I
H. T. HA"' Hanford Laboratories Operation, General Electric Co., Richland, Wash.
Uranium Oxide-Liquid Metal Slurries T
IN POWER REACTOR operation use of a molten metal as supporting medium for a fissionable material eliminates expensive fuel cladding, reduces decontamination requirements, permits continual withdrawal for chemical processing, and provides freedom from radiolytic decomposition. The liquid metal must have a relatively low melting point, fairly wide liquid range, and low neutron cross section. Because stability of the slurry varies inversely with differences in density benveen its constituents, the metal should have a density similar to that of the suspended material. Bismuth and lead, \vhich have densities of about 10 grams per cc., are promising. The molten metal may support the fissionable material in solution or as a slurry containing a suspended intermetallic or oxide. Solubilities of the fissionable metals are limited at practical temperatures, yet high enough to modify intermetallic particle size by solution and recrystallization. Oxides offer a wider temperature range of particle size stability. However, fissionable oxides may not be wetted by the liquid metal at reasonable temperatures. Whether wetting occurs is determined by the balance of forces at the interface formed by the solid and liquid. This relationship is described mathematically by ;SA
= ysL
+
yL cos
e
(1)
in which the surface tensions are those of the solid, the solid-liquid interface, and the liquid, respectively; contact angle 0 is measured through the liquid. When 0 is 90' C. or less, the surface is said to be wetted. Wetting is favored by a high solid surface tension, low liquid and interfacial tensions, or combinations. To attain maximum solid surface tension, the surface must be free of adsorbed atoms. The contact angle of bismuth on uranium dioxide decreases from 138.5' at 380' C. to 92' at 1242' C. in argon ( 4 ) . The wetting temperature is therefore greater than 1250' C. Similar angles were found for lead on uranium dioxide. Initial attempts to wet uranium oxide with bismuth failed. However, in 1956 experimenters a t the Knolls Atomic Present address, Phillips Petroleum Co.,
Idaho Falls, Idaho.
P o ~ e r Laboratory dispersed uranium oxide by adding various metals as oxygen getters (7), presumably raising the oxide surface energy. Although no uranium oxide was visible at the surface of the product in successful experiments, patches were noted in the interior. Magnesium. sodium, titanium, and uranium (as hydride) \\ere satisfactory; lithium and tin v ere not. Experimental
Methods and Materials. Two methods of slurry preparation were used successfully, magnesium gettering and in xitu preparation from uranium and bismuth sesquioxide. I n the former sufficient magnesium was added to the uranium oxide charge to reduce the oxygen-uranium ratio to 1.6, assuming that all the magnesium was oxidized. Preparation temperature was usually 700' C.; higher temperatures were used for comparison to the in situ method. The in situ method is based upon the reaction U
+
2/3
Bi203
+
UOz
+
"3
Bi;
AF = -150 kcal.
(2)
Above 840' C. the bismuth oxide is molten. The solubility of uranium in bismuth at this temperature is about 10 weight 7 0 , so that a completely liquid reaction path is possible. Materials were prepared as shown in Table I. ~~
Table I. Material
U UOS Bi203 Bi, Mg
Preparation of Materials Preparation Degreased, cleanedin8S "03, washed in chilled water and acetone, dried in air HZreduction of UOa; sieved to < 140 microns with mean particle size of 3 microns Oxidation of Bi in air stream Reagent grade
Slurries were produced in capsules machined from 304-L stainless steel? 2 X 1 to 5 X 2I/s inch in inner diameter. Capsule contents were added in air, no attempt being made to blanket the operation with inert gas except when the lids were welded into place. Capsules were heated either in a muffle furnace with rocker agitation or in a 9600-cycle-per-second Tocco induction unit with manual or rocker agitation.
