I
LEONARD W. NIEDRACH', ARTHUR C. GLAMM, MARY E. BRENNAN, and BURTON E. DEARING
Knolls Atomic Power Laboratory, General Electric Co., Schenectady,
N.Y.
Recovery of Uranium from Stainless Steel Fuel Elements This process, developed for processing the core from the submarine Seawolf, should be suitable with minor modification in processing the current APPR fuel vents normally used when highly radioactive materials are involved. As stainless steel is not normally susceptible to direct attack by nitric acid, other methods of fuel dissolution had to be considered. Where the stainless steel is used only as a cladding material, it appears feasible to cut the fuel element so as to expose the contained fuel to dissolution by a leaching reagent-e.g., nitric acid (8). Where the stainless steel is used also as a dispersant agent for the fuel (normally as oxide), the fuel particles are in suspension in a solid matrix of stainless steel. I n such a case, cutting the fuel element does not expose all the fuel and a complete disintegration of the fuel element must be considered. Active consideration has been given to the possibility of using aqua regia to attack such fuel elements (70). When this reagent is used, the chloride concentration must be reduced to the parts per million level at the end of the dissolution step, to avoid serious difficulties with corrosion during later steps in the processing. Several approaches have been taken to make stainless steel fuel elements susceptible to nitric acid
RECENT summarids of aqueous methods for the processing of reactor fuels (3, 4 ) have reviewed the philosophy of processing, the reasons for processing, and radioactive effects on processing schemes and cooling times. Most reports deal with processing of pure uranium fuels, or fuels that contain aluminum as an alloying or cladding agent, which can be readily removed or dissolved by simple processing methods. The present report deals with processing fuels that contain stainless steel as an important constituent and/or a cladding material.
Dissolution of Fuel In any aqueous processing method the first step is dissolution of the fuel element, to obtain an aqueous feed suitable for one of the solvent extraction methods used for the ultimate processing. In solvent extraction of spent fuel, nitric acid solutions are desirable because uranyl and plutonium nitrates are most readily extracted into the organic solPresent address, Research Laboratory, General Electric Co., Schenectady, N. Y .
ACID FOR
DILUTANT
attack-carburization with methane at high temperatures produces enough carbide in the alloy so that attack by nitric acid occurs (7). Stainless steel can be activated to nitric acid attack by making it the anode in an electrolysis cell and electrolytically dissolving the steel in the acid (9). The present work has studied sulfuric acid as a medium for dissolution of the stainless steel fuel element. Nitric acid is subsequently added to dissolve the actual oxide suspension that does not go into solution during the sulfuric acid attack.
Processing of UraniumStainless Steel Fuel The process described, wwhile not universally applicable, should serve as a guide for processing other stainless steel fuel elements. As the uranium dioxide- stainless steel fuel was to be processed in the chemical processing plant at the reactor test station in Idaho, attention was devoted only to a socalled "head-end" treatment. The principal purpose of this treatment was to prepare a feed compatible with existing
AX I O X T E P IN
,
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HC DILUENT
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STAINLESS STEEL
uoz (NO3 )e STAINLESS S T E E L CONSTITUENTS N 35q/R N4.5M
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.
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REL.VOL. 4.0
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STAINLESS STEEL CONSTITUENTS
MAKE.UP
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9EL.VOL.
