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R. G. WYMER and R. E. BLANC0
Oak Ridge National Laboratory, Oak Ridge, Tenn.
Uranium-Aluminum AIIoy Dissolution Some basic factors in processing spent nuclear fuel DIssommoiv of
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heterogeneous nuclear reactor fuel elements is the initial step in aqueous processes for the recovery of fissionable and fertile materials. This step converts the solid fuel to a liquid process feed material. Common fuel materials such as uraniumaluminum alloy and aluminum jackets for uranium or thorium core materials are readily soluble in nitric acid catalyzed by mercuric nitrate or in sodium hydroxide solutions. Jacket removal by caustic dissolution has the advantage of selective dissolution in that the core materials remain untouched. Sodium nitrate can be added to the caustic to suppress hydrogen evolution and reduce the explosion hazard. This paper reports a study of the nitric acid and caustic systems as applied to uranium-aluminum alloy dissolution. Nitric Acid-Mercuric Nitrate System The dissolution of aluminum and uranium-aluminum alloys in boiling nitric acid is catalyzed by mercuric ion, the dissolution rate of cast alloy being increased by a factor of 10 when mercuric nitrate is added to the nitric acid to make it about 5 X 10-4M in mercuric ion. An explanation of this behavior is that aluminum and its alloys, such as those described, are protected from all but very slight attack by nitric acid solutions by a thin, tenacious, film of aluminum oxide. Mercuric ion in the neighborhood of the aluminum being oxidized is in a reducing environment, and a small amount is reduced to metallic mercury. The free mercury, which has a transient existence in the nitric acid solutions, amalgamates the surface of the tenacious oxide film during an initial induction period. The amalgamated area then reacts vigorously with the nitric acid, leading to the production of more metallic mercury which causes further amalgamation, and so on. This process continues until a balance is reached between the reduction of mercury by the dissolving aluminum or its alloys and the dissolution of mercury by the nitric acid solutions. Experimental. Most of the experimental data were obtained by partially dissolving weighed me tal pieces of known
dimensions in solutions of mercuric nitrate and nitric acid at the boiling point. The length of time of immersion in the boiling solution, along with the weight loss and average dimensions of the metal, permitted the calculation of dissolution rates. In all the experiments reported, the metal pieces had been pretreated in a separate portion of the experimental solution in order to form the amalgamated surface and avoid the induction period. Upon removal from the acid solutions, the hot amalgamated aluminum or alloy was quite reactive and soon formed a bulky oxide coat. T o avoid handling difficulties caused by this coating, the specimens were immersed in tared bottles of 2-propanol to exclude air and weighed both before and after the rate-determining dissolution. Effect of Metallurgical Treatment. An important finding in this study was that dissolution rates could be changed by a factor of 5 or more when samples of aluminum alloy of different metallurgical histories-e.g., cast and extruded -were used. It was further found that different regions of a single piece of metal dissolved a t widely different rates. O n the other hand, the difference in dissolution rate caused by varying the uranium content is not distinguishable
over the range of uranium concentra, tions of 5 to 15 weight yo uranium. The differences in behavior exhibited by alloys of different composition and metallurgical history are shown in Figure 1. Curves I, 11, and I11 were obtained using small alloy rods, whose diameters were small compared to their lengths. Curve FV was obtained using alloy wafers which were obtained by cutting thin, flat cyclindrical slices from large alloy rods. Therefore, even though curves 111and I V were both obtained using cast alloy, the natures of the faces exposed to dissolvent were probably quite different, since the sides comprised most of the area of the alloy used to obtain curve I11 and the ends comprised most of the area of the alloy used to obtain curve IV. Effect of Nitric Acid a n d Aluminum Nitrate Concentration. The data as presented in Figure 1 show the characteristic maximum of the initial dissolution rate of aluminum and its low uranium alloys in nitric acid catalyzed by mercury. As the aluminum nitrate content of the dissolver solutions increases, the dissolution rate rises to a maximum and then drops off rapidly as shown in Figure 2. Slow dissolution of aluminum alloys occurs in solutions of aluminum nitrate containing a mercuric nitrate
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catalyst. The resulting solutions are represented as having negative values of acid Concentration. This mode of presentation is in accord with the convention of indicating the extent of hydrolysis of aluminum nitrate, with subsequent loss of the free nitrate ion through reduction, as “acid deficiency,” or “negative acidity.” Thus, a solution of -1iM nitric acid is 1M acid deficient-that is, it contains 1 mole of aluminum-bound hydroxyl ions per liter. The sequence of steps leading to acid deficiency is
HNO3
Figure 3.
