Gamma Radiolysis
I
L. L. BURGER and E. D. McCLANAHAN Hanford Atomic Products Operation, General Electric Co., Richland, Wash.
F
Tributyl Phosphate and Its Diluent Systems
ONE
FACTOR WHICH makes tributyl phosphate one of the most important solvents for processing nuclear fuels is its chemical stability. However, degradation reactions give butyl acid phosphates which may react with metal ions being separated ( 2 ) and must, therefore, be kept a t low concentrations. Because radiation damage may lead to similar products, radiolysis, not only of pure tributyl phosphate but also its mixtures with various diluents, was examined in detail. Data on radiolysis for mixtures of tributyl phosphate in petroleum naphtha have been published by Cathers (4) and Goode ( 5 ) . They employed the extraction behavior of uranium and plutonium to evaluate decomposition and estimated a G value (molecules of tributyl phosphate changed per 100 electron volts absorbed) of 0.24 for the 30% tributyl phosphate solution used. These workers also detected a peroxide in the irradiated mixture. Burr (3), Jones (7), and Williams, Wilkinson, and Rigg (70) have also reported data on tributyl phosphate radiolysis. Nichols (8) has reviewed the radiolytic production of chloride ion when carbon tetrachloride is used as a diluent, but decomposition of tributyl phosphate was not included. This work is concerned with extending previous data and identifying other products of radiolysis.
Experimental Tributyl phosphate, purified by distillation a t 10 mm. of mercury pressure, had a refractive index, , :n of 1.42252. Reagent grade carbon tetrachloride was refluxed with alkaline permanganate, dried over Anhydrone, and fractionated with a 6-foot column packed with stainless steel helices. The refractive index of the product boiling a t 76.6' C. was ny 1.4572. The hydrocarbons were reagent grade iso-octane and benzene, and Soltrol170, a synthetic kerosinelike hydrocarbon, manufactured by Phillips Petroleum Co., consisting of 100% branched chain paraffins. T h e other compounds were reagent grade materials. Two irradiation sources were employed-a laboratory cobalt-60 unit having interchangeable 500- and 1300-
either of the radiation sources used here, curie sources was employed for most Compton scattering makes the only of the work. Uniform exposure of 15- to important contribution to the absorption 25-ml. samples was provided by a planecoefficients for elements of low atomic tary-gear turntable which rotated and moved the samples in a circular path numbers. Thus, with the Fricke dosimeter as the standard; energy absorbed around the cobalt. For irradiating large by any other solution is given by multivolumes, gamma radiation, from freshly plying by the ratio of the electron discharged fuel elements from the density of the solution to that of the doHanford reactors was employed. Aversimeter solution. T h e latter value is age gamma energy from the fuel elements was slightly lower than the 1.253.41 X los electrons per cubic centim.e.v. cobalt-60 gamma. The Fricke meter. A similar calculation may be dosimeter, lOP3M ferrous sulfate in 0.4M applied to a particular component in the sulfuric acid, was the primary standard, solution. Most of the, work was carried and a G value of 15.5 for iron(II1) forrnaout with energy rates, as measured by the tion was used (6). dosimeter, of 0.46 and 1.22 watts per liter. Acid phosphates were determined by The latter corresponds to 7.62 X 1016 titration with standard base, At low electron volt cc.+ see.-' or 4.7 X IO5 concentrations, dibutyl phosphate was r hr.+ The yields have been caldetermined by the method of Brite (7). culated on the basis of energy absorbed Butyl alcohol was determined by by the component rather than that oxidation with 0.02M dichromate in 3M absorbed by the total solution. Thus, sulfuric acid. With some samples, the for any component A , energy absorbed is (exposure rate, watts) (time, hr.)(electron-density ratio)(vol. % A ) E = 100
alcohol was first removed from solution by azeotropic distillation with n-heptane. This method actually gives total reducing power and no attempt was made specifically to determine butyl alcohol. Its presence in many samples was ascertained by its unmistakable odor. Chloride liberated by radiolysis was determined in the organic samples by extracting into an aqueous sodium hydroxide solution containing a small amount of hydrogen peroxide. The p H was adjusted to approximately 4 and the solution was titrated potentiometrically with 0.01M silver nitrate. Low chloride concentrations were determined turbidimetrically. For the photon energies involved in
and the grams of product per watt hour absorbed is Y =
grams produced E
The latter is related to the conventional G value by Y 2680 G = mol. wt. of product molecules per 100 electron volts absorbed All mixtures are reported in volume per cent.