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TO W A S T E G A S FACILITIES
BW
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The head end process outlined has been tested VOL. 50, NO. 5
MAY 1958
763
47ay
450r 400L I
200 r
, 10
20
30
4'0
50
60
70
80
90
I00
11'0
,
,
20 30 40 P E R CENT R E D U C T I O N IN A R E A
50
60
I20
TIME IMINUTES)
Figure 1. Large variations in dissolution rates occur in 6M sulfuric acid
equipment, without sulfate or stainless steel constituents. Recovery of plutonium was not considered, because of the high uranium enrichment of the fuel. The bulk of the decontamination was to be achieved by subsequent passage of the fuel material through the existing processing plant. This plant has been described ( 6 ) . I n the head-end process the appropriate number of fuel elements are charged to a thoroughly rinsed dissolver vessel. Sulfuric acid is then added and brought to temperature. Varying concentrations* and amounts of excess sulfuric acid are required to dissolve various stainless steels. M'ith some steels, 4,tl sulfuric acid in 100% excess of the stoichiometric amount was satisfactory, in other cases a 400% excess of 6,M acid was required for dissolution at similar and reasonable rate. After the stainless steel has been dissolved (as evidenced by the diminishing rate of evolution of the hydrogen off-gas), nitric acid is slowly added to the dissolver solution. This dissolves the uranium oxide fuel. The off-gas from the sulfuric acid attack is largely hydrogen; nitrogen dioxide is the main product from the nitric acid step. If a niobiumstabilized steel is used a sludge of niobium carbide will be present at the end of the dissolution step. In certain types of contacting equipment-e.g., pulse columns and mixer-settlers-filtration or centrifugation is unnecessary, if the amount of sludge is not excessive. The dissolver solution is then adjusted to the desired feed concentration by adding water and nitric acid. Moderate amounts of nitric acid are required in the feed solution to give satisfactory extraction coefficients for the uranium into the solvent. The large quantities of iron salts present in a feed of this type require a n acid solution to avoid precipitation. I n acid solutions containing excess sulfate, however, iron sulfate is still not extremely soluble and feeds fairly dilute in uranium result for processing.
764
Figure 2.
Mechanical history
The need for diluting the feed can be minimized by minimizing the amount of excess sulfuric acid used in dissolution. For the solvent extraction, a tributyl phosphate (TBP) type of flowsheet was chosen because of stability under acid conditions. Tributyl phosphate (10 volume %) in a hydrocarbon diluent was chosen. Three separate contactors are used. I n the first, A, uranium is separated from the stainless steel components. Contactor B introduces a scrub, which reduces the nitric acid concentration of the organic phase. A low concentration in the ultimate aqueous product is desirable because of a subsequent concentration step required to cut down the volumes to be handled in later processing cycles. An extraction section is required in contactor B to extract back any uranium that has transferred to the aqueous phase in passage through the scrub section. I t is possible that contactors A and B could be combined into one unit, although the separate units are completely satisfactory. In contactor C, the uranium is stripped back into a n aqueous phase. A fourth contactor D, may be used, in which the aqueous phase is simply washed with the hydrocarbon diluent to remove the last traces of tributyl phosphate from the aqueous stream prior to the evaporation step. The head-end process has been tested on a pilot plant scale with regard to the dissolution and solvent extraction steps.
Material of Construction for Dissolver Vessel Aside from the dissolver vessel, normal materials of construction should be adequate, because the nitric acid in the streams acts as a corrosion inhibitor. I n the case of the dissolver vessel, corrosive conditions are exceedingly severe. After considerable research, Carpenter20 stainless steel was selected as best for resistance to corrosion and shock and feasibility of fabrication ( 5 ) . A small
INDUSTRIAL AND ENGINEERING CHEMISTRY
of the steel is important
test dissolver was cycled through over 70 dissolvings in the laboratory with no serious corrosion, the pilot plant equipment fabricated from this material performed satisfactorily.
Dissolution Step Both regular Type 347 stainless steel and a special melt from the uranium dioxide-stainless steel fuel were used in the dissolution step testing. Large variations in dissolution rates occur. The data in Figure 1 were obtained by dissolving a weighed amount of steel in a measured amount of acid and collecting the hydrogen off-gas. The precise factors involved in determining the rate of dissolution were never determined. but possible contributing factors are the concentration of major alloying agents and of impurities. especially carbon, and the past history of the steel with regard to cold working and heat treatments. That the past mechanical history of ' 1 ure the steel is important is shown by F'g 2, in which the rates of dissolution of steels having various degrees of cold work are plotted. Cold working the steel increases the dissolution rate. Having established that the history and nature of the stainless steel are important in determining the dissolution rate, the effect of sulfuric acid concentrations was investigated The typical data in Figure 3 were obtained in the same manner as those in Figure 1. Samples of the special melt (M-1 steel) were used with the 100% excess over the stoichiometric amount of sulfuric acid required for dissolution of the steel. Although the rate of dissolution increases with concentration up to 7 M acid, corrosion problems eventually limit the operating concentration to a maximum of about 6 M . Data are summarized in Table I on the behavior of two steels. The special melt is very resistant, whereas ordinary 347 steel is fairly readily attacked by sulfuric acid a t much lower concentrations and much smaller excesses.
