I
R. M. WAGNER, E. M. KINDERMAN, and L. H. TOWLE Stanford Research Institute, Menlo Park, Calif.
Radiation Stability of Organophosphorus Compounds Radiation damage to organophosphorus compounds used as extractants for uranium and plutonium can markedly influence process efficiency. Small changes in structure are important
RADIATION damage to solvents used in extracting uranium can be important. When reactor fuel elements have been cooled a short time, intense fission product radiation can change the composition of process reagents, and adversely affect performance of solvent extraction equipment or necessitate complete alteration of processing technique. Radiation doses as low as 0.5 watthour per liter produced changes in tributyl phosphate-hydrocarbon systems which noticeably affected fission product decontamination efficiency (6,g). However, they did not materially affect uranium extraction until levels of 10 to 30 watt-hours per liter were reached. Damage to solvents may result in emulsifiers or species which form insoluble or nonextractable complexes with uranium and fission products. The first causes entrainment in column operation ; the second, poor uranium yields during stripping or erratic distribution of fission product contamination in both phases. Either degradation product may impair or prevent efficient head-end treatment of fuel elements cooled only a short time. Consequently, when tributyl phosphate-hydrocarbon mixtures are used in high level radiation environments, Table 1.
4
the relationship of dose level to radiation damage must be known, as well as the relationship of this damage to process efficiency. Tributyl phosphate, tributyl phosphate diluted to 200 and 500 grams per liter in Amsco 125-82, and Amsco 12582 alone were irradiated with I-m.e.v. (nominal) electrons to dose levels of 1420, 822, 1455, and 780 watt-hours per liter (Table I). G values for dibutyl phosphoric acid production agree well with those reported (7, 2, 4). With y-rays, these G values were not reduced by diluting the tributyl phosphate with hydrocarbon diluent ( 2 ) . I n present studies definite decreases occurred with increasing dilution (Table I). G values for gas production did not behave similarly, because of increased gas yield with diluent concentration. Low G (alcohol) values indicated preferential radiolytic scission a t the C-0 rather than 0-P link. The effectiveness of a potential “electron sink” effect of an aromatic diluent was studied by irradiation of tributyl phosphate diluted with benzene or toluene. G (acid) values for monobasic and dibasic acids were reduced below those for pure tributyl phosphate; benzene was more effective than toluene in
suppressing monobasic acid, while toluene was more efficient in suppressing dibasic acid (Table I). With y-rays, there is a decrease in G (monobasic acid) with benzene dilutions of tributyl phosphate and an increase with carbon tetrachloride dilution (Halex process) (2). The effect of electron sink structures simultaneously present in extractant and/or in diluent was studied by electron irradiation of dibutyl phenyl phosphonate alone and diluted with Amsco 125-82 or toluene (Table I ) . This compound had been examined for uranium extraction efficiency by other investigators (3, 7). G values for monobasic acid were not influenced by dilution but were drastically reduced below the values previously found for tributyl phosphate. The phenyl group in the phosphonate conferred as much protection alone as diluted with toluene; hence it was concluded that diluent aromatic groups did not enhance stability. T o check the degree of stability of phosphonate structural types over that of phosphate types, dibutylbutane phosphonate, diamylpentane phosphonate, didecyldecane phosphonate, and Amsco 125-82 dilutions of two of these were irradiated with electrons (Table 11). Table I1 indicates that phosphonates
G Values for Irradiation of Extractant-Diluent Systems
Aliphatic or aromatic nature of diluentt markedly influences extractant rtabilitv TBP in Amsco Amsco TBP, 500 G./L. 500 g./l. 200 g./L 125-82 ’ In benzene I n toluene DBPP
Component TBP Total gas? 2.44 2.78 3.62 Hydrogen I 1.59 1.59 1.93 Methane 0.07 0.24 0.40 Acetylene 0.01 0.01 0.02 Ethylene 0.05 0.07 Ethane 0.07 0.08 0. IO Propene 0.02 0.08 0.21 Propane 0.16 0.16 0.21 Butene 0.19 0.18 0.30 Butane 0.38 0.35 0.30 Pentene 0.01 0.02 0.04 Pentane 0.02 -0.05 0.02 Hexene 0.01 0.01 0.11 Hexane 0.04 0.04 0.06 Heptane 0.04 0.05 0.40 Octene 0.08 0.23 Octane 0.08 0.23 Butanolb 0.13 0.11 0.12 Benzeneb Monobasic acidd 2.25 0.48 0.71 Dibasic acidd 0.39 0.11 0.15 Dose, watt-hr./liter 1420 1455 822 a All gas romponents determined by mass spectrometry. extractant. Determined by electrotitrimetry.