Discussion
Qualitative Observations. Preliminary distillation of tributyl phosphate
Figure 1. Yield of dibutyl phosphate from tributyl phospha te-benzene
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Table 1.
Radiolysis of Pure and Diluted Tributyl Phosphate
[Dose, 250 watt hr./l. (-lo8 r ) , 1.25 m.e.v. gamma] DBP
Y,
g./watt hr.
TBP Dry Water ~ a t d . ~ With 50% butyl alcohol Dry, with 70% iso-octane Water-satd. with 70% isooctane a
MBP Butyl Alcohol Y, Y, G, G, molecules/ g./watt molecules/ g./watt molecules/ 100 e.v. hr. 100 e.v. hr. 100 e.v. G,
0.14 0.091 0.093 0.15
1.8 1.2 1.2 1.9
0.02 0.02 0.01 0.03
0.3 0.3 0.2 0.5
0.11
1.4
0.03
0.5
Saturation results in a mole ratio of H*O/TBP
Table II.
N
0.02 0.01
0.7 0.4
0.02
0.7
...
...
...
...
1.
Radiolysis of Tributyl Phosphate and Its Mixtures (Dry systems, 0.6-1.0 m.e.v. gamma)
-DBP“
Butyl AlcoholQ G,
Y, Y, moleDose, g./watt cules/ g./watt R hr. 100 e.v. hr.
TBP Pure
104
106 107 108 30% in iso-octane
IO4
105 106 10’ 108
Soyo in Soltrol-170
104
105 106 lo7 IO8
30% in carbon tetrachloride
IO4
106 108
IO’ 108 a
Chlorideb G, moleY, molecules/ g./watt cules/ 100 e.v. hr. 100 e.v. G,
...
...
0.65 0.13 0.13 0.13
7 1.7 1.7 1.7
0.014 0.016 0.02
0.5 0.5 0.7
1.05 0.27 0.17 0.17 0.17
13 3.4 2.3 1.8 2.3
0.4 0.04 0.02 0.02
14 1.4 0.7 0.7
0.8 0.24 0.13 0.16 0.16
IO 3.1 1.7 2.0 2.2
0.4 0.05 0.03 0.03
14 1.8 1.0 1.0
4.1 1.3 0.78 2.3 1.1
52 17 10 31 14
...
... 14
...
I..
...
e . .
0.4 0.04 0.01 0.001
1.4 0.4 0.04
... ... ... ...
...
... ... I . .
...
..,
... ... ... ... ...
... ... ...
.. ...
...
... ... ...
..
..,
...
... 0.39 0.14 0.088 0.068 0.041
29 11 6.6 5.1 3.1
Yields based on energy absorbed by tributyl phosphate. Yields based on energy absorbed by carbon tetrachloride.
Table 111.
Effect of Tributyl Phosphate Concentration in Diluent Mixtures (Dry TBP) Chloride Yield
TBP, VOl. % 0
10 30 75 99 100
G./l.
DBP Yield y,
g./watt hr.
G./L
In cc14; Dose, 1.0 X 10’ R ; 0.6 to 1.0 M.e.v. Gamma 2.46 0.069 2.57 0.080 4.4 1.60 0.064 7.6 0.68 0.071 6.8 0.41 1.15 5.6 3.8
...