U R A N I U M RECOVERY 475 450
ORGANIC OILUENT AMSCO 123-15
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50 0
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20
30
40
50
60
70
80
90 100 110
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120
T I M E (MINUTES)
Figure 3. Rates of dissolution of annealed M-1-Type 347 stainless steel increase with concentration up to 7M acid
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0.1
All the work was performed in borosilicate glass equipment. When dissolving tests were made in a Carpenter steel vessel, however, the steel samples tended to remain passive in the acid. I t was necessary to trigger the dissolution reaction by touching the steel sample with a piece of active metal, such as a small nail or a piece of zinc, or to make steel cathodic momentarily by connection to a battery. Either procedure depassivates the surface of the steel and dissolution begins. This depassivation step was incorporated into the plant procedure by attaching steel wool or a band of pure iron to the fuel elements to be dissolved. This triggered the reaction without difficulty in the pilot plant tests. Because sulfuric acid dissolution of the stainless steel and the mixed acid dissolutionsstep on the residual fuel oxide material were to be done in the same vessel, the effect of residual nitrate from one dissolution on the following dissolution was studied. Efficient rinsing of the dissolver vessel was required to avoid passivation of the stainless steel in the next dissolution. If greater than 0.01M Table 1. Effect of Acid Concentration and Excess Acid on Dissolution Time of Type 3 4 7 Steel in Sulfuric Acid Dissolution Time, Hours HBO,, Excess Type M Acid, %" 347 M-1 2.0 19 4 100 '
5 5.5
200
0.8
13
400 400 300
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11
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..
8
1.5-5
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nitric acid was present in the sulfuric acid dissolving solution, the normal triggering action would not initiate dissolution of the steel. Because most of the tests on dissolution were made with nonirradiated materials, it was necessary to determine whether or not irradiation of stainless steel in a reactor wouId alter its dissolution rate. In Figure 2 there is no indication that irradiation markedly affects the dissolution rate. Once the sulfuric acid step is accomplished, further dissolution of the fuel oxides is no problem, Addition of nitric acid causes rapid oxide dissolution. I n actual practice, the rate of addition of this acid must be controlled to prevent excessive frothing of the dissolver solution. For complete dissolution, however, a t least a stoichiometric amount of acid must ultimately be added. Critical control of the actual amount was not necessary and the reaction went so easily that the details of the step were not studied.
t I
0
04
I n the extraction contactor A, the major variables of concern in establishing process conditions are the tributyl phosphate concentration of the organic phase and the sulfuric acid concentration in the aqueous phase (Figure 4). In all tests of the distribution, concentrations of uranium tin the solutions were low, so that data would be obtained in the linear portion of the uranium X-Y diagram and extraneous effects due to saturation of the tributyl phosphate phase avoided. The effect of the tributyl phosphate concentrations on the acid distribution coefficient is shown in Figure 5. For any given tributyl phosphate concentration the acid distribution ratio was constant over the tenfold range of sulfate concentrations studied.
12
2.0
1.6
MOLARITY
Figure 4. Uranium distribution between aqueous sulfate solutions and diluted tributyl phosphate systems
EOUAL VOLUME EXTRACTIONS AQUEOUS PHASE: 3.7 MH'
S s .!:;:.:3.0 M NOy(ADDED AS "03 ANO/OR NONOS)
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AQUEOUS SO:
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PERCENT TBP I N AMSCO 123-15
Figure 5. For any given tributyl phosphate concentration acid distribution was constant over the sulfate range studied
A wide latitude of choice in operating conditions for the flowsheet was not possible, because of the large excess of sulfuric acid required in the dissolution step. I t became a matter of minimizing the sulfate concentration in the feed. Following this, a tributyl phosphate concentration was chosen, to give satisfactory recovery with a moderate number of stages in the contactor. Equilibrium data for the design of contactor B were also obtained in the laboratory. Neutral aluminum nitrate was chosen as the scrubbing solution, to remove fission products as well as excess VOL. 50,
NO. 5
MAY 1958
765
.