...
... ... ...
...
...
4.62 2.77 0.62 0.05
...
0.13 0.21 0.39 0.37 0.16 0.05 0.07 0.03 0.12 0.06
0.06 0.10
... ... ... ...
780
1.96 1.20 0.06 0.02 0.05 0.05 0.02 0.14 0.18 0.24 0.01 0.01 0.01 0.04
... ...
...
0.10
...
1.54 0.41 1455
1.34 0.82 0.05 0.02 0.05 0.04 0.02 0.08 0.09 0.15 0.01 0.01 0.01 0.02
... ... ...
0.15 0.11 1.97 0.19 1455
0.45 0.34 0.03 0.01 0.01 0.02 0.01 0.03 0.01 0.02
D B P P , 500 G./L. I n toluene In Amsco 0.47 1.04 0.31 0.03 0.01 0.02 0.13 0.01 0.02 0.02 0.04
... ... ... 0.01 ... ...
...
... ...
... ... ... ...
...
0.80 0.10 0.01 0.01 0.02 0.01 0.03 0.07 0.07 0.03 0.01
0.01 0.03 0.04
... ...
0.08 0.18 0.36
0.07 O.3Oc 0.30
0.06 0.16 0.24
1507
1455
1455
...
...
Determined by gas liquid chromatography.
...
Produced from both diluent and
VOL. 51, NO. 1
JANUARY 1959
45
Table II.
Comparison of G Values and Radiation Parameters Shows Phosphonates Are More Radiation-Resistant Than Phosphates Dose,
System
Gas
Total Acid
WattHr./Liter
Tributyl phosphate Dibutylbenzene phosphonate Dibutylbutane phosphonate Diamylpentane phosphonate Didecyldecane phosphonate Tributyl phosphate‘ Dibutylbenzene phosphonatea Dibutylbutane phosphonateO Diamylpentane phosphonatea
2.44 0.45 1.79 1.30 2.61 2.78 1.04 2.05 1.79
2.64 0.36 1.40 1.90 1.17 0.60 0.24 0.80 1.00
1420 1507 1230 1220 1210 1455 1455 1325 1325
500 g./liter in Amsco 125-82.
produce lower G values for acid production than phosphates; increase in alkyl chain length in phosphonate increases G gas values; G (acid) values for phosphonates decrease with increasing hydrocarbon dilution, but not as markedly as those for phosphates; the phenyl protective effect is operative in dibutylphenyl phosphonate when the compound is diluted with hydrocarbon and the change from alkyl to aryl phosphonate is more effective in reducing radiation damage than the change from phosphate to phosphonate. The liquid phase from irradiation of tributyl phosphate was examined for polymeric constituents. Acidic components were removed from the mixture with an anionic resin column, and residual tributyl phosphate was removed by vacuum distillation. Polymeric material obtained possessed a titrimetic acidity of 6y0by weight and a cryoscopic molecular weight of 840. The G value for polymer of 0.91 is lower by a factor of 2 or 3 than values reported by other investigators. (5) This is attributed to the fact that the “polymer” fraction obtained in these studies contained fewer acidic components (Table 111). Because dibutyl phosphoric acid is one of the primary products of radiolysis or hydrolysis of tributyl phosphate, it was irradiated with electrons to compare polymer production values with those obtained from tributyl phosphate. The G (polymer) value of 1.42 (Table 111) indicated that dibutylphosphoric acid produces polymeric species under irradiation more readily than does tributyl phosphate. The polymer from this study was not isolated as in the tributyl phosphate case; the amount of polymer was calculated by material balances based upon titrimetric measurements. I n the solvent extraction processes for uranium recovery from reactor fuel elements, the magnitude of uranium distribution coefficients in favor of the organic extractive phase is important. I t is equally important to strip the uranium efficiently from the organic phase into a n alternate aqueous phase. The efficiency of the “backwashing” operation is limited by the degree to which radiolytic products of solvent degrada-
46
INDUSTRIAL AND ENGINEERING CHEMISTRY
tion allow formation of uranium-retentive nonaqueous soluble species. Dibutylphosphoric acid and “phosphate radiation polymer,” which are known to form such species, were added in known amounts to tributyl phosphateAmsco 125-82 mixtures. Concentrations were adjusted to correspond to specific radiation doses, using the composition of 1900 watt-hours per liter of tributyl phosphate as a basis for calculation. These solutions were loaded with uranium by contact with aqueous simulated feed and then stripped of uranium by contact with 0.OlN nitric acid. A spinner-type column ( 7 7 ) was used. The organic raffinate and aqueous product phases were then analyzed for uranium, and uranium distribution coefficients calculated (Table IV). The data indicated, when compared with those derived from similar studies with virgin and 1900 watt-hours per liter tributyl phosphate, that 20% of the uranium retention was due to polymer, 65% to dibutyl phosphoric acid. and 15% to other componenents. I t is concluded that both polymer and dibutylphosphoric acid contribute to the uranium-retentive
Table IV.
