nuclei wash out, during the low growth period, as was discussed by Randolph and Cise (19'72). In order to explore this further, experimental conditions should be varied so that more than 2% (by weight) of the total crystals formed are smaller than 47 p. Also the population density of the smaller particles would need to be measured. Suspension Density. As observed from the correlation, the nucleation rate varies almost linearly with the suspension density. This indicates the presence of secondary nucleation due to dislodging of the number of aggregates of solute molecules which are loosely held at a crystal surface. The importance of secondary nucleation was also confirmed by experiments on the mode of nucleation and growth where supercoolings of greater than 3 "C were required. Effect of Stirring Rate. The above correlation indicates a strong dependence of the nucleation rate on stirrer rpm. This provides strong evidence that secondary nuclei are generated by collisions of the seed crystals with the impeller blades. These observations are consistent mechanistically with the work of Strickland-Constable et al. (1969), Clontz and McCabe (19711, and Evans et al. (1974).The rpm or the power input of the stirrer are a measure of the energy input for crystal-crystal, crystal-impeller, and crystal-wall collisions. Thus, the results indicate that an increase in the energy input for crystal-crystal, crystal-impeller, and crystal-wall collisions increases the nucleation rate. This indicates the presence of collision breeding on contact nucleation.
CAS = equilibrium concentration J = nucleation rate, number/L min Jcal = calculated nucleation rate using eq 4 J e x p = experimental nucleation rate K N = constant MT = suspension density, g/100 cm3 of slurry N = number of crystals per unit volume n = population density, number11 L no = population density of nuclei ( L = 0) L = characteristic crystal length, p r = growth rate, k/min rpm = stirrer speed, revolutions per minute T = turnover time, min N = supersaturation ratio, C.A./CAS
Literature Cited Bennett, R. C., Van Buren, M., Chem. Eng. Prog. Symp. Ser., 65 (95), 44-49 (1969). Canning, T. F., Randolph, A. D., AlChEJ., 13, 5 (1967). Clontz, N. A., McCabe, W. L., Chem. Eng. frog. Symp. Ser., 67 (110), 6 (1971). Evans, T. W., Margolis, G., Sarofim, A. F., AlChEJ., 20 (5), 950 (1974). Lal, D. P., Mason, R. E. A,, Strickland-Constable,R . F., J. Cryst. Growth, 5, 1-8 ( 1969). Larson, M. A., Randolph, A. D., Chem. Eng. Prog. Symp. Ser., 61 (55). 1-13 (1965). Lee, Fu-Ming, Lahti, L. E., J. Chem. Eng. Data, 17, 304-306 (1972). Lodaya, K. D., Ph.D. Dissertation, University of Toledo, 1975. McCabe, W. L., lnd. fng. Chem., 21, 30-33 (1929). Randolph, A . D., Cise, M. D., AlChEJ., 16, 806 (1972). Randolph, A . D., Larson, M. A,, AlChEJ., 8, 639 (1962).
Nomenclature CA = bulk concentration
Received for reuieu: April 21, 1976 Accepted February 24, 1977
Development of an Electrolytic Dissolver for Plutonium Metal Earl J. Wheelwright* Chemical Technology Department, Battelle Memorial hstitute. Pacific Northwest Laboratories, Richland, Washington 99352
Richard D. Fox Research Department, Atlantic Richfield Hanford Company, Richland, Washington 99352
A critically safe dissolver was fabricated and demonstrated during the dissolution of ten plutonium buttons and two plutonium ingots totaling 23.9 kg of metal. Maximum dissolution rates exceeding 400 g/h and an average rate exceeding 300 g/h were demonstrated with a dissolver current of 150 A or 12 A/in.2 of anode surface. The production of a solids-free dissolver solution was demonstrated by maintaining a dissolver solution composition of 10 M HN03-0.05 M HF. High dissolver efficiency was achieved by the unique design of a "traveling" cathode which maintained a minimum, constant separation from the plutonium metal surface as that surface dissolved.
