1 I
D. A. VAUGHAN, J. R. BRIDGE, A. G. ALLISON, and C. M. SCHWARTZ
,
Battelle Memorial Institute, Columbus 1 Ohio
Processing Variables, Reactivity, and Sinterability of Uranium Oxides b Current interest in the application of uranium dioxide as a reactor fuel has stimulated investigation of methods which yield oxide bodies of high density. One objective of this investigation is to produce an oxide powder that can be fabricated into shapes having at least 90% of theoretical density by forming at moderate pressures and temperatures-about 20,000 pounds per square inch and 1650' C.
The crystallite size and the unit-cell dimension before and after sintering were determined. The phase compositions of active and inactive uranium oxides were investigated by x-ray diffraction examination and electrical-conductivity measurements. An analysis of the oxygen-uranium ratio of the uranium dioxide powder and sintered pellets was also obtained. Commercial Uranium Trioxide
Examination of commercial uranium COMMERCIALLY prepared uranium ditrioxide revealed two major phases: oxide sinters to only 70YGdensity a t moderate pressures and temperatures. "Commercial" as used here refers to oxide normally produced by the Mallinckrodt Chemical Works. Early experiments indicated that to obtain a dense body (90% of theoretical density) by sintering commercial uranium dioxide powder, a high forming pressure of 100,000 pounds per square inch and a high sintering temperature of 1900O C, were required. Subsequent investigations were directed toward a better understanding of the effect of method of oxide preparation on sinterability. This report describes a correlation between sinterability and "oxygen reactivity"-Le., susceptibility toward sorption of oxygen a t room temperature. Experimental Procedure
Structural and compositional differences among various preparations of uranium trioxide and of the uranium dioxide reduced from them were related to uranium dioxide sinterability. In commercial preparations, milling methods, reduction temperatures, and atmospheres were varied. The uranium trioxide was milled wet or dry, and reduced in dry hydrogen, dry carbon monoxide, or ethyl alcohol in argon, a t temperatures ranging from 400' to 700' C. The commercial uranium trioxide is identified by Katz and Rabinowitz (7) as Type 111, yellow oxide. In laboratory preparations, the structure type was varied. Type I11 was prepared, as well as an amorphous type and that listed by Katz as Type 11, red oxide. Hydration reactivity was measured by the rate of water pickup on exposure to water-saturated air a t 30' C. Compacts of the dioxide powders were formed a t 20,000 pounds per square inch, with camphor as a binder, and were sintered a t 1650' C. for 1 hour in dry hydrogen.
about 90 wt. 010 Type I11 uranium trioxid and wt. % a-UOa.H2O. Less i a n 1 wt. % of UaOs was detected.
ib
2 60
cr
a
+
250
P E 3 v)
2
240
\
W X
cc w +
4 230
A U O j 2 H20 T Y P E m MCW UO, 0 TYPE E l UO, (250-C.) + TYPE IIl UO, (400 C,I AMORPHOUS UO,
0
F
d
5
2 220
z
d3
0
7
210
5
?
::2 00 I
400
i
500
600
I
700 TEMPERATURE, C.
, 800
Figure 1. Relation of oxygen-uranium ratio of UOa to reduction temperature for various types of UOB
Reduction of the as-received uranium trioxide in various atmospheres had little or no effect on the sinterability of the resultant uranium dioxide. Sintered densities were 70 to 80% of theoretical. Dry milling of the uranium trioxide prior to reduction increased the sinter density of the uranium dioxide only slightly. When the uranium trioxide was milled in water, 98% of the powder was converted to UO%.H%O, and uraniumdioxide reduced from the wet-milled uranium trioxide sintered to a bulk density of 9570 of theoretical. Structures and Hydration Reactivity
The methods of preparing modifications of uranium trioxide and their hydrate phases produced on exposure to 30' C. water-saturated air are given in Table I. The equilibrium water contents of the hydrates show the range of water content of the hydrate phases. Drying the hydrated oxide a t 300' C. results in Type I11 (normal Mallinckrodt) uranium trioxide. However, if residual nitrate in the trioxide is removed by water leaching, the resulting hydrate dries to an amorphous uranium trioxide when heated a t 300' C. The hydration reactivity of uranium trioxides is shown by the amount of water adsorbed during exposure to 30' C. water-saturated air, the amorphous being approximately three times as reactive as the crystalline modifications. Reactivity of Uranium Dioxide
The modifications of uranium trioxide were reduced in flowing dry hydrogen. Reactivity measurements were made on the resulting uranium dioxide powders.
Table I.
Polymorphic Modifications of Uranium Trioxide and Hydrate (Formed on exposure to 30' C. water-saturated air) (The monohydrate is the major phase in low-temperature denitrated and hydrate UOs; the dihydrate is obtained during hydration of high temperature UOS)
UOa Structure Type Amorphous
I11
I1
Method of Preparation Dehydration of UNH" in vacuum or dry air before denitration at 300' C. Mallinckrodt production simultaneous dehydration and denitration of UNH, 250' to 400' C. Flash decomposition of UNH at 400' to 500' C.
Equil. Water Content, Hydrate Phase % ' Water UOa.HaO
25 to 30
U03.Hz0 and/or UOa.2H20 UOa.2H&
7 to 12
10
Uranyl nitrate hexahydrate.
VOL. 49, NO. 10
OCTOBER 1957
1699
D
701
+
+
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AMORPHOUS U03 TYPE II UO3 TYPE III MCW UOA UOs' 2 H20
*
6 500
1000
1500
::
8o
1 u,/
V W
AIR-OXIDIZED ACTIVE UO, BEFORE SINTERING OXYGEN-TO-URANIUM RATIO AFTER SINTERING OXYGEN-IMPREGNATED SINTERING MCW UO2 BEFORE
k I
2000
CRYSTALLITE SIZE, A.