A specially designed rocker was used with the Tocco unit ( 3 ) . T o avoid sectioning of each capsule and tedious analyses, a gamma absorptiometer was constructed for nondestructive examination of the slurries. I t consisted of a thulium-170 source, an automatic traversing platform, a 1/16 X 3/8 inch collimating slit in a lead block, and a sodium iodide scintillation crystal with associated electronic equipment. The instrument was capable of determining the uraniumcontentof a uraniumbismuth capsule within 1% absolute. Measurements on uranium oxide systems were invalidai.ed by gas pockets in the slurry. The experiments at the Knolls Laboratory were confirmed in these laboratories at higher uranium concentrations. A 50-gram batch of slurry containing 13.2 iveight 7 0 uranium as dioxide, plus sufficient magnesium to reduce the oxygen to uranium ratio to 1.6, was capsulated and heated 3 hours at 700' C. with rocking. The capsule was air-cooled and cut open. The oxide had dispersed within the melt. The casting was trisected and found to contain 13.3, 12.1, and 14.0 weight 7 0 uranium in descending order. (Uranium concentrations are henceforth expressed parenthetically as weight per cent in descending order.) In an identical experiment: save for replacement of bismuth with the bismuth-45.5% lead eutectic composition, analysis confirmed (15.2, 12.7, 11.7) that the oxide could be dispersed by this technique and that the suspension was reasonably stable. Gallium Experiments. To facilitate observation and measuremen:? a dispersion which is unreactive to\vard glass is desirable, but free uranium is undesirable in the gettered product. Gallium will not attack glass, but \vi11 remove oxygen from bismuth and reduce higher oxides of uranium to the dioxide. It is low-melting and mobile. However, a charge of uranium dioxide in bismuth plus gallium was unwet when heated 3 hours at 1100' C.: 16 hours at 900' C.? and 8 hours at 1100" to 1175' C. with periodic oscillation. In a second attempt a sealed \-).cor tube, previously evacuated to 3 microns at 500' C., was heated to 850' C. for 6 days with periodic shaking by rods attached to the tube ends. The upper portion of the casting contained high concentrations of uranium-up to 26 weight yO-and a spotty distribution of VOL. 51, NO. 2
0
FEBRUARY 1959
197
segregated oxide partially coated with gallium. I t was concluded that uranium dioxide itself is not wet by bismuth a t these temperatures. In Situ Method. The desire to prepare a dispersion free of additives with tailored oxygen to uranium ratio suggested a n in situ preparation. A charge Qf uranium, bismuth oxide, and bismuth was capsulated and heated (Table 11, No. 1). When the capsule was opened, the bismuth oxide had disappeared and no uranium was found in the original form. Finely dispersed uranium oxide was visible as red particles during micro-
scopic examination under polarized light. I n other experiments in which the oxygen-uranium ratio was close to 2 (Table 11, No. 2 ) , a considerable amount of unwetted oxide was found at the top and throughout the upper portions of the casting. The amount of oxygen present in the free space of the capsule increased the oxygen-uranium ratio to less than 1.97. Because lack of wetting was not due to ' oxygen gain, either some uranium was removed by combination with the capsule, or oxides with oxygen-uranium ratios close to 2 cannot be wet at these temperatures. I n experiments in which the original ratio was 1.95 an essentially unwet powder was found on the surface of the product; in no experiment in which the ratio was 1.67 or less has the unwet powder occurred. As no magnesium is present in the in situ preparations, and uranium dioxide is not wet a t these temperatures, it is concluded that in this case wetting is produced by reduction of the oxide-uranium ratio. Magnesium Gettering. As gettering of excess oxygen is not solely responsible for the wetting obtained with magnesium, alternatives were sought. From thermodynamic data ( 2 ) uranium monoxide is more stable than the dioxide in the range 300' to 800' C. Because of its oxygen deficiency the surface energy of the monoxide is probably higher than the dioxide, a condition which would promote wetting. However, uranium monoxide has not been identified in bulk. Surface films have yielded x-ray diffraction data, but these lines were not found in diffraction patterns of the dispersion. Although no evidence exists of free reduced forms of uranium, magnesia may exist as a coating or bridge between the oxide particles and bismuth-e.g., Bi..Mg-O..U-0. This mechanism received support from later experiments ( 3 ) , in which wetting did not occur with a higher oxide (U308), but did occur with addition of magnesium to reduce the oxygen-uranium ratio to 2.0. Stability of Slurries. To estimate the relative stabilities of slurries prepared by both methods, capsules containing 8 Table II.
198
Charge, Grams Biz03 Mg
u
1 2
30.6 9.40
... ...
20.0 12.0
3
...
21.6
4 5
18.8 47.0
6
...
O/U = 1.6 INDUSTRIAL AND ENGINEERING CHEMISTRY
weight % uranium (Table 11, 3 and 4) were subjected to essentially identical treatment. T h e .regular heating period was followed by 30 minutes at 600" C. with intermittent shaking. After dispersion, the capsule was allowed to stand 10 minutes without movement. The furnace was then carefully removed and the capsule quenched by a directed stream of water. The magnesium gettered casting (Figure 1) showed no oxide powder a t the surface or in the interior. This particular capsule showed a clean separation o f ' slurry and bismuth inch from the base. Chemical analysis (10.5, 11.5, 11.0, 10.1, 5.2, 0.0; weighted average 7.9y0) confirmed that the slurry segregated upward-i.e., the effective density of the oxide was less than that of the bismuth. I n Figure 2 the large "balloons" are gas pockets. Microscopic inspection revealed many small uranium dioxide particles clustered about the periphery. The region below the interface was completely free of uranium dioxide. The casting from the in situ preparation (Table 11, No. 4) revealed no region
-
Slurry Preparations W e r e Selected to Illustrate Points O/U
KO.
Figure 1 . A magnesium-gettered uranium oxide-bismuth slurry showed no oxide powder
Figure 2. A micrograph of Figure 1 casting shows dispersion in segregated region (50X)
UOZ
c, %
... . ..
Bi 182.5 213.5
Wt. 14 4
...
0.78
213.0
8
... ...
20.5
51.1
... ...
195.7 136.9
8 20
53.3
...
1.916
179.8
20
~~l~ Ratio
1 1.96
Heating Condition. Hours c.
