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Ind. Eng. Chem. Process Des. Dev. 1984, 23, 122-125
Dissolution Rates of U308Powders in Nitric Acid Aklhlko Inoue' and Takeshl Tsujlno Division of Nuclear Fuel Research, Japan Atomic Energy Research Institute, Tokai-Mura, Ibaraki, Japan
The dissolution rates of U308powders in nitric acid have been investigated in the temperature range 30-95 O C and the acid concentration range 0.1-10 N under atmospheric pressure in order to obtain the basic design data for the dissolution process of voloxidized spent LWR fuel. The dissolution rate was strongly affected by stirring speeds. The highest dissolution rate was obtained in an unstirred run. Above 500 rpm almost identical rate curves were obtained. The rate data obtained at the constant stirring speed of 500 rpm showed that the dissolution proceeded through two stages. The dissolution rate constant during the second stage varied with the 0.8-1.7 power of acid concentration below 6 N HNO, in the temperature range 30-95 O C . The activation energies of 13.9-19.2 kcal/mol were obtained in the acid concentration range 1-4 N HNO, during the second stage. The simulated fission products contained in U308powder had little depressing action on the dissolution at 2 N HNO,.
Introduction Recently in the field of fuel reprocessing much attention has been paid to the dissolution of U308powder in nitric acid, as it represents a dissolution stage in reprocessing of LWR fuel. It incorporates voloxidation as a head-end step, which is a candidate process to remove tritium from spent fuel by oxidative heat treatment. It has been developed at ORNL (Burch et al., 1980), SRL (Stone, 1980), JAERI (Kitamura et al., 1981), etc. In order to develop the above dissolution process, information about the dissolution rate will be especially necessary. Several workers have investigated the dissolution rate of UOz pellets (Taylor et al., 1963; Shabbir and Robins, 1969), U 0 2 spheres, and powders (Shabbir and Robins, 1968) in nitric acid. However, much less information is available for that of U308powder in nitric acid. The purpose of this work was to obtain reliable rate data on the dissolution of U308powder in nitric acid and to determine quantitatively the effect of process variables. Variables investigated in this work were temperature, acid concentration, and agitation. The effect of simulated fission products contained in U308 powder was also examined. This work is considered to offer useful information on the choice of favorable operating conditions for the dissolution process. Experimental Section Materials. UOz pellets (0.93 cm diameter, 1.53 cm long, 93% T.D.) and UOz pellets containing simulated fission products (0.93 cm diameter, 1.53 cm long, 74% T.D.), which were supplied by Mitsubishi Metallic Co., were used for the production of each type of U308 powder. U 0 2 pellets containing simulated fission products were produced by the following procedure. Simulated fission products were mixed with UOz powder in oxide powder forms. The powder mixtures were compacted in a pellet form and sintered at about 1750 "C. Oxidation treatment of UOz pellets was carried out in a rotary kiln type reactor (20 cm diamter, 40 cm long) at 450 "C and in a flowing 15% 02-85% Nzgas mixture for about 4 h as a voloxidation process experiment (Kitamura et al., 1981). In the experiment, UOz pellets, about 500 g, were used for one run. Details of each type of U308 powder are indicated in Tables I to 111. Chemicals used were AR grade reagents. Distilled water was used to make up all the solutions. Apparatus. The dissolution vessel used in the experimental work was a flask 9 cm in diameter and 11cm high 0196-4305/84/1123-0122$01.50/0
Table I. Details of U30,Powder Used for the Dissolution Experiments
powder
sp surf. area, m*/g (BET O/U method) ratio 1.47 1.72
U30,
U,O,-F.P. a
compn
part. size distrna
Table I11
Table I1 nearly same with U,O,
-2.66
Sedimentation method.
Table
a.
Particle Size Distribution
wt %
part. size, km
6.5 36.5 30.5 17.5 6.5 2.5
0- 2 2-4 4-6 6-8 8-10 10-14
mean particle size
4.8 p m
Table 111. Composition compna Sr Y Zr Mo Ru Pd
0.08 0.04 0.31 0.30 0.20 0.09
wt % for U metal
La Ce Pr Nd Sm Ba
0.11 0.23 0.11 0.35 0.06 0.12
a This composition is that of the original UO, pellet, which simulates 3.5% enriched UO, fuel, burned to 30 000 MWd/T, and cooled for 180 days.