...
...
Y,
g./watt hr.
...
1.8 1.0 0.37 0.23 0.15
In Benzene; Dose, 8.7 X 107 R ; 1.25 h1.e.v. Gamma 2 10 20 30 50 80 90 96 99
1 54
INDUSTRIAL AND ENGINEERING CHEMISTRY
0.10 0.62 1.56 2.60 5.75 15.5 22.7 27.7 31.7
0.025 0.029 0.036 0.041 0.054 0.091 0.12 0.14 0.15
given an exposure of 250 watt hours per liter (about 108 r) gave evidence of butyl alcohol and ether, di- and monobutyl phosphate, and a high molecular weight polymer. Distillation was carried out at 21-mm. pressure to avoid thermal decomposition. A second sample was distilled at less than 1-mm. pressure and yielded the same products. Infrared absorption curves assisted in identifying the butyl ether. Phosphoric acid was not found in any samples given an exposure up to 250 watt hours per liter. Tributyl phosphate, severely degraded a t 3000 watt hours per liter, gave a sirupy mass from which phosphoric acid was extracted. No peroxides were detected whether or not air was present. Thus, the peroxide reported by Cathers (4) probably came from the diluent used. Gas production rate was measured by water displacement and the gases were analyzed by mass spectrometry. Approximate yields expressed as G values were, hydrogen 2.5, butane plus butene 0.08 (total), propane 0.05, ethane 0.02, and methanr 0.05. Because of solubility of the hydrocarbon gases in the tributyl phosphate, only the values for hydrogen and perhaps methane can be considered realistic. Gas, pumped from the residual tributyl phosphate, was 50% butane. A more thorough investigation of gas production from the irradiation of tributyl phosphate has been carried out by Burr a t the Oak Ridge National Laboratories ( 3 ) . Dibutyl Phosphate Yield from Pure Tributyl Phosphate. Interest was centered on liquid products of the radiolysis, especially dibutyl phosphate which is formed in much larger amounts than any other compound (Tables I and 11). The analysis of monobutyl phosphate i s difficult in these mixtures. However, a t an exposure of 250 watt hours per liter, the titration curve with sodium hydroxide showed a second inflection typical of monobutyl phosphate and this. curve was compared with that for synthetic dibutyl phosphate-monobutyr phosphate mixtures. The curves were identical except for a slight additional buffer effect a t a pH of about 4 in the irradiated samples. Difficulty in locating the titration end points makes the individual yields of dibutyl phosphate and monobutyl phosphate more uncertain than the total yield of both compounds. G values for the latter quantity are 2.2 and 1.5 for dry and wet tributyl phosphate, respectively. The decreased dibutyl phosphate yield on adding water to tributyl phosphate was unexpected. T o test the hypothesis that hydrogen bonds may be involved, butyl alcohol was used as a diluent. The results (Table I) indicate yields similar to those for the watersaturated solution. The low energy associated with hydrogen bonding makes
NUCLEAR TECHNOLOGY protective action from this questionable. These solutions were not thoroughly examined for other radiation products. Dibutyl Phosphate Yield from Tributyl Phosphate Diluent Systems. Dilution of dry tributyl phosphate with iso-octane to concentrations of 75, 40, 30, and 10% tributyl phosphate did not significantly change the dibutyl phosphate yield. Likewise, the water-saturated solutions gave results similar to pure wet tributyl phosphate. Substitution of other paraffin hydrocarbons -e.g., Soltrol-170-for iso-octane did not change the results. * By contrast, use of benzene as a diluent gave a lower yield of dibutyl phosphate. The data from Table I11 are shown in Figure 1. As benzene is added the yield drops rapidly to about one sixth that of pure tributyl phosphate. The stability and protective action of a benzene ring as part of the molecule being irradiated is