Introduction The conversion of plutonium metal to an aqueous solution suitable for reprocessing by conventional methods has been a continuing problem. Small, thin pieces of metal can be satisfactorily dissolved in 15 M "03 containing up to 0.1 M H F at boiling temperatures, but the passivity of plutonium metal in such systems greatly restricts the dissolution rate. Rapid dissolution of plutonium metal in 3-4 M "03-0.13 M HF was reported by Miner et al. (1969) at the Rocky Flats Plant, Golden, Colo.. but they reported that the reaction is difficult
to control. In the process currently used in metal reclamation facilities at the Hanford Atomic Plant, Richland, Wash., plutonium metal buttons (hemioblate spheroids with diameters up to 4 in.) are mechanically sectioned into smaller pieces and burned to oxide. The oxide is then screened, pulverized when necessary, and dissolved in 15 M HNO, containing H F at boiling temperatures. The oxide is difficult to dissolve, often requires several hours per batch, and frequently an insoluble residue remains when the dissolution is terminated. Electrolytic dissolution of normally passive metals in "03 was first demonstrated by Pitzer (1951). He anodically dislnd. Eng. Chem., Process Des. Dev., Vol. 16, No. 3 , 1977
297
YF’IT
LINE
.~(ETURN
rh ELECTRICAL COYNECTOR
OYERF.3:’
TEFLOY
iNSULAT0RS
0.bTIY:M FACE T 4 i T A L U M ANODE
Figure 1. Components of the electrolytic dissolver system. solved stainless steel-clad reactor fuel in “ 0 3 on a small scale. The work was developed and demonstrated on a large scale at both the Savannah River Laboratory, Aiken, S.C., and the Idaho Chemical Processing Plant, Idaho Falls, Idaho. This work has been extensively reviewed by Caracciolo and Owens (1970) and will not be discussed in detail. Two separate electrolytic processes have been developed. In one, contact with the dissolvable material is established through the electrolyte, the “liquid contact” principle, and special precautions are taken to prevent direct contact of the dissolving material with either the cathode or the anode. In the alternate process, the “metal contact” principle, the dissolvable material is physically attached to the anode or is contained in a conducting basket which serves as both the anode and a container for the dissolvable material. The “metal contact” process has shown less promise because of the difficulty in maintaining a good high-conduction contact between the anode and the dissolving material. When the contact deteriorates, electrical arcing occurs which can damage the electrodes or lead to fires if the dissolving material is pyrophoric. In preliminary laboratory tests utilizing the “liquid contact” mode, Fox (1971) suspended plutonium metal in a horizontal configuration between the two electrodes and demonstrated that plutonium metal could be dissolved. In the work described here, the “liquid contact” principle was maintained, but the electrode assembly was converted from a horizontal to a vertical configuration. The plutonium metal was confined between the two electrodes but physically isolated from both, and the entire assembly was immersed in the dissolver solution. With current flowing through the dissolver, the side of the plutonium metal facing the cathode electrode becomes the anode and dissolves. There are two parallel electrical paths between the electrodes: (1)directly through the electrolyte, and (2) electrolyte to plutonium metal to electrolyte. The efficiency of the dissolver depends upon the success achieved in maximizing the second path. The much higher resistivity of the electrolyte, compared to plutonium metal, is helpful in achieving a proper path maximization.