+ A I HzO d AI(OH),(N0a)3-z + nitrogen compounds
Dissolution rates as function of mercuric nitrate concentration
(2)
Effect of Mercury Concentration, The concentration of mercuric ion is an important factor in determining dissolution rates at low mercury concentrations. However, increasing the mercuric nitrate concentration above 0.005M did not show any significant increase in dissolution rates. Figure 3 shows how the dissolution rate of extruded alloy varies with initial mercuric ion concentration in boiling solution of 8M nitric acid. Similar curves are obtained in nitric acid of other concentrations using metals with different metallurgical histories. As suggested, the catalytic activity of the mercury in-
volves its cyclic reduction and oxidation after an initial induction period. Figure 4 shows the amount of amalgamated mercury present under steady-state conditions on the surface of dissolving aluminum alloy from two different sources using nitric acid solutions with an initial mercuric nitrate concentration of 0.005M. The ratios of solution volumes to alloy areas were such as to provide a threefold excess of ionic mercury over metallic mercury in the extreme cases, and in general, the excess was much larger than threefold. Stoichiometry of the Catalyzed Dissolution. The stoichiometry of the reaction between boiling nitric acid and aluminum or its alloys depends on the acid concentration and probably on the metallurgical history of the metal. The extremes which can be approached are shown in Equations 3 and 4.
done at the boiling temperatures of the dissolvent solutions, which ranged from about 102’ to 115’ C. .Also, no attempt was made to control the ionic environment of the solutions, so ionic strengths covered a large range. The kinetic data. which are summarized in Figures 1 to 5, can best be explained in terms of two competing reactions. The first reaction is that leading to the formation of metallic mercury, which makes the aluminum susceptible to nitric acid attack as discussed previously. The second reaction is the dissolution of the amalgamated mercury. The mercury amalgamates the surface of the aluminum as long as it remains undissolved and so makes it possible for the aluminum to continuously react with the nitric acid. The rate of reduction of mercury to the metal, when the fraction of the total mercury reduced is negligible, is a function of the reducing potential of
0 161
40
>
M HN03
I 5,
I
012-
At acid concentrations around 4iM the reaction has approximately the stoichiometry catalyst
A1
+ 3.75 HNOi -+Al(N0s)z + 0.225 N O ( 9 ) + 0.15 NsO ( 8 ) 0.1125 Nz (8) 1.875 HzO ( 9 ) ( 5 )
0 04
TOTAL NOS
Figure 2. Dissolution rates of extruded 15% uranium-85% aluminum alloy in aluminum nitrate-nitric acid containing 0.005M mercuric nitrate
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The heat of reaction is approximately 190 kcal. per mole of aluminum. Figure 5 shows that the acid requirement per mole of alloy decreases with increasing acid concentration. The metal used to obtain the data for Figure 5 v,as very reactive extruded aluminum. Kinetic Interpretation. Only a qualitative discussion of the dissolution kinetics can be made from the data at hand, Nearly all the experiments were
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 4. acidity
Free mercury as function of
Figure 5. Stoichiometry as function of acidity
the solution provided by the dissolution of the aluminum or alloy and of the nitrate ion concentration. The rate of dissolution of aluminum, however, is dependent on both the nitric acid concentration and the previous metallurgical treatment of the aluminum. Thus, for any specified nitric acid concentration, the rate of reduction of mercury and hence the extent of aluminum amalgamation varies with the history of the metal. The rate of dissolution of the amalgamated mercury, however, will be independent of the history of the alloy. Therefore, for a particular piece of metal in a large excess of boiling nitric acid containing mercuric nitrate, a steadystate condition will be reached in which a definite fraction of the metal will be amalgamated. The rates of oxidation and reduction of the mercury will be equal. The amount of free mercury present under a given set of conditions will depend on the alloy, as is shown in Figure 4. The fact that the amount of amalgamation drops off as the acid concentration increases shows that the rate of reaction leading to free mercury oxidation increases more rapidly than the overall rate of aluminum dissolution-mercury reduction. The maxima shown in Figure 1 result, at least in part, because the amalgamated aluminum dissolves a t rates which increase quite naturally with increasing acid concentration u p to the point where the area amalgamated becomes so small that the limited reactive surface becomes the dominant rate-controlling factor.
