Separation of Mixtures of Tritium and Hydrogen Using Hertz Pumps FRANK J. DUNN, JOHN R. MOSLEY, and ROBERT M. POTTER Los Alamos Scientific Laboratory, University of California, Lor Alamos,
N. M.
Tritium of a purity in excess of 99.9% has been prepared from hydrogen-tritium mixtures by means of a 12-Hertz-pump system and a 16-pump system employing continuous gas flow. Satisfactory separations were accomplished at pressures greatly in excess of those described by other workers.
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H
YDROGEX-3, or tritium, has in recent years become available for experimental purposes. This paper describes a method suitable for obtaining isotopically pure tritium in quantities satisfactory for many laboratory purposes. Performance data pertinent t,o the operation of the two systems are given. In t,he analysis of tritium-hydrogen mixhres it was found that the equilibration of such mixtures, according to the equation
4-
~I
HBf Tn S OIIT
Figure 2.
posed a problem. This reaction proceeds at room temperature with a measurable velocity, and its course can be followed by the thermal conductivity method of analysis. Therefore, in order to obtain unambiguous analyses it was necessary to charge t,he analysis cells only with gas mixtures which had already undergone equilibration. This reaction probably occurs via a free-radical mechanism, induced by the radioactivity of tritium. I t is known that the analogous equilibration of hydrogen and deuterium can be accelerated by exposure to external radiation (3). Figure 1 illustrates the practical effect of this self-equilibration on analysis by thermal conductivity. For this figure, the self-equilibrated points were obtained by allowing a gas mixture to stand a t 35' C. until several successive thermal conductivity analyses became constant. This equilibrated mixture was reacted with cold, finely divided uranium, and then re-evolved by pumping at low pressure on the warm (approximately 175 ") hydride. Fractionation of the mixture was avoided by evolution and analysis of the entire gas sample. A change in the analyses after this treatment might be expected because of equilibration a t different temperatures, but none was observed, possibly because the time required for evolution was sufficiently long for self-equilibration by 8particle action to mask a small change. This self-equilibration also means that the separation of a hydrogen-tritium mixture containing little hydrogen is primarily a
r,i,o7 I
Hertz Pump Separation Line
separation of tritium and H T molecules, so that the differences in physical properties favorable to separation are reduced. Even so, the mass ratio of 6 to 4 is still considerably more favorable than one ordinarily encounters in isotope separations. Another difficulty attributable to the p-decay of tritium is that the helium-3 produced in uranium tritide is removed completely only with difficulty. For this reason, freshly prepared uranium tritide was used to minimize helium content. EXPERIMENTAL PROCEDURE
Apparatus. The diffusion pumps employed (Figure 2 ) were described by Sherr ( 4 , 5 ) , who modified Hertz's earlier pumps ( 2 ) . In the static separation line, 12 such pumps were connected in series, the first and last pumps being connected also t o bulbs of 400-ml. volume (later increased to 1 liter). Provision was made for the introduction and removal of gas a t either end and a t the center of the line. The initial charge for the line was obt,ained from uranium hydride storage furnaces and transferred by automatic Toepler pumps. The operation of these pumps was controlled by an RF oscillator circuit, in order to eliminate glass t o metal seals. Coils wrapped around the inlet and outlet arms of the Toepler pumps changed in inductance as mercury filled these arms, interrupting the oscillation and thereby operating the appropriate solenoid valves. In the event of a n accident, these controls would not provide the spark necessary to ignite a hydrogen-air mixture. When the line and end volumes contained gas a t the desired pressure, the pumps were started. After equilibration was efitablished, the end volumes Tere isolated from t,he rest of the line by stopcocks and their contents transferred to the analysis system, using Toepler pumps. The line vias then ready for refilling. In the 16-pump, continuous-flow system these interruptions were unnecessary. Feed gas was continuously supplied by a uranium furnace maintained a t a temperature sufficiently high to supply gas at t h e desired pressure. The end volumes were absent, having been replaced by ceramic variable leaks. The back pressure in these controlled leaks was maintained at 20 to 50 microns by Toepler pumps, which transferred the enriched or depleted gas into storage uranium furnaces. When mixtures richer than 95y0 tritium were being further purified, the light-end controlled leak x a s ordinarily closed entirely and feed gas was introduced a t this end. The lighter gases were simply allowed t o accumulate in the end pumps and in the feed uranium furnace for the duration of the run. Analysis. Two analytical procedures were employed in these experiments: thermal conductivity and mass spectrometry. For the thermal conductivity measurements, Leeds and Northrup thermal conductivity bridges, x i t h the cells modified to reduce their volume t o 10 ml., were used. An improvement in this apparatus was the addition of a photoelectric servo-mechanism, which made precise control of the bridge current possible ( 1 ) . With this control, annlyses were reproducible to 3~0.02%.
I 0 SELF-EQUILIBRATED BY (3-RADIATION
00441
ELECTRIC HEATERS
0 EQUILIBRATED BY REACTION WITH URANIUM
004 0 . 5 % IN COMPOSITION
0 041 70
80
90
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PRESSURE I m m H g l
Figure 1.