Equipment A schematic of the solution-handling components of the dissolver system is shown in Figure 1. The equipment was installed within a gloved box and individual components were connected together with polyethylene tubing. All dissolving tests were performed with plutonium metal. Dissolver. The design objectives included maximization of the electrical path through the plutonium metal button by providing for: (a) minimum separation distance between the 298
Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 3, 1977
Figure 2. Schematic of the electrolytic dissolver. electrodes and the two button surfaces a t all times during dissolution, (b) removal of evolved gas bubbles from the surface of the plutonium metal, and (c) prevention of formation of acid-depleted zones at or near the button surfaces. Imposed upon these objectives were the requirements of geometrically safe criticality conditions and provision for cooling the electrolyte. The physical arrangement of dissolver components to achieve these objectives is shown in Figure 2. The anode was formed by electron-beam welding a 4.6 in. diameter by in. thick tantalum disk around the end of a 3,$-in. 0.d. tantalum tube. The i.d. of the tube was 3/16 in. A tantalum flat washer was used at the joint to provide a greater weld area. The anode was faced with a 4.0-in. diameter by 0.020 in. thick platinum disk to maintain electrode conductivity after anodization of the remaining surface of the anode. A l/4-in. hole was drilled through the center of the platinum disk, and the disk was bonded to the tantalum by inside, intermediate, and outside circular passes with the electron-beam welder. The dissolver basket was machined from a block of polypropylene. Eight projections, I/s-in. long, equally spaced around the bottom diameter, supported the basket upon the tantalum surface of the anode. Eight equally spaced external flutes provided solution transfer space between the walls of the basket and the dissolver vessel. The floor of the basket was perforated with an array of l/a-in. holes and contained eight equally spaced radial ribs, & in. high, to support the Plutonium metal button and provide space for the flow of solution between the plutonium metal and the perforated floor of the basket. For similar reasons, the inside surface of the basket wall contained eight equally spaced flutes. The basket was designed to serve two functions: (1)to physically contain the plutonium metal button and to facilitate transfer of the button into the dissolver; (2) to confine the plutonium button between the two electrodes in a manner that would minimize the distances separating the plutonium metal surfaces from the faces of the two electrodes, but prevent direct contact or arcing conditions, without restricting the removal of evolved gas bubbles by the radial flow of dissolver solution entering the dissolver through both the anode and the cathode. The tantalum cathode was fabricated in a manner similar to the anode, but a platinum surface was not required. Eight
Figure 3. Exploded view of the electrolytic dissolver showing the component parts
press-fit Teflon insulators, inserted into holes in the face of the cathode, provided a Y' in. minimum physical separation between the cathode and the surface of the plutonium metal. The cathode was free to move in a vertical motion under the influence of the gravity. The inside surfaces of the dissolver basket, and the center-line hole through the dissolver lid served to guide the downward travel of the cathode as the plutonium metal dissolved. The dissolver vessel was fabricated from 5-in. i.d. glass pipe with 1-in. flanged glass pipe openings. Machined brass connector blocks were used to connect the cathode and anode to neoprene-insulated welding cables. The exposed brass and copper were covered with silicone ruhber to prevent corrosion by acid fumes inside the gloved box. Stainless steel tubing fittings were used to connect polyethylene tubing to the tantalum electrodes. An exploded view of the dissolver is shown in Figure 3, and the assembled dissolver is shown in Figure 4. The removable lid was machined from polypropylene. The lid and cathode assembly was easily removed for inserting the basket. A circular stainless steel ingot was used as a stand-in for plutonium in Figures 3 and 4. A Teflon plug with a machined taper served to seal the anode in the bottom of the dissolver vessel. With the basket in place, the volume of the dissolver, a t overflow conditions, was 800 mL. The total volume, with the overflow blocked, was 1800 mL. Power Supply. The dc power supply, not shown in Figure 2, was rated a t 200 A, and the voltage was limited to about 14 V. Power was transmitted into the gloved box and to the electrodes by size 2lO flexible welding cables. Off-Gas System. A vigorous evolution of oxygen, nitrogen, oxides of nitrogen, and a small amount of hydrogen accompany the electrochemical dissolution process. The off-gas was
Figure 4. Photograph of the assembled dissolver.
controlled by venting the storage tank back to the dissolver and drawing a negative pressure on the dissolver through, first, a condensation trap and then a caustic scrubber. The caustic was replenished after each run. The scrubbed gas was passed through a high efficiency filter, and then discharged into the Ind. Eng. Chem.. Process Des. Dev.. Vol. 16, No. 3. 1977 299
Table I. Pilot Plant Dissolution of Pu Buttons Button weight,
Run no.
1923 1950 1959 1867 2018 2008 2117 2026
1 2 3 4 5 6 7 8
Dissolution current, A Dissolution time, h 12.8
.
