This is why, in order to demonstrate the scientific feasibility of caesium partitioning from acidic nuclear waste streams, efforts at the CEA Cadarache (France) have focused on the substitution of o-nitro-phenyl-alkylethers by mixtures of hydrogenated tetrapropene (HTP), the diluent used in the French PUREX process, and modifiers allowing the use of calix[4]arenes — monocrown-6 without third-phase occurrence. Two reference systems pre­senting sufficiently high and selective caesium extraction from acidic nuclear waste have been optimized (Fig. 11.6):

1. The first is based on 1,3-(di-n-octyloxy)-2,4-calix[4]arene-crown-6 (0.065 mol. L-1) and tributyl phosphate (1.5 mol. L-1) as phase modifier.

System 1

Calix[4]arene-Crown-6 )o o£

1,3-[di-n-octyloxy]-2,4-calix[4]arene-cr-6 Modifier: TBP

TributylPhosphate

11.6 The two reference systems chosen at the CEA to selectively extract caesium from acidic nuclear waste streams.

2. The second is based on 1,3-(2,4-diethyl-heptylethoxy)-2,4-calix[4]arene- crown-6 (0.1 mol. L-1), more difficult to synthesize, and methyloctyl-1,2- dimethylbutanamide (1 mol. L-1) as phase modifier.

Process flowsheets have been developed for both systems and counter­current tests have been performed implementing laboratory-scale contac­tors (e. g., centrifuges, Taylor-Couette effect columns, and 15 mm diameter pulsed columns) and surrogate feeds. More than 99.9% of caesium was extracted and back-extracted in the centrifuge test, in agreement with flow­sheet modelling. Although satisfactory, the hydrodynamic behaviour of the second system (calixarene + monoamide) appeared more emulsifying in the pulsed and Taylor columns than the first system.

In the light of these promising results, two hot tests were performed in the hot cells of the Atalante facility on a genuine DIAMEX raffinate[11] (see Fig. 11.7 for the flowsheet implemented). By adding oxalic acid to the genuine feed, the extraction of molybdenum and zirconium was prevented and very high caesium recovery yields (>99.9%) were observed, thus con­firming the ability of calix[4]arenes-monocrown-6 based solvents to parti­tion caesium efficiently and selectively from acidic nuclear waste streams (Madic et al., 2002).

11.7 Flowsheet for caesium partitioning from genuine DIAMEX raffinate implemented in the Atalante facility of the CEA Marcoule (Madic et al., 2002).

The radiolytic degradation of 1,3-(di-n-octyloxy)-2,4-calix[4]arene — crown-6 was studied in the presence of nitric acid. High-performance liquid chromatography, directly coupled with ESI-MS, allowed more than 50 dis­tinct degradation products to be observed, and about 30 of them to be identified (as products issued from radical cleavage or addition, oxida­tion, and aromatic substitution) in aliphatic and aromatic diluents (Lamouroux et al., 2004). Despite the severe degradation conditions tested ([HNO3] = 3 mol. L-1 for the acidic hydrolysis and a gamma delivered dose of 106 Gy for radiolysis), 1,3-(di-n-octyloxy)-2,4-calix[4]arene-crown-6 appeared remarkably stable as illustrated by the limited losses of compound observed: 33.5%.