Radiolytic degradation

Many tracer chemicals are inherently unstable even as the unlabeled forms. Susceptibility of a chemical to hydrolysis, oxidation, photolysis, and microbiological degradation needs to be evaluated when designing suitable storage conditions for the labeled compound. Eactors that reduce radiolytic degradation, such as dispersal in solution, are apt to increase chemical degradation or instability. 

Yokoya, A. Kobayashi, K. Usami, N. Ishizaka, S. Radiolytic degradation of cystathionine irradiated with monochromatic soft X-rays at the K-shell resonance absorption of sulfur. J. Radiat. Res. (Tokyo) 1991, 32 (2), 215-223. 

TBP is a sufficiently powerful extractant for actinides that it may be used in diluted form. Dilution improves the hydrodynamic properties of the solvent, allowing more complete and rapid phase disengagement. Typically concentrations of 20-30 v/o TBP in OK are used in process flowsheets. Although TBP is relatively stable as an extractant, radiolysis does lead to the formation of some acidic phosphate esters, HDBP and H2MBP, which can impair process performance.289 An aqueous alkali wash of the recycled solvent is usually carried out to remove those by products. Radiolytic degradation of the diluent in the presence of nitric acid can result in the formation of hydroxamic acids,290 which can lead to fission product retention by the organic phase. Again the solvent wash is used to prevent the accumulation of such species. A comprehensive account of the industrial utilization of TBP has recently been published.291... 

Tripathi, S.C., Sumathi, S., Ramanujam, A. 1999. Effects of solvent recycling on radiolytic degradation of 30% tributyl phosphate- -dodecane-HN03 system. Sep. Sci. Technol. 34 (14) 2887-2903. 

Ikeda, H., Suzuki, A. 2001. Empirical correlations for radiolytic degradation of -dodecane density viscosity and phase separation time. J. Nucl. Sci. Technol. 38 (12) 1138-1140. 

Berthon, L., Morel, J.M., Zorz, N. et al. 2001. DIAMEX process for minor actinide partitioning Hydrolytic and radiolytic degradations of malonamide extractants. Sep. Sci. Technol. 36 (5 6) 709-728. 

Zhang, P, Song, C.L., Liang, J.F., Xin, R.X. 2003. Identification of radiolytic degradation products of y-irradiated 30% trialkylphosphine oxide-kerosene solution. Solvent Extr. IonExeh. 21 (1) 91-108. 

Hydrolytic and radiolytic degradation of TAP solution in normal paraffinic hydrocarbon (NPH) in the presence of nitric acid was investigated. Physicochemical properties such as density, viscosity, and phase-disengagement time (PDT) were measured for undegraded and degraded solutions (197). The variations in these parameters were not very different from those obtained with degraded TBP. Thus, the hydro-dynamic problems expected during the solvent-extraction process with TAP would be similar to those encountered with TBP/NPH system. The influence of chemical... 

Since the pioneering work of Siddall, /V./V-dialkyl amides have been evaluated extensively as alternative extractants to TBP (200, 201). The salient features of amides as extractants are (i) low volume of secondary waste generated (completely incinerable), (ii) innocuous nature of chemical and radiolytic degradation products (better decontamination from fission products andregeneration/clean up easier), (iii) low aqueous-phase solubility, (iv) final U and Pu products streams are free of P contamination, and (v) ease of synthesis. However, LOC values of U and Pu as well as viscosity are... 

Venkatesan, K.A. Robertselvan, B. Antony, M.P. Srinivasan, T.G. Vasudeva Rao, PR. Physiochemical and plutonium retention properties of hydrolytic and radiolytically degraded tri- -amylphosphate, Solvent Extr. Ion Exch. 24 (2006) 747-763. 

The radiolytic degradation of solvents was usually performed by irradiation of synthetic solutions rather than industrial samples. In more than 90% of studies, samples were exposed to y-irradiation with a 60Co source, and sometimes with a 137Cs... 