13.9 9.9 15.3 17.4 15.0 13.1 14.5
Table 11. Dissolution Data for Experiment No. 1" Dissolutior, time periods, h
Dissolver current, A
Av rate of dissolution in time period, g of Pulh
449 270 150 158 150 9 a Button weight, 1750 g; initial dissolver solution composition, M HF; composition of incremental additions 10 M "03-0.05 t o maintain volume, 10 M "03-0.05 M HF; terminal acid concentration, 5.3 M. 2 2 2 1
150
150
building exhaust system for additional filtration. Sufficient air was drawn through the dissolver to prevent the accumulation of an explosive mixture of hydrogen and oxygen. Process Demonstration Equipment of the size described above is usually not considered to be a pilot plant, but because of nuclear safety requirements, a larger dissolver cannot be used and, indeed, a major plant facility would but consist of two or more such dissolver systems operating in parallel. Because of an urgent need to dissolve eight plutonium metal buttons, an initial production campaign was performed, followed by a more detailed investigation of processing parameters and of problems uncovered in the first campaign. Some of the information obtained during the 8-button campaign is given in Table I. The dissolver system was initially filled with 7.4 1 of 10 M "OB, and additional increments of acid were periodically added to maintain that volume. One button was dissolved a t a time, and all dissolver solution was removed from the system before the next button was placed into the system. The power supply was operated in a constant-current mode of operation and did compensate, within the voltage limitations of the machine, for minor fluctuations in conductivity caused by changes in the distribution of gas bubbles, changes in dissolver solution composition, or changes in the position and geometric configuration of the button as it dissolved. The conductivity of the system increased, and then very slowly decreased as the button was dissolved. The intent was to maintain the dissolver current as close to 150 A as possible, within the limits of the power supply. That power level usually could not be reached a t the start or near the end of a run. The average current values given in Table I represent the average of 12 to 20 readings for each run. The cathode cracked during Run 3 and a section broke off during Run 5 . Hydrogen embrittlement a t low acid concentration ( 5 to 6 MI was suspected. See Bull and Koonce (1970). Correcsive action included reducing the power level to 100 A, 300
Ind. Eng. Chem.. ?:ociss DES. Dev., Voi. 16, No. 3, : S i 7
Min
'4v
M2.x
137 130 120 100 100 100 112 95
146 144 135
150 155 145 100
100
101 102 120 103
Terminal acid concn, M 4 6 7 3 5 7 6
103 105
125 125
10
Table 111. Dissolution Data for Experiment No. 2" Av rate Dissolution Dissolver of dissolution Dissolver solution time periods, current, in time period, H N 0 3 concn, h A g of Pulh M
150 150 145 145 150 150 150
224 218 180 120
10.4 10.1 9.6 8.9 100 8.9 99 9.8 80 10.5 150 67 11.3 74 150 11.0 Ingot weight, 2204 g; initial dissolver solution composition, 12 M "03-0.05 M HF.
starting with Run 4, and replacement of the cathode and use of 15.6 M H N 0 3 for incremental acid additions starting with Run 6. No further cathode problems were observed. The most serious problem encountered was the generation of substantial quantities of very fine dark green-to-black solids. X-ray analysis performed on a single sample indicated most of the material was amorphous, but some crystalline PuO2 was present. Other diffraction patterns, including that for plutonium metal, were absent. The amount of solids per run varied from 3 to 10% of the total plutonium button. The exact nature or mechanism of formation of the solids was not determined; it was found that they dissolved very quickly in 10 M "03 at 80 OC if made 0.03 M in HF. Process Development The amount of development work performed on the electrolytic process was limited to the dissolution of only four pieces of plutonium metal. The four runs were designed to investigate the possible existence of insoluble impurity inclusions in one specific kind of plutonium, to minimize or eliminate the generation of solids during dissolver processing, to provide more quantitative information about the dissolver performance, and to investigate the composition of the dissolver off-gas. Unfortunately, it was necessary to use two kinds of plutonium metal, two hemioblate spheroid buttons, similar in size and shape to the eight buttons previously dissolved, and two right circular cylindrical ingots, 2.25 in. in diameter by 2 in. high. Correlation of dissolver performance was complicated because the buttons occupied most of the area between the faces of the two electrodes, while the ingots occupied only about one-third of the area. The results given in Table I1 show that, excepting the final 9 g, the average rate of dissolution for experiment no. 1 w s 293 gin. A large amount of dark green
Table IV. Dissolution Data for Experiment No. 3a
Dissolution time periods, h
Dissolver current, A
1 1
150 150 130 150 130 100 150
l b
1 l b l b
1 l b
95
1
150 85 150 100 100
l b
1 l b l b
Av rate of dissolution in time period, g of P u b
Dissolver efficiency, g of Pu dissolved/ (Ah)
266 247 218 173
1.77 1.65 1.68 1.15 1.45 1.84 1.04 1.57 0.77 1.72
188
184 156 149 116 146 76 161 134
Dissolver solution "03 concn, M 10.4 10.7 10.3 10.1 9.7 9.8 9.7 9.9 9.9 10.2 10.0 10.3 10.2
0.51
1.61 1.34
a Ingot weight, 2170 g; dissolver solution: initial concentration, 12 M HN03-0.05 M HF; concentration of additions, 14 M "03-0.05 M HF. Flat washer in place around ingot.