with a separable cover. Temperature was measured with a mercury-in-glassthermometer. A semicircular blade type glass stirrer of 5 cm width was fitted through a gas-tight gland in the central neck of the cover and driven by a motor with a speed controller. The motor speed was adjusted at the proper rpm and checked by a photoachometer. A reflux condenser and a water trap to prevent the evolution of NO, gas to the air were fitted to the side neck of the cover. Dissolution Rate Experiment. A typical dissolution rate experiment was carried out as follows. Initially 400 mL of nitric acid solution of a known concentration was placed in the dissolution vessel and then heated to a desired temperature. Approximately 800 mg of U30s powder 0 1983 American
Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 1, 1984 123
g l ' , '0
10
,
,
20
30
50 60 Time l m i n l
40
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0.04
I
- 002c I
I
- 100
0011
ap
g 80
,
2
4
I
6 8 10
1
20
40 6080100 200
Figure 2. Log [l - (1 - X ) l I 3 ]vs. log t plots for the dissolution of U308powder in nitric acid at 55 O C .
0
-
1
Time ( m i n )
-
2
010 0.08 0.06
60
c
40
0
20
'0
20
IO
30
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I
fraction. This is probably due to the very rapid dissolving of small U308particles or active sites on the surface of U308 particles. The unreacted-core-shrinkingmodel for a sherical particle was employed to analyze the dissolution rate curves (Wen, 1968). Among these models two possible reaction models shown below were considered for the dissolution. When the diffusion of reactants or products through the layer of reaction products is the rate-controlling step, the rate equation in an integral form becomes approximately 1 - (1- X)1/3 = hltl/Z (1) where X is the fraction of dissolution of solid particles and k1 is the overall rate constant. When chemical reaction a t the solid-liquid interface is the rate-controlling step, in this case particle size reduces with reaction, the rate equation becomes
1
/'ON
80
90
100
110
120
1 - (1 - X)li3
r
'0
20
l
40
,
60
80
100
I20
140
160
180
200
1
Time I min 1
Figure 1. Dissolution of U308powder in nitric acid (a) at 95 O C ; (b) at 75 "C; (c) at 55 OC;(d) at 30 O C .
was charged into the solution a t a desired stirring speed. Samples (5 mL) of the solution were withdrawn a t appropriate intervals and immediately diluted with a large excess of water to stop any further dissolution. Uranium(VI) in the solution was determined by the titration method with alkali solution described in the literature (Motojima and Izawa, 1964), using an automatic titrator. The dissolution fraction, if expressed in percentage, is considered to be accurate within *3% from the comparison of initially charged quantity and that abtained by the titration method.
Results and Discussion Analysis of Rate Curves. Dissolution rate was strongly affected by stirring speed, so all the experiments were carried out a t a constant stirring speed of 500 rpm, above which almost identical rate curves were obtained. The effect of agitation on the dissolution rate will be discussed in a later section. Figures l a d show the rate curves for the dissolution of U308powders in nitric acid a t 95, 75, 55, and 30 OC,respectively. The rate curves do not extraporate back to zero
k2t (2) where k2 is the overall rate constant. When a log-log plot of [l - (1 - X)1/3]vs. t is made, the slope determines whether it is chemical reaction controlling or diffusion controlling. Plots of log [I - (1- X)1/3]vs. log t at 55 O C are shown in Figure 2. The results show that the dissolution proceeds through two stages. The slopes are in the range 0.2-0.7 during the first stage and 1-1.7 during the second stage below the concentration of 6 N HNO, in the covered temperature range; however, the difference of the slopes is minor at the same temperature during the second stage. This indicates the major effect of diffusional control during the first stage. On the other hand, the chemical reaction at UsOs surface seems to be the rate-controlling step during the second stage. Higher power of time than unity during the second stage may indicate the progressive increase of nitrous acid concentration at the surface of U308powder as the dissolution proceeds. Nitrous acid is one of the reduction products of nitric acid and known to act as an "autocatalyst" for the dissolution of uranium metal (Lacher et al., 1961) and uranium dioxides (Taylor et al., 1963) in nitric acid. The minor effect of chemical reaction during the first stage is considered to be due to a low concentration of nitrous acid. In order to confirm the effect of nitrous acid on the dissolution of U30s in nitric acid, NaNO, was added to nitric acid solution as NaNOZ generates HNO, reacting with HN03. The results at 4 N HN03 and 55 OC are shown in Figure 2. By adding 0.005 M NaNO, and 0.01 M NaNO,, the first stage disappeared in the covered time range and the dissolution fraction at a fixed time increased with the concentration of NaNOz during the second stage. Also, the rate constant k z evaluated on the basis of eq 2 was 1.7 times at 0.005 M NaNOz and 3.5 times at 0.01 M NaNO, that in the case of 4 N
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Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 1, 1984 10
7
1
-
i
a
I '
c
L
0021 ~
0011
1
2
1
6 8 10
4
-
20
40 6080100
200
Time imin 1
, 20
40
60
I
1
01
Figure 3. Log [l - (1- X)1/3]vs. log t plots for the dissolution of U308powder in nitric acid at 30 "C.