Uranium Distribution Ratios
Both polymer and dibutylphosphoric acid contribute to uranium retention Dose,
WattTBP Organic
Hr./
Phase Compn.
Litera
f?(U)b
1900 watt-hr./liter
100 200 300 400
0.13 0.25 0.37 0.49
Solutions of DBP
100 200 300 400
Solutions of polymer
0.08 0.17 0.25 0.32 0.03 0.05 0.07 0.09 0.01 in all cases
100 200 300 400 Virgin 0 ‘ “Composition simulated” except 1900 watt-hr./liter TBP (see Table 111). Ratio of concentration of residual uranium in stripped organic phase t o concentration of uranium in aqueous strip phase. Size of parameter is indicative of stripping efficiency.
Table 111. Dibutylphosphoric Acid Produces Polymeric Species under Irradiation More Readily Than Tributyl Phosphate G
Item Gas
DBP MBP Target (destroyed) Polymer Dose, watt-hr./liter
TBPa 2.07 1.37 0.03 0.01 2.15* 0.91 1900
DBP 3.38
...
2.81 0.42 3.92* 1.42 1277
a Values differ from data in Table I because of higher dose and longer irradiation time. G.w values--molecules of parent material destroyed per 100 e.v. absorbed.
character of irradiated tributyl phosphate-hydrocarbon systems. The polymer retention is one fourth that of the dibutyl phosphate retention; the sum of both retentions accounts for 85% of the total observed with gross radiolysis mixtures (Table 11’). All irradiations were performed using a General Electric resonant transformer of fixed-beam type rated at 1 k.r+.,which produces 1-m.e.v. (nominal) electrons. Dosimetry was based upon ceric values (8, 70). All G values were compuwd on the basis of energy absorbed by the 100 grams of solution used in each case and not on considerations of energy absorbrd by individual components. The dose rate used in all irradiations was 6.5 x lo8rrgs per gram per minute. literature Cited (1) Baldwin, W. H., Oak Ridge N:itl. Lab., Central Files No. 57-4-9 (1957). (2) Burger, L. L., McClanahan, E., IKD. ENG.CHEM. 50, 153 (19581. (3) Burger, L. I ., Waqner, R . SI., Jones, B. R., General Electric Co., Richland, Wash., HW 44888. (4) Burr, J. G., Oak Ridge Natl. Lab.,
2046 (1955). (5) Burr, J. G., Radiation Rfsearch 8, 214--21 (1958).
(6) Goode, J. H., Sucleonits 15, N o . 2, 68-71 (19571. (71 Hipgins, C. E., Baldwin, W. H , Ruth, G. M., Oak Ridge S a t l . Lab., 1338 (1952). (81 Kinderman. C. hl., LVripht .Air Dcvelopment center, i i r RGsearch and Development Command, US.L\F, Tech. Rept. 57-465, 3 (July 5, 1957). (9) Swanson, J. S.,General Electric Co., Richland, Wash., HW 38262 (1955). (10) Taimuty, S. l . , Glass, R. A , , DeLa Rue, R., American Suclrar Society, Pittsburgh, Pa., June 1957, abstracts of meeting, p. 72. (11) Tolbert, B. M . ; Zebroski, E. I,., Wood, W. C . , Kindcrman, E. M., Univ. California Radiation Lab., 4.9.10 (1945). RECEIVED for rei-iew April 7, 1958 ACCEPTEDNovembcr 7, 1958
Division of Industrial and Engineering Chemistry, Symposium on Rrprocrssing Chemistry for Irradiated Fuel, Aqueous Methods, 133rd Meeting,- .ACS, San Francisco, Calif., April 1938. Work done under Contract W-7405-eng-26 with O a k Ridge National Laboratory.
