1
B. M. ABRAHAM, H. E. FLOTOW, and R.
D.
CARLSON
Argonne National Laboratory, Lernont, 111.
Properties of Uranium Dioxide-Sodium-Potassium Alloy Slurry Fuels
AN of uranium dioxide (UO?) suspended in the eutectic alloy of IKVESTIGATION
sodium and potassium (KaK) was begun as part of a program to develop a fluid fuel for a fast reactor. No specific reactor application was under consideration; the motivation was to obtain basic information on suspensions that used liquid metal vehicles, so that the reactor engineer could make effective use of the design flexibility offered by slurry fuels. Wetting of the particulate matter by the vehicle is essential for the preparation of any slurry. I t is important, then, to know the contact angle, which is the quantitative measure of !vetting, between the liquid and the solid. All evidence so far indicates that the contact angle between UO?.oo and either sodium or N a K is greater than zero. As a result, one would expect finely divided UOZ to flocculate in S a K , a condition lvhich has been observed. Much information useful for characterization of a slurry can be obtained from the consistency curve, rate of shear us. shearing stress. With a rapidly settling suspension, as with UO?-A-aK, and with one that requires turbulent flo1.r.conditions to maintain homogeneity, it becomes difficult to obtain the true consistency curve; however, much information can be obtained from the pseudoconsistency curve-i.e , pressure drop us. velocity. A forced convection loop was constructed to determine the pseudoconsistency curves a t various temperatures, to establish the effect of prolonged pumping, heating, and metal additives.
the accuracy was to 0.1 mm. of mercury over the calibrated range of 50 mm. Density measurements could be made with a n accuracy to rt0.006 gram per ml. from 25' to 550' C. a t velocities u p to 2.13 meters per second. The difference between pressure drops in the vertical and horizontal tubes was directly proportional to the difference between the density of slurry and of K a K a t 2.5' c.: 300 ApV Apfl = - ( f $ - fPiaK)
-
PH%
where p i is the slurry density a t tem~ Kp~~ are the peratures t o C. and ~ N and densities of N a K and mercury, respectively, a t 25' C. T h e UOZ used for constituting the slurry was Mallinckrodt PW grade, which contained 22 p.p.m, of nitric acidinsoluble impurities, 17 p.p.m. of iron, and 9 p.p.m. of nickel. The total uranium assay was 88.0170 (stoichiometric UOz contains 88.15% uranium) and the O/U ratio was 2.04, which accounted for the low uranium analysis. T h e oxygen content was obtained by weight change on oxidizing UO2+, to US08 (5). T h e 'CTO2.o4 was ground in a ball mill with steel balls, using isopropyl alcohol as the lubricant. This introduced 3400 p.p.m. of iron into the UO?.ot, which was subsequently extracted by leaching with diethyl ether saturated with anhydrous hydrochloric acid. T h e powder was then washed with acetone. After this treatment the iron content was
reduced to 50 p.p.m. T h e UOz.od was vacuum dried and reduced with hytlrogen a t 500" C. to U02.01: the usual composition for the loop experiments. T h e particle size distribution, determined in Fvater (7), showed 90% less than 10 microns and 257c less than 2 microns. T h e U 0 ? . o l charge of 125.8 grams was sufficient to produce a 5.25 volume (40.8 weight %) slurry. which was calculated assuming a circulating volume of 222 ml. Of this charge 4.5 grams were irradiated in the Argonne CP-5 reactor. so that a scintillation counter could be used to monitor changes in slurry compositions by counting 7-rays from the activity induced in the UOz (2). The U 0 z . 0 1 charge was then added through the vacuum lock. -4fter the first series of measurements was made, it was ascertained from pressure measurements that the complete charge had not dropped into the loop; the slurry contained 4.29 volume C;:, U02.01. This suspension was heated to 550' for 2 hours to obtain data a t the higher temperature and the loop was cooled. T h e remainder of the charge was added by judicious tapping of the charging pot. T h e final composition was 5.09 volume yo U02.01. 122 grams; apparently 3.8 grams remained on the walls of the charging pot and in the valves. Pressure drop data were taken as a function of velocity for NaK: the 4.29 volume slurry, and the 5.09 volume slurry a t 50" and 550' C. No pro-
YQ
Experimental T h e loop, constructed of Type 347 stainless steel tubing 1.05 cm. in inside diameter, was approximately 54 cm. square \vith a circulating volume of 222 ml. Its essential feature was provision for making pressure measurements so that the viscous loss and the density of the circulating fluid could be determined. The pressure taps in the horizontal and vertical legs were spaced 30 cm. apart and M-ere h\-draulicallv identical up to velocities of 2.13 meters per second. T h e pressure sensors were belloivs that actuated the core of a linear differential transformer ( 6 ) . 'The pressure sensitivity was 0.05 mni. of mercury and
SUMP, LIQUID NoK LEVEL.