Caustic System Aluminum and aluminum-rich alloys will dissolve in aqueous solutions of sodium hydroxide according to the reaction A1
+ NaOH + HzO +NaAlOz + 3/2 Hz ( 6 )
The sodium aluminate forms a stable solution only in the presence of excess caustic. Experimental. All dissolution rate measurements were made at the boiling temperature. Rates were measured by taking periodic readings on a differential flow manometer through which the hydrogen from the dissolution reaction flowed. Hydrogen evolution was generally so rapid that the pieces of aluminum or alloy were floated by the gas after the pieces became fairly small. Reduced dissolution rates resulted because the tops of the floating pieces were not dissolving. For rate determinations the aluminum was wrapped in a coarsemesh stainless steel screen whose weight held the aluminum beneath the surface of the dissolvent. The screen had no effect on dissolution rates. Kinetic Interpretation. When caustic was used as dissolvent the dissolution, to a good approximation, followed heterogeneous, first-order kinetics. The rate law may be written as - - de , = k - s~
(7)
whefe c is the caustic molar concentration at time t; s is available surface area of aluminum or alloy, in square centimeters; t is time, in minutes; and u is the dissolvent volume, in liters. The value of the specificreaction rate constant, k , is 2.6 X 10-4 liter/min. x sq. cm. If either s or u is not constant (s is obviously not constant when a dissolution is carried to completion), this variable must be introduced into the differential rate expression as the appropriate function of caustic concentration. The metallurgical history of the metal had little effect on the dissolution rate in the caustic system. Effect of Dissolvent Impurities. In uranium-aluminum alloy dissolution it was important to learn if probable impurities in the dissolvent would affect the dissolution rate. The most important impurities were thought to be sodium carbonate and hydrogen peroxide. Sodium carbonate is an almost unavoidable contaminant of sodium hydroxide solutions, especially when prepared in amounts required for large scale chemical processing. Hydrogen peroxide is likely to be formed by fission product radioactivity when spent uranium-aluminum alloy reactor fuel is dissolved. I t was found that high sodium carbonate concentrations in the dissolvent caused no noticeable uranium loss to the resultant sodium aluminate solutions and that dissolution rates were those which would be expected from the caustic alone. In a series of experiments
where greater than tenfold excesses of hydrogen peroxide over that which could be formed by fission product radioactivity were continuously added during the dissolutions, extensive solubilization of the uranium was experienced ; however, no change in dissolution rate was observed. The losses were greater in dilute than in more concentrated caustic solutions when the same aluminum/ sodium hydroxide mole ratio was used. Also, catalytic amounts (6 X lOW4M) of sodium iodide reduced uranium solubility as long as the system was held at the boiling temperature. The uranium oxide solids resulting from pure caustic dissolutions were generally about 70% uranium, by weight, and were primarily quadrivalent uranium. They contained only traces of aluminum and sodium. X-ray analyses showed the solids to be nearly amorphous but to give a diffuse pattern of uranium dioxide. The solids were not gelatinous and were easily washed and dried. Solution Stability. A convenient dissolvent concentration for caustic dissolutions is in the range 3 to 8M sodium hydroxide. The sodium aluminate solutions resulting from the dissolutions had to contain excess sodium hydroxide to stabilize them. Solutions containing 3M sodium hydroxide and 3M sodium aluminate were stable indefinitely a t room temperature as supersaturated solutions. At low caustic concentrations the sodium hydroxide/sodium aluminate mole ratio had to be increased to reach the same degree of stability. Fission Product Distribution. Great importance is attached to the way the fission products released into a caustic dissolver solution from irradiated uranium-aluminum alloy are distributed between the uranium oxide solids and the caustic-aluminate supernatant. The bulk of the fission products is precipitated with the uranium solids, leaving cesium, iodine, zirconium, and niobium as the major contaminants of the liquid phase. Zirconium and niobium are not present to more than several tenths of 1% of their total amounts. Dissolution of Uranium Solids. The usual process requirements for spent fuels involve formation of the nitrate salts for solvent extraction steps. The uranium oxide solids from caustic dissolutions were shown to be readily soluble in boiling 6M as well as 3M nitric acid at uranium concentrations from 2 to 60 grams per liter. Five moles of nitric acid is required to dissolve 1 mole of uranium oxide solids when the solids are about 7Oy0uranium (as they generally are) and when most of that uranium is quadrivalent.
RECEIVED for review January 12, 1956 ACCEPTED July 2 , 1956 VOL. 49, NO. 1
JANUARY 1957
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