well known in radiation chemistry. Merely employing the benzene as a diluent is also effective. The mole fraction plot is linear, suggesting that the protective action is proportional to the number of molecules of benzene around the tributyl phosphate molecule rather than to the mass fraction. Carbon tetrachloride as a diluent gave higher dibutyl phosphate yields which increase as the concentration of carbon tetrachloride increases (Table I11 and Figure 2). Phosgene and chlorine, two products of radiolysis for carbon tetrachloride, could be responsible for the foi-mation of dibutyl phosphate. However, experiments where phosgene was added to a tributyl phosphate-carbon tetrachloride system produced hydrolysis a t a rate which would only account for about 10% of the increased radiolysis yield. L'se of chlorine in a similar manner produced an even slower dibutyl phosphate formation. The yield of dibutyl phosphate is based on the energy absorbed by the tributyl phosphate. This does not account for the energy imparted to its molecules from hot radicals produced from irradiated carbon tetrachloride which has a high free radical yield and consequently is able to transfer gamma energy absorbed through collisions of these free radicals to other molecules. Hence the strong dependence of the dibutyl phosphate yield on carbon tetrachloride concentration. The fact that the dibutyl phosphate yield in a tributyl phosphatehydrocarbon system does not vary with the diluent concentration must indicate that the free radical yields for iso-octane and Soltrol-170 are low and comparable with that of tributyl phosphate. Table IV gives some of the free radical yields obtained by Prevost-BCrnas (9). Several workers have reported yields of dibutyl phosphate from both pure
tributyl phosphate and that in kerosine solutions. The G value for dibutyl phosphate formation given by Cathers (4) and by Goode ( 5 ) , if calculated on the basis of energy absorbed only by the tributyl phosphate, would be 0.80considerably lower than that given in Table I. Burr's (3) results from 1.66m.e.v. electron bombardment were 2.78 and 2.44 for dibutyl phosphate and 0.065 and 0.14 for monobutyl phosphate a t doses of about 3 X l o 7 and 6 X 107 r, respectively. T h e dose rate was about 400 times that used in the present study. Williams, Wilkinson, and Rigg (70)recently reported G values of 1.5 for dibutyl phosphate and 0.17 for monophosphate. Like Burr, they used electron bombardment but a t a slightly lower dose rate. Both of the latter studies can be assumed to be on pure dry tributyl phosphate and thus can be compared with the G values of 1.8 for dibutyl and 0.3 for monobutyl phosphate of Table I. Jones (7) reported a G value of 0.62 for dibutyl phosphate formation from cobalt-60 gamma radiolysis of 30y0 tributyl phosphate. O n the basis of the energy absorbed by the tributyl phosphate alone, this is equivalent to 2.07. T h e analysis was based on uranium
Table V.
Table
IV.
Free Radical Yields from Gamma Radiolysis (9) (Radium source) No. Free
Radicals Produced/ Compound Carbon disulfide Benzene Toluene Ethylbenzene Nitrobenzene n-Heptane n-Octane Cyclohexane Methanol Ether Ethyl acetate Chlorobenzene o-Dichlorobenzene Ethyl bromide Chloroform Carbon tetrachloride
100 E.V. 0.85 1.8 3.1 9.0 4.5 9.9 11.4 14.3 24.0 24.5 32.0
17.5 30.0 28.0
57.5 70.0
extraction, and the method probably determines both monobutyl and dibutyl phosphates. The lack of agreement among several of the above investigations may result partly from difficulties in analysis, particularly for monobutyl phosphate, and partly from dosimetry errors. The latter should be a minimum for radiation
Effect of Dissolved Air and Water on Chloride Yield (Irradiated Ccb, 0.6-1.0 m.e.v. gamma) Chloride Y , g./watt hr. G./l. absorbed by cc14 Dose, R
Air satd.