The radiolytic degradation of a molecule or the formation of new species can be quantified by a radiation-chemical yield related to the energy absorbed, and the term G- value represents the number of molecule changes for each 100 eV of energy absorbed. Thus, G(X) refers to the number of molecules of a product X formed on irradiation per 100 eV of energy absorbed and G(-Y) refers to the loss of a material Y destroyed on irradiation (78). 

Di(2-ethylhexyl) phosphoric acid (HDEHP) is an extractant molecule used for An(III)/Ln(III) separation. Used in TALSPEAK-type processes in a mixture with TBP, or in the DIAMEX-SANEX process in a mixture with a malonamide (154-157), it has also been proposed, in a mixture with TBP, to remove strontium from PUREX acid waste solution in the Hanford B plant (158). Therefore, numerous studies have focussed on the radiolytic degradation of HDEHP and its effects on the extraction of Sr(II), lanthanides(III), and actinides(III) (10, 158-163). 

Degradation Products from the Radiolysis of HDEHP Systems. The main radiolytic degradation products are in the organic phase, mono(2-ethylhexyl) phosphoric acid (H2MEHP), 2-ethylhexanol, and polymeric species, but also a certain amount of ortho phosphoric acid (H3P04) was detected in the aqueous phase. The nature of the short compounds identified in the gas fraction was classical H2, unsaturated and saturated hydrocarbons (from 1 to 4 carbons), 02, and N2 (10, 74, 158-160, 164). 

Effects of Degradation The effects of radiolytic degradation are highly dependent on the experimental conditions ... 

FIGURE 8.7 Simplified scheme for the hydrolytic and radiolytic degradation of DMDOHEMA. (Redrawn from Berthon, L., Carnes, B., CEA Report CEA-R-5892, 206-211, 2000.)... 

FIGURE 8.10 Schematic diagram of the radiolytic degradation of TODGA. (Drawn from Sugo, Y., Sasaki, Y., Tachimori, S., Radiochim. Acta, 90 161-165, 2002.)... 

Extraction of U(VI) and Pu(IV) from 1 to 8 mol L 1 HN03 solutions by radiolytically degraded DCH18C6 in toluene was studied (256). A decrease in the distribution ratios for both U and Pu was observed for irradiation in the range 0.010-0.071 MGy, with a higher effect for Pu(IV). For 0.2 mol I. DCH18C6-toluene solution in contact with 3 mol I. nitric acid, D, decreased from 0.21 to 0.12 and /.)hl from 64.3 to 6.42 after a dose of 0.07 MGy. This behavior was explained by both diluent and extractant degradation. 

FIGURE 8.16 Major radiolytic degradation compounds of (octMC6) in NPOE/HNOj. (Redrawn from Lamouroux, C., Aychet, N., Lelievre, A., Jankowski, C.K., Moulin, C., Rapid Commun. Mass Spectrom., 18 1493-1503, 2004.)... 

To limit the radiolytic degradation of extractants, the influences of free-radical inhibitors have been measured. The addition of dimethoxybenzaldehydes (DMBA), particularly 3,5- and 3,4-DMBA, to the PUREX solvent could improve its stability and decrease its contamination (307). DMBA has a double effect, including a protective effect for the excited molecules of TBP (because of its low ionization potential), and the aldehyde radiolysis products could react with the HDBP present and therefore inhibit its complexing properties. 

The selection of a suitable diluent is important to limit radiolytic degradation. Diluents currently used in nuclear applications are hydrocarbons, despite their well-known sensitization effect on radiolysis, as mentioned for alkylphosphates or amide extractants (90, 182, 183, 199), and as discussed in Section 8.4.2. To avoid this negative effect or to enhance the solubility of ligands and metallic complexes, other diluents have been selected and their influence on degradation investigated. 

In the TRUEX process, the presence of TBP decreases the radiolytic degradation of CMPO (41) ... 

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