,HEATER' - c
The loop and pressure gages were the essential equipment features VOL. 51, NO. 2
FEBRUARY 1 9 5 9
189
vision was made for backflushing the pressure taps and they became plugged with slurry, so that the response was sluggish after 50 hours and completely absent after 100 hours. Operation of the loop was continued even though pressure measurements could not be made. Gross changes in slurry behavior were recorded by erratic or anomalous flowmeter readings and by changes in radioactivity emanating from various segments of the loop. After the data on 5.09 volume yo were taken, the velocity fell very gradually from 2.09 to 1.88 meters per second, while the loop was kept a t 550’ C. Tapping the loop brought the velocity u p to the original value and then it fell again. T h e effect became more pronounced with time, i n that the rate of drop and the extent were greater. This indicated that UOZ was collecting in some spot in the loop and restricting the flow, although the counter did not show a significant drop in the counting rate. T o counteract this behavior 2.1 grams of powdered uranium metal admixed with 10.1 grams of U 0 2 . 0 1 were added to the loop. For approximately 24 hours there was no drop in velocity and tapping the loop had little effect when operating a t GOO0 C. After the 24-hour period, again the velocity dropped slowly by about 10% when the loop was held a t 600’ C. A second addition of powdered uranium brought the total to 3.5 grams, but this had no effect; considering the difficulty of adding a small amount of powder, this uranium may not have dropped into the loop. T o test the hypothesis that sodium monoxide was responsible for the observed (2) flocculation, 0.7 gram of sodium peroxide was added to the loop. At 600’ C.: the flocculation increased so
that flow was almost stopped ; there were wide fluctuations in the velocity, which had a period equivalent to the transit time around the loop. T h e counting rate indicated little or no UOZ was circulating and scanning the loop with the counter showed most of the UOz collected on the pump inlet. When the temperature was lowered to 500’ C., the UOZwas completely suspended and the flowmeter behaved normally. After a third addition of 3.9 grams of uranium metal, the condition of the loop returned to that before addition of sodium peroxide, as shown by a normal velocity pattern. Before the loop experiment was discontinued, tellurium metal was added to the slurry to see if it would act as a wetting agent. Tellurium is miscible in all proportions with sodium (4) and forms NaZTe. When the loop was heated after addition of 14 grams of tellurium, the velocity remained steady u p to 500°, then began to fall, but in a very short time reversed direction, began to rise to a slightly higher value than that a t 500°, and remained steady u p to 650’. This was repeated several times before the loop runs were discontinued. Discussion T h e data plotted below bring out two interesting points.
Although a consistency curve can be interpreted only for the region of laminar flow which is below 0.1 meter per second for these curves, any reasonable extrapolation of the curves indicates that a large yield value exists for these slurries. T h e consistency of the 5.09 volume 7o slurry changed after only 5 hours of heating. T h e slurry became more flocculated, so that the fluidity approached
PSEUDO -SHEAR DIAGRAM PSEUDO-SHEAR DIAGRAM AT 50’C
AT 550 ‘C I
I NoK
-I
I NaK 2 4 2 9 % ( 2 h r a155O’Cl 3 5 0 9 % I 5 hr at 550°C)
2 4 2 9 v o I % ( G h r at550’C.)