0.064 1.0 2.5 4.1 0.49 0.78 3.3
0.18 0.28 0.069 0.11 0.20 0.027 0.011
107 107 107
1.6 1.7 1.7
0.064 0.069 0.069
107 106
l.Ja 1.7
0.064 0.086
105 106 107
Water satd. Degassed
107 6.8 X 106 7.8 X 106 8.6 X 107
With 30% TBP Air satd. Water satd. Degassed With 40% TBP Air satd. Degassed 9.4 Interpolated from Table 111.
x
Table VI. Chloride and Dibutyl Phosphate Yields (In two-phase and single-phase systems irradiated with cobalt-60 gamma rays) Yield, G./Watt Hr. DBP System" Dose, R c1DBP initial after 3 daysb Two-phase, 30% TBP in CCt aqueous UOa(NOs)2 104 0.27 1.0 1.3 2
Single-phase, 30% TBP in CCl4 equilibrated with aqueous UOz(NO&
x
106
0.36
104 0.24 2 x 105 0.35 4 x 106 0.33 Aqueous phase consisted of 0.7M UOs(N0s)z and 2M "0s. Corrected for chemical hydrolysis (-0.004 gram DBP per liter
After two days.
VOL. 50, NO. 2
0.3
0.9c
0.8 0.24
0.8 0.9c 0.6c
0.15
per day).
FEBRUARY 1958
155
2 .o
2.0
a m 1.0
’0 .L 0 2 1.0 u
al
9 >
i
0
IO0
50 Vol.
O/o
0
100
50
TBP
Vol.
O/o
TBP
Figure 2. Yields of dibutyl phosphate and chloride from tributyl phosphatecarbon tetrach1oride“solutions sources employing cobalt-60 where the calibration using the Fricke dosimeter has been well standardized. Chloride Yield. Chloride production was measured as a function of carbon tetrachloride concentration in its mixtures with tributyl phosphate. Chloride yield was constant from 25 to 100% carbon tetrachloride (Table HI), but very high for 1% (Figure 2). The calculation does not take into account energy imparted to the carbon tetrachloride by the excited tributyl phosphate molecules. This may be appreciable at low concentrations of carbon tetrachloride. Yields of chloride from both pure carbon tetrachloride and that containing 30% tributyl phosphate agree with those compiled by Nichols (8). For the pure carbon tetrachloride system the yield was increased when water or oxygen was present. As seen from Table V, in the tribut)] phosphate-carbon tetrachloride system the presence of dissolved oxygen or water did not affect the chloride yield a t a dose of 10’ r. The data of Table VI1 suggest that for low doses, if there is any effect from water saturation in the tributyl phosphate-carbon tetrachloride system, it is obscured by analytical uncertainties. Effect of Uranium and Nitric Acid. The presence of uranium and nitric acid reduces dibutyl phosphate yields (Table
VI) more than the similar effect from adding water to tributyl phosphate. The gas produced in tributyl phosphate saturated with uranyl nitrate was about 30% less than for pure tributyl phosphate. The chloride yields are slightly higher in the presence of uranium and nitric acid. Postirradiation Effect. A postirradiation effect was found in the formation of dibutyl phosphate in tributyl phosphate-carbon tetrachloride solutions with or without uranium, nitric acid, and water present. Dibutyl phosphate continues to form after the irradiation is stopped and may double its concentration in a few days (Tables V I and VII). Ordinary chemical hydrolysis occurs at a much lower rate ( 2 ) . The possibility that photolysis was responsible was excluded by protecting the samples from the laboratory light. Thus, dibutyl phosphate which appears after irradiation must have been produced from an intermediate formed during radiolysis. No postirradiation effect was found in the formation of chloride ion. Effect of Dose a n d Dose Rate. An increase in irradiation is generally accompanied by a decrease in yield. The largest effect is at the low irradiation levels, about lo4 r. Unfortunately, in this region the analytical results are not of sufficient accuracy to permit a quanti-
Table VII.