8 840 1.5 950-1170 2.0 1170-1220 0.5 600-700 1.60 1 875-930 1 1000-1090 ' 1 1090.-1140 As in No. 3 1.67 20 900 1.67 1 1000-1200 1.60 2 850 22 900 72 600
NUCLEAR TECHNOLOGY of uranium freedom after 10 minutes a t 600" C. (15.4, 10.3, 5.2, 5.3, 4.9, 5.3; weighted average 7.4%), and no unwetted powder. I t was clear that the effective density of the oxide was less than the bismuth. The stability may be expected to increase with uranium concentration, because of higher viscosity-e.g., a n l 1,570 in situ preparation with an oxygenuranium ratio of 1.67 was heated at 700" to 900' C. for 32 hours with rocker agitation and 1 hour at 1200' C. with manual shaking. The capsule was allowed to cool without movement. It is estimated that the contents were fluid for 15 minutes. Upon sectioning, no unwet oxide \vas found and analysis showed relatively little segregation (12.2, 12.5, 11.3, 11.2, 10.6; weighted average 11.6%). Figure 3 is a micrograph of upper and lower segments of this casting. The oxide particles are 3 to 4 microns. T h e kidney-shaped inclusions have a diamond hardness of 303 under 50-gram load. The corresponding value for uranium is 200 and for uranium ferride (UeFe) is 319. It is probable that the particles are uranium ferride, a product of corrosion. The lowest segment shows uniformity of dispersion. To identify the appearance of uranium bismuthide, a casting was prepared containing 11.77, uranium in bismuth (Figure 4). The large crystals have a hardness of 58 and do not resemble any formations in the oxide systems. The largest uranium concentration achieved in a slurry fluid a t 600" C . has been 20 Lveight 7 ' (Table 11, 5 and 6). After a quiescent period of 92 hours at 600' C. the slurries showed surprisingly little segregation-e.g., 22.2, 25.5, 22.0, 21.1, 19.3, 10.9; weighted average 19.9%. Though still mobile, these slurries apparently are slushlike. Sodium Addition. The upward segregation of oxide apparently results from incomplete wetting-i.e., 0" < 0 < 90'. One method of countering this effect is to add a light metal to the suspending medium, to decrease its density. Addition of up to 3 weight 70 of sodium lowers the melting point of bismuth (5). Sodium has also proved effective as a wetting agent ( 7 ) . Addition of 1.76 grams of sodium to a previously described composition (Table 11, No. 3) increased stability markedly after 10 minutes' quiescence at 600' C. (7.7, 7.3, 7.8, 8.4, 8.3, 7.8 us. 10.5, 11.5, 11.0, 10.1, 5.2; 0.07'). Use of sodium as a stabilizing agent therefore looks promising ( 3 ) . Fluidity. Viscosity determinations are complicated by the presence of rising solid particles, high temperature, reactivity toivard container materials, and anaerobic requirements. ,4 work-
Figure 3. In situ preparation of uranium oxide in bismuth shows uniformity of dispersion (250X). Upper segment at left, lower at right
able viscometer has not been developed. Encouraging results have been obtained with a variable-speed solid cylindrical graphite rotor with tapered base rotating in a graphite pot. The assembly is set in a vertical tube furnace enclosed in a helium-swept glove box. Direct comparison with substances of known viscosity should reduce systematic errors. Comparison at 400" C. of the 8% in situ preparation with bismuth indicated a viscosity about 1.5 times greater or approximately 21 mp. Observation of the 11.5 weight 7 0 uranium slurry in a porcelain crucible under IO-* mm. of mercury pressure confirmed the fluidity of the system. Gentle agitation easily redispersed any oxide which appeared at the surface of the melt. Fluidity is felt during manual shak-
ing of the capsules. Occasionally above 1000" C. movement ceases, but fluidity returns below red heat. Conclusions
Dispersions of up to 20 weight (ro uranium as oxide in bismuth are fluid at 600" C. and may be prepared by the magnesium-gettering or in situ procedure. An oxide-bismuth slurry may be prepared by partial reduction of Us08 with magnesium. Wetting is apparently not complete, as evidenced by upward segregation of the solids. The mechanism of wetting varies. I n the gettered slurry wetting apparently occurs as a result of magnesia coating or bridging; in the in situ preparation it results from reduction of the oxygen-uranium ratio below 2. The stability of the slurry increases with increasing oxide concentration due to higher viscosity. A promising method of stabilizing low concentration slurries involves addition of a light metal, such as sodium, to reduce liquid metal density. literature Cited
(1) Davidson, .J. K., others, U. S. Atomic Energy Comm., Rept. KAPL-1686 (1956). (2)'-GlaSsner, A , , Ibid., ANL-5107 (1953). (3) Hahn, H. T., Ibid., HW-57161 (1958). (4) Livey, D. T., Murray, P., Atomic Energy Research Estab. (Gt. Britain), AERE-M-R-1746 (19551. (5) "Metals Reference Book," Interscience, New York, 1049. RECEIVED for review April 7, 1958
ACCEPTED November IO, 1358
Figure 4. Uranium bismuthide crystals d o not resemble oxide formations
Division of Industrial and Engineering Chemistry, Symposium on Chemistry and Reprocessing of Circulating h-uclear Reactor Fuels, 133rd Meeting, ACS, San Francisco, Calif., April 1958. VOL. 51, NO. 2
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