3 509voI%(Zhr o t 5 5 0 ’ C )
I
I
i
6
A
i--0
. I I -1 1 . .
10
20
30
40 50 60 70 8 0 A P (rnrn H g / m l
90
100
0
I0
20
30
40
A P
50
EO
(FlTp.
’0 BO P3/m.J
90
100
A large yield value exists for these slurries and flocs did not break down even at highest rates of shear 1 90
INDUSTRIAL AND ENGINEERING CHEMISTRY
that of N a K and the flocs did not break down even a t the highest rates of shear. If the behavior exhibited in curve 3 of the figure ( B ) , were characteristic of u0z.00 and NaK, it should have shown UP before the 2-hour heating period. Sodium monoxide increases the flocculation of UOZin N a K to the extent that the powder can n o longer be dispersed. Addition of uranium metal reverses this effect, presumably by reaction with sodium monoxide to form UOZ. T h e following mechanism is proposed to explain the role of NazO and tellurium. Uranium dioxide has the fluorite structure; sodium monoxide and NazTe have the antifluorite structure. Experimental observations suggest that a n epitaxial relation exists between UOZ and sodium monoxide or NazTe, such that sodium atoms take u p oxygen positions in the lattice with either oxygen or tellurium oriented toward the NaK. With oxygen oriented toward the N a K the wetting becomes poorer and the dispersed phase flocculates; with tellurium, the wetting is improved and the UO1 is more easily dispersed. T h e oriented wetting agent concept was fruitful in suggesting tellurium as the first additive to try, but additional work is required to validate this hypothesis. References (1) Abraham, B. M., Flotow, H. E., Carlson, R. D., Anal. Chem. 29, 1058 (1957). (2) Abraham, B. M., Flotow, H. E., Carlson, R. D., Nuclear Sci. and Eng. 2, 501 (1957). (3) Abraham, B. M., Flotow, H. E., Carlson, R. D., unpublished. (4) Atomic Energy Comm. and Department of Navy, TID-5277 (1955). (5) Bright, N. F. H., Ripley, L. G., Rowland, J. F., Lake, R. H., Can. Dept. Mines and Tech. Surveys, Mines Branch MD-207 (1956). (6) Flotow, H. E., Abraham, B. M., Carlson, R. D., Reu. Sci. Znstr. 29, 869 (1958). (7) Green, H., “Industrial Rheology and Rheological Structures,” p. 74, Wiley, New York, 1949. (8) Kruyt, H. R., “Colloid Science,” vol. 1, 79-84, Elsevier, New York, 1952. (9) Livey, D. T., Murray, P., “Wetting Properties of Solid Oxides and Carbides by Liquid Metals,” Plansee Proc. 1955, pp. 375-404, ed. by F. Benesovsky, Pergamon Press, London, 1956. (10) Taylor, J. W., Ford, S. O., Atomic Energy Research Estab. (Gt. Brit.), AERE-M/R-l729 (1955). (11) Weyl, W. A,, “Structure and Properties of Solid Surfaces,” p. 164, ed. by R. Gomer and C. S. Smith, University of Chicago Press, Chicago, 1953. (12) Woodrow, J., Atomic Energy Research Estab. (Gt. Brit.), AERE-ED/M1 3 (1954). RECEIVED for review April 7, 1958 ACCEPTEDDecember 8, 1958 Division of Industrial and Engineering Chemistry, Symposium on Chemistry Reprocessing of Circulating Nuclear Reactor Fuels, 133rd Meeting, ACS, San Francisco, Calif., April 1958. Based on work performed under the auspices of the U. S. Atomic Energy Commission.