solids remained in the dissolver system after the button had been dissolved. Most of the dissolver solution was removed by decantation through a medium porosity glass filter, and the solids were dissolved in 12 M "03-0.05 M H F heated to 70 "C. A material balance accountability of 100.7% confirmed the absence of major quantities of impurities in the metal button. During the second, third, and fourth experiments, dissolution was briefly interrupted after each hour of operation, 1 L of dissolver solution was removed from the dissolver system and filtered through medium porosity glass fritted funnels, and the dissolver solution volume increased to the normal level by the addition of fresh acid. Analytical samples were taken each hour for determination of the dissolver solution acid concentration. During each down-period or during alternate down-periods, depending upon need, the plutonium metal was removed from the dissolver and weighed. These steps permitted more accurate analyses of dissolver operations and control of the acid concentration. In the second experiment the initial acid concentration was increased to 12 M "03-0.05 M H F (see Table 111) in an attempt to reduce the amount of solids formed. The slower dissolution rates reflect the approximate one-third utilization of the cross-sectional area between the electrodes, compared with the button dissolved in the first experiment. Solids could not be seen in the dissolver solution a t any time during this experiment, but a t the end of the fifth hour, the rate of filtration became slower as a thin dark-colored film accumulated on the surface of the filter. Starting at the ninth hour, the acid concentration of the incremental additions was increased from M H F to 15.6 M "03-0.05 M HF. The 12 M "03-0.05 filtration problem was corrected when the acid concentration in the dissolver solution was increased above 10 M. In experiment no. 3 the acid concentration in the dissolver solution was maintained very close to 10 M by starting with 12 M H N 0 3 and replacing the hourly withdrawals with 14 M HN03. Initially, the dissolver solution contained no HF. However, after only 10 min, large amounts of brown solids were present in the system, and H F was added to give a concentration of 0.05 M HF. The solids were dissolved in a short period of time and were not thereafter observed. Solids were not found on the filters. A flat polyethylene washer, designed to fit around the Pu ingot and blind the uncovered area between the electrodes, was used in those dissolving periods marked with an asterisk in Table IV. The washer limited the dissolver current by impeding the current path directly through the solution, but had no effect upon the rate of dis-
420
360
320
. i
r
280
L
. 240
oi
Y
4 c
z
E
200
c 3 i
0
m y1
160
120
80
40
0
2
4
6
a
DISSOLVING TIME,
10
12
14
hr
Figure 5. Effect upon the dissolution rate of a Pu ingot of using a flat washer to blind unneeded electrode surface area.
solution until the ingot height was decreased to about 3/4in. At that point, with the electrodes increasingly closer together, the washer was effective in increasing the rate of dissolution compared with no washer. This effect is illustrated in Figure 5. The data in Table IV show that use of the washer significantly increased the current efficiency of the dissolver. The fourth experiment was designed to provide dissolver off-gas samples and to investigate the possibility of using HF concentrations less than 0.05 M. The experimental results from the dissolution are given in Table V. In contrast to the ingots dissolved in the two previous experiments, the nearly 4-in. diameter of the button dissolved in this experiment intercepted most of the area between the electrodes and dissolved at a much faster rate. Unfortunately, Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 3, 1977
301
Table V. Dissolution Data for Experiment No.4 " Dissolver solution Dissolution time periods, h
Dissolver current, A
Av rate of dissolution in time period, g of Pulh
"03
concn, M
HF concn, M
150 150
406 10.8 0.025 377 10.0 0.025 150 348 9.5 0.025 150 306 8.8 0.050 150 176 8.4 0.050 150 92 0.050 150 16 8.6 0.050 a Button weight, 1722 g; dissolver solution "03 concentration: initial, 12 M; incremental additions, 14 M; dissolver solution HF concentration: initial, 0.005 M; changes noted above. 1 1 1 1 1 1 2