E.M.F.
us. Pressure with Fixed Composition of
Hydrogen-Tritium Showing pressure dependence of e.m.f. and apparent self-equilibration
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ANALYTICAL CHEMISTRY
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To test the behavior of t h e separation and analysis systems, several experiments were made with hydrogen-deuterium mixtures. As a result of these experiments, i t was believed t h a t if 90 to 95% tritium were cycled through the line several times, t h e heavy-end product would be essentially pure tritium. Using pure tritium obtained in this manner, mixtures of tritiumhydrogen of accurately known composition were prepared and used to calibrate t h e thermal conductivity cells. For convenience, curves of e.m.f. against composition and e.m.f. against pressure at fixed composition were constructed, so as to simplif . the interpolation. I n general, t h e pressures employed for ana?ysis were approximately 140 mm. Figures I and 3 are calibration curves constructed in this manner. The fact t h a t correct results were obtained by this procedure was subsequently indicated by comparison with mass spectrometer analyses (thermal conductivity 99.44 A 0.02% tritium, maas spectrometer 99.44 =!= 0.05% tritium). Another independent check was made with the cooperation of Louis Rosen, who measured proton-proton scattering of a sample using the Los Alamos cyclotron (thermal conductivity 98.60 & 0.02% tritium; proton-proton 98.8 & 0.2% tritium). The agreement among t h e three methods of analysis led to the belief t h a t in the thermal conductivity method interpolation was possible even a t t h e higher tritium concentration end of the curve. Since this is t h e case, t h e tritium whose thermal conductivity was unaffected b y subsequent reprocessing in the Hertz pump line was substantially pure. The mass spectrometer employed was a Consolidated-Sier Model 201. For these analyses, samples were withdrawn from the systems and transported to the mass spectrometer in sample bulbs fitted with greased pressure stopcocks. Although the authors’ experience indicated t h a t i t was advantageous to minimize the time during which highly purified tritium was exposed to stopcock grease (Apiezon type S),because of the radioactivity-induced exchange, this method of analysis was frequently used for samples obtained from the continuous-flow line. Because comparatively small gas samples were required, the operation of this line could then be more easily controlled. RESULTS
T h e literature dealing with Hertz pump separations has invariably stressed the necessity of operating such pumps at low pressures (2, 4,5 ) . This leads to low over-all separation rates, because the volume of gas obtained at the completion of a run is small. Consequently, once the lines were known t o perform satisfactorily at low pressures (5 to 8 mm. operating pressure), a series of experiments on the static line was undertaken to determine the highest pressure a t which a useful separation rate was
Table I.
Typical Separation Data for HydrogenDeuterium in Static Line
Table 11. Running Time, Hr. 2.5 2.5 5.0 12.0 47.5
Table 111. Pressure, Jlm. 9.8 8.3 8.4 5 3 8
9
Typical Separation Data for TritiumHydrogen in Static Line
(End volumes, each 1000 ml.) Tritium, ’?& Pressure, Heavy Starting Mm. end material 18.5 29.7 20.5 22.0 13.6
44 85
96.5 97 99.4
75 99 99.8 99.5 99.9f
Light end 10 41 65
h-ot removed Not removed
0.05
1 0
Initial gas
0,064 0.053 0.120 0.015 0.015 0.052 0.072
95.85 99.1 97.9 98.9 98.9 98.9 99.2
Tritium, ”% Heavy end 99.75 94.7 99.44 99.56 99.65 99.01 99.87
Light end Not removed h-ot removed 89 . 5 h-ot removed Not removed Not removed Not removed
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50
60
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100
%1
Figure 3.
E.M.F. rs. Tritium Concentration P
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140 mm. H g
t o be obtained. Surprisingly, it was found that a much highcr running pressure could be used with moderately longer running times. Subsequent static runs ITere therefore made at operating pressures of 20 to 28 mm., rvith running times of 2.5 to 3 hours. Runs could not be made at pressures above 35 mm. because enough heat could not be provided to obtain mercury vapor jets. (Operating pump pressures were those measured while the pumps were running. T h e difference between “operating” and “inivas greater than could tial”-Le., room temperature-pressures be attributed solely to temperature change of the gas. The discrepancy was attributed to the fact that, in operation, the stream of mercury vapor in the individual pumps efficiently expelled the gas above the liquid mercury surface, largely compressing it into the lines between the pumps.) T h e folloiving conditions were found to be satisfactory for the operation of the static line when 1-liter end bulbs were installed: Time, hours Operating pressure (measured in center of line), mm. Heat input (pumps insulated), watts/pump
2.5-3 20-25 300
Under these conditions, it was possible to obtain 25 ml. (standard temperature and pressure) of enriched tritium per run. The average rate of gas removal from the continuous-flow line \vas 150 ml. (standard temperature and pressure) per day, compared with 50 nil. per day with the manual line, even though the operating pressure was considerably lower in the former. This advantage results in part from the fact that the former was run continuously and from the relatively few interruptions necessary to recharge or remove gas from the uranium furnaces, as compared to removal from the glass end volumes of the static line. Tables I, 11, and I11 contain examples of the separations obtained in a number of runs under varying conditions of operation. Separation factors are not included in these tables because conditions were adjusted to obtain satisfactory separation rates. As a result, the separation factors vary. ACKYOWLEDGMENT
The authors wish to express their indebtedness to P. JT. Byington, of this laboratory, for the design and construction of the oscillator controls.
Separation Data for Tritium-Hydrogen in Continuous-Flow Line Flow Rate, hll /Min.
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LITERATLRE CITED (1) Dunn, F. J., Nann, J. B., and AIoiley. J. R.,
. ~ N % L .CEimf., in
nreqs
(2) H&-G., Z. Physiic, 91, 810 (1934). (3) Mund, W.. Lories, R., and Huyskens, P., Bull. SOC. chim. belges, 57, 469 (1948). (4) Sherr, R., J . Chem. Phys., 6 , 251 (1938). (5) Sherr, R., and Bleakney, W., Phys. Rev., 49, 882 (1936). RECEIVED for review M a y 4, 1954. Accepted October 13, 1954. Work performed under the auspices of the U. S. Atomic Energy Commission.