0
I 1
,
1
I 10
I O
HN03 iN) Figure 5. Plots of rate constants during the second stage vs. acid concentration for the dissolution of U30s powder in nitric acid.
1 80
100
120
Time ( m i n 1
Figure 4. Plots of [I - (1 - X)lI3]vs. time for the dissolution of U308 powder in nitric acid during the second stage at 55 "C.
HN03 These results must indicate the role of nitrous acid as "auto-catalyst" for the dissolution of U308in nitric acid. An abrupt change of the slope during the second stage occurs around 8 N a t 30 "C (Figure 3). The slope decreases from 1.7 at 6 N to 1.0 a t 8 N and 0.7 at 10 N. This may suggest that the rate of regeneration of nitrous acid reduces a t high acid concentration or the decomposition rate of nitrous acid increases a t high acid concentration as these are considered to suppress the increase of nitrous acid concentration a t the surface of U308. Effect of Acid Concentration and Temperature. Dissolution rate curves were analyzed by plotting dissolution fraction vs. time according to eq 1 during the first stage and eq 2 during the second stage. However, it was difficult to evaluate the rate constants during the first stage because of a lack of data. Typical plots during the second stage are shown in Figure 4. From the slopes of these lines the rate constants can be evaluated. These rate constants can be considered to correspond to those at averaged nitrous acid concentration between the initial value and the final value during the second stage as the nitrous acid concentration a t the surface of U308may increase progressively during the dissolution in some cases as pointed out in the above section. Logarithmic plots of the rate constants vs. acid concentration are given in Figure 5. The slope of each plot was calculated by the least-square method. The results show that the slope decreases with increasing temperature and ranges from 0.8 to 1.7. These slopes are about one-half of those in the case of UOz pellets (Taylor et al., 1963) and about one-third of that in the case of uranium metal (Lacher et al., 1961) under similar conditions. This may be due to the higher oxidation state of uranium in the U308 phase than in UOz and uranium metal phases.
I 25
27
29
31
33
35
Reciprocal Temperoture 103/TIKI
Figure 6. Arrhenius plots for the dissolution of UsOs powder in nitric acid.
Figure 6 shows the Arrhenius plot of the rate constant of the dissolution during the second stage. From the slopes of these lines the values of 13.9, 16.9, 19.2 kcal/mol were calculated for the activation energy of the reaction in 4, 2, and 1 N HN03, respectively. These values strongly suggest that the dissolution rate of U308powder in nitric acid is chemically controlled during the second stage. Similar values have been reported for the dissolution of U02 pellets (Taylor et al., 1963),U02 powder and spheres (Shabbir and Robins, 1968), and also for that of uranium metal (Lacher et al., 1961). It seems that the difference in the oxidation state of uranium and the form of a sample does not cause a large change of activation energy. The dissolution mechanism of U308in nitric during the second stage is considered to be a heterogeneous chemical and nitric acid solution containing various reaction of U308 reduction products of HNO, such as HN02and NO, gases. However, detailed discussion about the mechanism is beyond the primary objective of this work. Further work is needed to elucidate the dissolution mechanism. Effect of Simulated Fission Products in U308 Powder. The simulated fission products are seen to reduce the dissolution rate of uranium considerably a t low acid concentration (Figure 7). However, the simulated fission products have little effect on the dissolution rate at high acid concentration of 2 N HN03, even if the temperature is decreased. The reduction of dissolution rate at low acid concentration is probably due to the inhibition
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 1, 1984
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nitrous acid at the surface of Us08 will become highest in an unstirred solution because the diffisuion rate of nitrous acid from the surface to bulk solution is considered to become smallest in that solution. The dissolution rate curves a t 100 and 300 rpm are not smooth. This is considered to indicate that the nitrous acid concentration a t the surface of U308varies irregularly with time.
40
20
-0
60
80
100
120
140
Time I min.)
