A recently developed tridentate ligand, N, N,N,,N’-tetraoctyl diglycolamide (TODGA) is a promising extractant being considered for the separation of An(III) and Ln(III) from HLLW solutions (Sasaki et al., 2001, Narita et al., 1998, Sugo et al., 2002, Tachimori et al., 2002). The tridentate functionality provided by the additional etheric oxygen has led a number of researchers to consider TODGA over the commonly used neutral diamides with biden­tate properties (Ansari et al., 2005). The TODGA ligand structure is shown in Fig. 13.11.

Hoshi et al., (2004) evaluated the use of TODGA impregnated SiO2-P solid-phase extraction resin for performing group separation of MA and Ln elements from HLLW. This is one step in the proposed ERIX (Electrolytic Reduction and Ion exchange) process for reprocessing spent fast breeder reactor mixed oxide fuel (FBR-MOX). Column tests using simulated HLLW showed that group separation of Am and some Ln ele­ments relative to other fission products was satisfactory. Zirconium, Ru, Sr, and Y were also extracted; however, all were effectively separated from the An/Ln elements using appropriate elution schemes, i. e. using different concentrations of HNO3 and oxalic acid for Zr. The authors found the uptake and elution kinetics of Ru to be slow compared to the other ele­ments and were only able to recover 85% of the Ru fed to the column. The loss of TODGA to the aqueous phase was also investigated and the researchers concluded that <0.02% (w/w) of the extractant was lost during one separation cycle.

A solid-phase extraction resin consisting of 30% (w/w) TODGA and 10%(w/w) TBP adsorbed onto Amberchrom CG-71 was evaluated for the separation of An and Ln elements from a simulated PUREX raffinate solu­tion (Modolo et al., 2007). The simulant represented a PUREX raffinate (5000 L/t HM) from fuel having an initial 235U enrichment of 3.5%, 33 000 MWd/tHM burn-up, and a 3-year cooling period. Small column (22.9 cm3) studies indicated that the An and Ln elements could be effec­tively separated from the light fission products using four nitric acid scrub­bing steps followed by a strip with 0.01 M HNO3 and a H2O wash. Oxalic acid and HEDTA were also included in the first and second scrub steps to effect the complete removal of Zr and Pd from the column. Similar to comparable investigations (Hoshi et al., 2004), a portion of the Ru remained on the resin, as well as a small fraction of Cf and Y. It was noted that this
issue needs further investigation and that a more radiolysis resistant sub­strate (e. g., silica) may also be needed for large scale applications.

Various flow sheets using CMPO and TODGA sorbed onto SiO2-P have also been proposed and tested for partitioning An(III) and Ln(III) ele­ments as part of the proposed Minor Actinides RECovery (MAREC) process based on solid phase extraction. (Zhang et al., 2004, Wei et al., 2004b, Zhang et al., 2005c, Zhang et al., 2008). The stability of the TOGDA-SiO2-P resin as a function of nitric acid concentration, temperature and y-radiation was investigated by Zhang et al., (2005a). The authors evaluated the effects of these parameters by comparing the batch adsorption of Nd onto the resin. Nitric acid concentrations up to 3M did not decrease the adsorption properties of the resin at 25°C. The capacity of the resin was, however, reduced by approximately a factor of three in 3 M HNO3 at 80°C, presum­ably due to losses of the extractant to the aqueous phase as evidenced by an increased total carbon concentration in the aqueous fraction. Irradiation of the resin from approximately one to four MGy resulted in a linear decrease in extraction capacity. The highest dose (4.1 MGy) resulted in a 70% reduction of capacity, which was also determined to be from extractant losses. Studies to quantify these losses as intact TODGA molecules or its radiolysis products were inconclusive. Similar investigations were con­ducted by these researchers (Zhang et al., 2005b) to assess the effects of HNO3, temperature and y-radiation on CMPO-SiO2-P solid phase extrac­tion resin. Nitric acid (up to 3 M) and increased temperature (80 °C) did not cause a significant decrease in capacity for Nd. The y-radiation experi­ments indicated a linear decrease in capacity, with an approximate decrease of 50% at the maximum dose of 4 MGy.