the button had been quartered before delivery, and the bottom of the dissolver basket had deteriorated sufficiently that the pieces had to be placed around the outer edge of the basket as they became smaller. This prolonged the dissolution of the last 150 g of metal. The fourth experiment was initiated with the dissolver H F concentration at 0.005 M. Brown solids were generated during the first 10 min, and the H F concentration was increased to 0.025 M. This change alleviated the solids problem for about 2 h. When dark green solids were then observed, the H F concentration was increased to 0.05 M. This increase did not completely eliminate the solids. The small amount of solids remaining in the system a t the end of the run was rapidly M HF. The generation of a small dissolved in 14 M "03-0.05 amount of solids in this experiment, compared with the absence of solids in the two previous experiments, could be attributed to the decrease of the dissolver acid concentration below 9.5 M, to the higher concentration of plutonium in the dissolver solution, or directly to the higher dissolution rate which leads to the other two effects. Analyses of dissolver off-gas samples taken during the first, third, and sixth hours of operation revealed a wide range of compositions. Hydrogen compositions of 3.5, 6.2, and 1.4 mol % indicate the need for a good air sweep through the system to keep the hydrogen composition a t a safe level and a detection system to verify the composition. Conclusions There were no apparent problems with the platinum-faced tantalum anode or with the tantalum cathode a t higher acid conditions. However, because of the relatively small amount of material needed for each dissolver and the limited number of dissolvers required, the possibility of circumventing po-
302
Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 3, 1977
tential problems by utilizing all-platinum electrodes should be considered. The bottom of the polypropylene dissolver basket did deteriorate, and the basket could not have been used for another experiment. This damage could have resulted from chemical attack, mechanical shock, or from the passage of electrical current through the bottom. Such baskets might be considered expendable, but fabrication from other plastics should be investigated. An electrically isolated tantalum or niobium basket might prove durable. The gross rate of dissolution of plutonium metal depends upon several factors, including the cross-sectional area of the plutonium metal piece, and the electrode current density. Excepting the final 100 g of metal, the average dissolution rate for the 4-in. diameter button in the fourth experiment was 323 g/h. Even with the 2.25-in. diameter ingot dissolved in the third experiment, the average dissolution rate exceeded 160 g/h. In both cases, the dissolver was operated a t 150 A. The generation of solids during the dissolution can be eliminated or reduced to an insignificant level by maintaining the nitric acid concentration in the dissolver solution above 10 M and adding hydrofluoric acid to a level of 0.05 M. The effects of higher concentrations were not investigated. In a production unit, the acid concentration could most easily be controlled by a continuous removal of dissolver solution and replenishment with fresh acid. A solution of 14 M "03-0.05 M H F should prove to be close to optimum for this purpose. Acknowledgments The authors thank R. D. Dierks for assistance in designing and security fabrication of some of the dissolver components and T. R. Myers for his participation in the experimental work. Literature Cited Bull, H., 111, Koonce, J. E., Jr., "Performance of an Electrolytic Dissolver at the Savannah River Plant,'' USAEC Report No. DP-MS-70-41, presented at the Symposium of Chemical Reprocessing of Irradiated Nuclear Fuels, American Institute of Chemical Engineers Meeting, Chicago, Ill., Nov 29-Dec 3, 1970. Caracciolo, V. P., Owen, J. H., "3.1 Electrolytic Dissolution of Power Reactor Fuel Elements," "Progress in Nuclear Energy Series Ill.,Process Chemistry." Vol. 4, C. E. Stevenson, E. A. Mason, and A. T. Gresky, Ed., Pergamon Press, New York. N.Y. 1970. Fox, R. D., "Electrolytic Dissolver for Plutonium Metal," USAEC Report No. ARH-2203, Sept 20, 1971. Miner, F. J., Nairn, J. H., Berry, J. W., Ind. Eng. Chem., Prod. Res. Dev., 8, 402-405 (1969). Pitzer, E. C., "Electrolytic Dissolution of Stainless Steel Clad Fuel Assemblies," USAEC ReDOrt No. KAPL-653, Dec 1951
Receiued for reuiew May 3, 1976 Accepted March 24,1977
This work was conducted by Batelle-Northwest,Richland, Washington, for the U.S. Energy Research and Development Administration under Contract No. AT(46-1)-1830.