Figure 2. Relation of sinterability of UOz to crystallite size for Types ll, 111, and amorphous UOaand U 0 3 .2Hz0
Air Oxidation. The trioxide was reduced at specified temperatures over the range 480" to 800" C., cooled to room temperature, and exposed to room-temperature air. The more reactive dioxide reacted immediately with air. Structure and crystallite size of the exposed uranium dioxide were determined by x-ray diffraction. The oxygen-uranium ratio was obtained to determine the combined effect of reduction temperature and reaction with air. At the lower temperature of reduction, the black color of some of the oxides prior to air exposure showed that reduction was incomplete. This was most evident for the inactive dioxides. Figure 1 shows that the susceptibility of uranium dioxide toward oxidation is related to the temperature of preparation as well as the structure type of the prior uranium trioxide; the structure type is more important. X-ray diffraction studies of these reduced oxide powders revealed that the uranium dioxide structure was retained after air exposure for all samples except that reduced a t 480" C., which had an oxygen-uranium ratio of 2.60. As this sample did not evolve heat or change color on exposure to air, incomplete reduction must be assumed in this case. Uranium trioxide preparations resulted in uranium dioxide of differing crystallite sizes for a given reduction temperature. This difference is due in part to reaction with air after reduction. Sinterability. The effect of uranium trioxide structure on hydration reactivity and on the oxidation tendency of the subsequent uranium dioxide makes it evident that the more active the uranium trioxide, the more active is the uranium dioxide. The sintered densities of compacts of uranium dioxide powders made from these preparations are shown in Figure 2. Although no significant effect of crystallite size on the sinterability of uranium dioxide was observed, the structural modification of source uranium trioxide has a marked effect on sinterability of the resulting uranium dioxide. A definite increase in sinterability was obtained for the more active uranium dioxide. To show that sinterability was related to activity rather than oxygen content, oxy-
1700
60 200
I
205
,
I
210
2J5
220 225 230 215 OXYGEN-TO- URANIUM RATIO
I
I
240
2.45
Figure 3. Relation of oxygen-uranium ratio to sinterability of UOz made from various types of U 0 3 or UO, hydrates and from MCW UOZ oxygen impregnated at 180" C.
gen-impregnated inactive uranium dioxide was sintered. Figure 3 shows that sinterability is not related to only the oxvgen content of the uranium dioxide. As the theoretical density is approached the oxygen-uranium ratio of the sintered body approaches the stoichiometric composition of uranium dioxide (Figure 3). Active and Inactive Uranium Dioxide The high-temperature structures of active and inactive uranium dioxides having an oxygen-uranium ratio of 2.30 to 2.40 were investigated, to aid understanding of the mechanism of sintering. X-ray diffraction analyses of these two oxides showed that the inactive oxide was a tetragonal phase, while the active oxide retained the cubic symmetry of stoichiometric uranium dioxide. When the inactive oxide was annealed at 45OoC., its structure reverted to cubic symmetry. However, annealing the active oxide did not change its structure. The deviation from a linear expansion above 500" C. for the active UO2.4 has been interpreted as a loss of oxygen from its lattice. The inactive oxide had a linear expansion from 200" to 700' C., suggesting no loss of oxygen from its lattice. Sinterability may be related to the bonding energy of the excess oxygen in the uranium dioxide lattice. Conclusions The history of the preparation of uranium oxides influences structure and chemical reactivity. Reactivity of uranium dioxide powder is related to its sinterability in a way to indicate a difference between the structures of active and inactive oxides. Although further investigation of this difference is needed to understand the mechanism of sintering, these conclusions were established.
Modifications of uranium trioxide production processes influence the reactivity of both uranium trioxide and uranium dioxide. Marked differences in the structure of uranium trioxide are
INDUSTRIAL AND ENGINEERING CHEMISTRY
observed. The structural differrnces in uranium dioxide are harder to detect. Commercial uranium trioxide has a low reactivity toward hydration, and reduction produces unreactive uranium dioxide of poor sinterability. Hydration of commercial uranium trioxide by wet milling increases the reactivity and sinterability of the resultant uranium dioxide. Compacts having 95% of theoretical density can be obtained by forming at 20,000 lb./sq. in. and sintering {or 1 hour a t 1650" C. Commercial uranium dioxide is unreactive to air oxidation and has poor sinterability under ordinary forming pressures and temperatures. Compacts formed at 20,000 pounds per square inch have bulk densities 70% of theoretical after sintering for 1 hour a t 1650" C. The oxidation ease of uranium dioxide is a measure of its sinterability. Crystallite size of uranium dioxide powder in the range 200 to 2000 A. does not affect sinterability. Sintered bodies of highest density approach the stoichiometric composition uranium dioxide. More oxygen is lost from active than from inactive oxide, upon sintering. High temperature x-ray diffraction analyses show that active oxide (UO2.41) begins to lose oxygen when heated above 500' C., while inactive oxide (U02.32) does not lose oxygen u p to 700" C. Electrical conductivity studies reveal active oxide (UO2.41) to be a @-typeconductor, the inactive oxide (UO2.32) is an n-type conductor. Acknowledgment The assistance of Collin Hyde in carrying out the sintering experiments and Robert Willardson in making the electrical conductivity measurements is gratefully acknowledged. Special appreciation is given to A. E. Ruehle, Mallinckrodt Chemical Works, for supplying the uranyl nitrate and for his interest. Literature Cited (1) Katz, J., Rabinowitz, E., "Chemistry of Uranium," 1st ed., p. 276, McGraw-Hill, New York, 1951. RECEIVED for review December 3, 1956 ACCEPTED April 12, 1957