Postirradiation Effect CCla, cobalt-60 gamma) Immediate Analysis Analysis after 2 Days DBP c1DBP c1Y. 1’. Y, Y. &/watt g.,fwatt g./watt g./watt G./l. hr. G./l. hr. G./l. hr. G./I. hr. Dry 0.0055 0.2 0.011 1.6 0.8 0.005 0.004 0.2 0.24 0.10 1.5 0.055 0.78 0.056 0.051 0.22 0.15 0.38 0.32 0.65 0.24 0.78 0.30 0.19 (30% TBP in
Dose, R 9 . 4 X IO* 9.4 X IO*
6.6 X 10’ 9 . 4 X IO4 6.6 X 106
156
0.068
0.97
0.30
0.61
Water Saturated 0.026 0.11 0.21 0.13
INDUSTRIAL AND ENGINEERING CHEMISTRY
0.11 0.45
1.6 0.92
0.050 0.27
0.21 0.16
tative evaluation of this trend. ,It n radiation level of 106 to 108 r, thc yields are independent of the dose. For pure tributyl phosphate, a factor of 2 in the dose rate did not significantly change the yields. However, dose rate was not studied as a variable. I t might be a more significant factor in systems where the free radical yields are highere g . , solutions containing carbon tetrachloride. Application to Fuel Processing. In terms of total radiation damage, tributyl phosphate can be considered a normal organic compound. One compound, dibutyl phosphate, is produced in considerably higher yield than any otlicr. Although this compound is an objectionable impurity in tributyl phosphate used for solvent extraction of plutonium or uranium, the amount of radiation received in most processing applications is less than 0.1 watt hour per liter. The small amount of dibutyl phosphate formed under such conditions is not objectionable. A concentration buildup can be prevented by periodically washing the solvent with an alkaline solution. T h e limitations imposed by higher radiation exposures have been discussed by Cathers (4) and Goode (5).
Literature Cited W., “Determination of Dibutyl Phosphate,” Hanfoord Atomic Products Operation, General Electric Co., HW-30643, 1954. (2) Burger, L. L., “Progress in Nuclear Energy,” Pergamon Press, London, 1958. ( 3 ) Burr, J. G., Jr., “Semiannual Progr. Rept. for Period Ending Dec. 20, 1955,” Union Carbide Il’uclear Co., Oak Ridge National 1,aboratory, ORNL-2046, p. 71 (confidential). (4) Cathers, G. I., “Progress in N L ~ ~ C X Energy,” vol. I, Ser. 111, p. 68, McGraw-Hill, New York, 19.56. (5) Goode, J. H., Nucleonics 15, No. 2, 68 11957). ( 6 ) Haybittle, ‘J. L., Saunden, K. D , Swallow, A. J., J . Chem Phjbs. 25,
(1) Brite, D.
1211 .- ( 4 9 % ) (7) JonesI, S. S., “Rept. of Chemistrv and \ - - - - I -
Chemical Eng‘lneering Section for November. December 1955, Jnnuary 1956,’’ General Electric Co., Knolls Atomic Power Laboratory, KAPL-1491, p. 30, 1956 (secret). (8) Nichols, G. Starr, “Radiation Dainapc to Carbon Tetrachloride,” Savannah River Laboratory, E. I. du Pont de Nemours & Co., DPC-294, Dec. 16, 1952 (secret). ( 9 ) Prevobt-BCrnas, k., Chapiro, h., Cousin, C . , Landler, Y., Margot, M., Discussions Faraday Soc. 12, 98-1 32 (1 952). (10) Williams, T. F., Wilkinson, K. LV., Rigg, T., Nature 179, 540 (1957). RECEIVED for review Piovember 7, 1957 ACCEPTEDDecemhrr 9, 1957 Division of Industrial and Enginccring Chemistry, Symposium on Nuclear ’Iechnology in the Petroleum and Chcmicaf Industries, 131st Meeting, ACS, Miami, Fla., April 1957.
