Radiolysis of TBP

The effect of irradiation on the extractability of sulfoxides towards plutonium, uranium and some fission products were studied by Subramanian and coworkers . They studied mainly the effect of irradiation on dihexyl sulfoxide (DHSO) and found that irradiation did not change the distribution coefficient for Ru, Eu and Ce but increases the distribution coefficient for Zr and Pu. When comparing DHSO and tributyl phosphate (TBP), the usual solvent for the recovery and purification of plutonium and uranium from spent nuclear fuels, the effect of irradiation to deteriorate the extraction capability is much larger in TBP. Lan and coworkers studied diphenyl sulfoxides as protectors for the gamma radiolysis of TBP. It was found that diphenyl sulfoxide can accept energy from two different kinds of excited TBP and thus inhibits the decomposition of the latter. 

The primary separation of plutonium and uranium from the fission products involves a solvent extraction with 30 vol % TBP at room temperature. The activity levels in this separation are quite high ( 1700 Ci/L for the fission products) and the aqueous waste, which contains 99+% of the fission products, is a high-level waste. Am and Cm are not extracted and Np is partially extracted. Because of the high radiation levels, there are radiolysis problems with TPB, leading to solvent degradation. Primary products of the radiolysis of TBP are the dibutyl- and monobutylphosphoric acids along with phosphoric acid. These degradation products are removed in the solvent purification steps. 

Degradation Products from the Radiolysis of TBP/diluent/nitric Acid... 

Qualitative Analysis. Degradation products from the radiolysis of TBP include di- -butyl phosphoric acid (HDBP), mono-n-butyl phosphoric acid... 

Nature of the Irradiation Source. The degradation of TBP-diluent in contact with an aqueous phase did not reveal any qualitative influence of the nature of the radiation (y, p, a, accelerator radiation). Moreover, the yield value of the decomposition products of TBP (8, 9,11, 26, 84), nitro and carbonyl products (RN02, RN03, RCOOH, and RCOR ) (16), alkanes (C5-C8), and butanol (8) were almost identical, whatever the nature of radiations. Nevertheless, after radiolysis of TBP in n-dodecane without aqueous solution, the yield of H2MBP was substantially lower under a-irradiation than under y-irradiation (Ga (H2MBP) = 0.158 and Gt(H2MBP) = 0.443) (5). 

Concentration of the Phosphorus Degradation Products Observed after Radiolysis of TBP... 

According to Kerr and Webster (279), the radiolysis of TBP leads to alkyl radicals R and OP(OR)2OR . Investigations of the radical intermediates, by ESR examination or by use of electron scavengers, provided clear evidence regarding the formation of R radicals by dissociative electron capture (Equation 8.1) (294). 

Nowak, Z., Nowak, M., Seydel, A. 1979. The radiolysis of TBP-dodecane-HN03 systems. Radiochem. Radioanal. Lett. 38(5-6) 343-354. 

Kulikov, I.A., Kermanova, N.V., Vladimirova, M.V. 1985. Gamma-radiolysis of TBP and DBPA in the presence of HN03. Soviet Radiochem. 27 58-63. (Translated from... 

Radiolysis. Radiation degrades both TBP and hydrocarbon diluent in Purex systems, with formation of molecular fragments, polymers, and nitration products. The main product, however, is the same as from hydrolysis, namely, DBP. The yield of DBP in radiolysis of TBP varies somewhat with the diluent used, water content, type of radiation, and dose rate. Baumgartner and Ochsenfeld [B6] cite production of 20 to 30 mg DBP/liter in 30-min exposure of 30 v/o TBP to 0.2 Wh/liter of radiation in mixer-settlers processing fuel cooled 240 days after 33,000 MWd/MT burnup. Because the density of DBP is 1065 g/liter, the volume percent DBP was... 

Radiolysis of hydrocarbons has been investigated and there is a review [108]. n-Dodecane is one of the saturated hydrocarbons, and its radiation degradation is rather known as compared with that of TBP. Main degradation products are H2 and dimers, as summarized in Table 3 [109,110]. Again, the values are dependent on the reports. Recently, systematic study on the radiolysis of n-paraffins has been reported [111-113]. The formation process of dimers and double bonds is assumed as follows. 

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... 

Accumulation of Radiolysis Products in the Organic Phase during Irradiation of TBP Solutions... 

Concentration of TBP. The rate of TBP decomposition and the yields of major acids G(HDBP) and G(H2MBP) increased with the concentration of TBP (8, 85, 88). On the other hand, the yield of gaseous radiolysis products coming from the diluent decomposition decreased with an increasing fraction of TBP (88, 96). 

Intensive studies on hydrolysis and gamma radiolysis of OOD(iB)CMPO were carried out in decalin (40), n-dodecane (20, 41, 46), TCE (tetrachloroethylene) (42), and CC14 (40). The stability of CMPO in dodecane was greater than in chlorinated diluents, as shown by G-values established in the presence of an acidic aqueous phase for the disappearance of CMPO in the presence of TBP in the organic phase (to simulate solvent in TRUEX process conditions), G(-CMPO) = 1.2 0.3 in dodecane, and 4.5 0.3 molecules/100 eV in tetrachloroethylene (42). This protective effect of dodecane on CMPO was quite unusual. 

In the presence of TBP molecules, the effect of radiolysis on Z)A 11 at high and low acidities was reduced, which allowed authors to declare a protective effect of TBP (40, 42). 

According to Zhang, the excited site of TBP was located on the P = O bond (287), and several active species were formed by y-radiolysis, such as excited singlets, excited triplets, and positive ions (293). 

With pulse radiolysis and flash photolysis, Jin et al. have recently focused on the examination of TBP excited species (291). The authors concluded that the decomposition of TBP occurred through two processes dissociative electron capture (Equation 8.1) and decomposition of TBP excited molecules (Equation 8.2). 

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. 

Nevertheless, with the oldest extraction system in the nuclear field, the TBP-alkane solvent has been studied since the 1950s, and authors have succeeded in proposing a detailed qualitative and quantitative description of the radiolysis. Many authors have focused on the primary process of the mechanism for pure TBP (18, 279-281, 283-287). The formation rate of TBP degradation products has been estimated under several experimental conditions and a degradation scheme proposed with rate constants (9, 311-313). An empirical equation allowing the calculation of the yield of TBP decomposition and degradation product formation in the presence of HN03, Pu(N03)4, and U02(N03)2 has been proposed (9, 11). 

Pulsed radiolysis has been used to study the primary process of TBP, TOPO (292), and TODGA (182) degradation. This approach allowed an experimental proof of the charge-transfer reaction from the alkane cation to the extractant, already previously proposed by various authors (90), to be obtained. Recent studies with a series of aliphatic alcohols have demonstrated the interest of pulse radiolysis to measure the rate constants for radical production and recombination (314). 

Lesage, D., Virelizier, H., Jankowski, C.K., Tabet, J.C. 1997. Identification of minor products obtained during radiolysis of tributylphosphate (TBP). Spectroscopy 13 275-290. 

Barelko, E.V., Solyanina, I.P. 1975. Radiolysis of solutions of TBP in contact with nitric acid. II. Processes of oxidation and nitration. Atomic Energy 38(1) 25-29. (Translated from Atomnaya Energiya 38(1) 23-26.)... 

Faugeras, P, Talmont, X. 1968. Radiolysis and hydrolysis of TBP and their effects. Proceedings of the 5th International Conference on Solvent Extraction Chemistry, Jerusalem, Israel, 16-18 September, 411 —423. 

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