Figure 7. Dissolution of U30spowder containing simulated fission products (solid curves) and U30spowder (dashed curves) in nitric acid at 95 "C.
-0
10
20
30 40 Time l m i n I
50
60
Figure 8. Effect of agitation on the dissolution of U308powder in nitric acid at 95 OC.
by insoluble fission products such as Ru, Zr, and Mo contained in U308powder. Effect of Agitation. The effect of agitation on the dissolution rate of U308 powders was investigated in the range of stirring speed 0-800 rpm in 0.3 N HNO, at 95 "C, 4 N HNO, at 55 "C, and 10 N HNO, at 30 "C. The results in the case of 0.3 N HNO, a t 95 "C are indicated in Figure 8, which shows that the dissolution rate is strongly affected by the stirring speed. The dissolution rate was the highest in the unstirred run. The slow stirring of 100 rpm, at which speed almost all the powder was accumulated in the bottom of the flask, is seen to reduce the dissolution rate to a considerable amount. Above 100 rpm, total dissolution time does not change greatly; however, dissolution rate curves change with increase of stirring speed up to about 500 rpm, a t which speed all the U308 particles were kept in suspension in the solution, and further increase has no effect. Similar results were obtained in the solutions of 4 N HNO, at 55 OC and 10 N HNO, a t 30 "C. The nitrous acid concentration a t the surface of U308powder will be the main factor of these variations of the dissolution rate curves with stirring speed as the speed of diffusion of nitrous acid from the surface of U308powder to bulk solution largely depends on agitation. The concentration of
Conclusion The dissolution of U308 powder in nitric acid strongly depended on the speed of stirring. This is though to be due to the variations in nitrous acid concentration a t the surface of U308 powder. Based on the unreacted-core shrinking model, the rate data at the constant stirring speed 500 rpm, above which speed almost identical rate curves were obtained, were analyzed. The results show that the dissolution proceeds through two stages. It seems that the diffusion of reactants or products through the surface ash layer of the U30s particle is the rate-controlling step during the first stage. On the other hand, the chemical reaction a t the surface of U308 particles seems to be the rate-controlling step during the second stage. A rate dependence to the 0.8-1.7 power of nitric acid concentration was observed during the second stage. The activation energies calculated from the rate data during the second stage were 13.9,16.9, 19.2 kcal/mol at the nitric acid concentrations of 4, 2, and 1 N, respectively. The simulated fission products contained in U308powder decreased the dissolution rate to a considerable amount at low acid concentration below 0.3 N, whereas at the high acid concentration of 2 N HNO, they had little depressing action on the dissolution. The results obtained in this work will contribute to the design of a large-scale dissolver for voloxidized spent LWR fuel, especially in the case of a continuous dissolver. However, in order to find the optimum operating conditions for the dissolution process, the effect of other factors such as the concentration of uranyl ion and fission products impurities in the solution, oxygen gas bubbling, on the dissolution rate should be determined. Also, it is necessary to investigate the dissolution behavior of plutonium particles and fission products and the release behavior of gaseous fission products such as iodine. From the more basic point of view, the dissolution mechanism of U308is considered to be an interesting research item. Acknowledgment The authors would like to thank all the members in the authors' laboratory for their helpful discussions. Registry No. U308, 1344-59-8; HN03, 7697-37-2.
Literature Cited Burch, W. D.; Fekiman, M. J.; Groenier, W. S.; Vondra, B. L.; Yarbro, 0.O., Unger, W. E. ORNLlTM - 7192 (1980), Oak Ridge National Laboratory, Oak Ridge, TN. Kitamura, M., private communcations, 1981. Lacher, J. R.; Sattzman, J. D.; Park, J. D. I n d . Eng. Chem. 1961, 53, 282. Motojima, K.: Izawa, K. Anal. Chem. 1964, 3 6 , 733. Shabbir, M.; Robins, R. G. J . Appl. Chem. 1968, 18, 129. Shabbir, M.; Robins, R. G. J . Appl. Chem. 1969, 19, 52. Stone, J. A., DP-MS-80-9 (1980), E. 1. du Pont de Nemours 8 Co., Savnnah River Laboratory, Aiken, SC 29801. Taylor, R . F.; Sharratt, E. W.; de Chazai, L. E. M.; Logsdall, D. H. J . Appl. Chem. 1963, 13, 32. Wen, C . Y. Ind. Eng. Chem. 19689 6 0 , 34.
Received for review February 17, 1981 Revised manuscript received May 25, 1982 Accepted May 31, 1983