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EQUAL VOLUME EXTRACTIONS TOTAL URANIUM 10-3M
075M ANN1
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P ORGANIC DILUEVT. AMSCO 123-15 EQUAL VOLUHE EXTRACTIONS TOTAL U R A N I U M 10%
Y 0
05
1 10 15 2 0 M O L A R I T Y “0, I N AQUEOUS
25
30
7. Nitric acid equilibrium between 10% tributyl phosphate in Amsca 123-1 5 and various aluminum nitrate solutions
Figure 0 05 IO 15 20 25 MOLARITY “03 I N AQUEOUS AT E Q U I L I B R I U M
Figure 6. Uranium distribution between 10% tributyl phosphate and nitric acid-ANN aqueous solutions
acid from the organic stream before the uranium was stripped back into the water. Data obtained show the effects of varying the aluminum nitrate and nitric acid concentrations on the uranium and acid distribution coefficients. Batch experiments were performed at room temperature using 10% tributyl phosphate in Amsco 123-15. All acid, uranium, and salts were originally present in the aqueous phase and equal phase volumes were employed. The original uranium concentrations in the aqueous phases were 0.001M. The data for uranium extraction are shown in Figure G and for the acid in Figure 7. Variations in the extraction coefficient for the acid are straightforward, with increases in acid concentration paralleling increases in salting strength. In the case of uranium, the situation is more complicated. There is an inversion of trends a t 0.75M aluminum nitrate, associated with competition between the uranium and nitric acid for the tributyl phosphate in the organic phase. In contactor C uranium is stripped from the organic phase by rinsing countercurrently with water or very dilute nitric acid. High distribution coefficients favoring the aqueous phase are maintained for uranium and transfer can occur in a small number of stages. Adequate equilibrium data for this contactor are available (7).
Demonstration of Flowsheet with Irradiated Material Fission product decontamination obtainable across the extraction cycle of the head-end flowsheet was demonstrated through the use of a miniaturemixer settler ( 2 ) . I n these runs contactors A, B, and C of the pre-extraction
766
cycle were arranged in a three-bank cascade. The ruthenium decontamination was less than the average beta decontamination in both contactors A and B. Actually the ruthenium decontamination is a limiting factor in determining both the beta and gamma activity of the product stream. The niobium and zirconium activities are important contributors to the gamma activity of the products. Because the extraction coefficients for the fission products in tributyl phosphate are extremely low, the decontamination factor across contactor A is almost completely determined by conditions at the feed stage. The number of stages used in such a contactor is therefore unimportant from the point of view of decontamination, although it determines the extent of uranium loss. Most of the actual decontamination occurs in contactor A, but a small additional amount occurs in contactor B. In this case, much depends upon the number of stages in the scrub section, but each successive scrub is less efficient in enhancing the decontamination. O n the basis of the miniature mixer-settler data, the resulting over-all decontamination factor obtained in the head-end process was less than that normally obtained in the absence of sulfuric acid.
Summary and Conclusions Sulfuric acid is an adequate reagent for dissolving fuel elements made of stainless steel. Its use, however, introduces complications in the subsequent solvent extraction process for uranium purification. The sulfate markedly decreases the extraction of uranium into tributyl phosphate. When large amounts of sulfuric acid are required to effect dissolution, steel concentration in the feed solution is limited and fairly dilute streams are required for processing. A proper choice of stainless steel, when sulfuric acid is the
INDUSTRIAL AND ENGINEERING CHEMISTRY
dissolution agent, can reduce these difficulties. I t should be possible to design a reasonable flowsheet employing sulfuric acid for recovery of fuel from a stainless steel element. The advantage of the sulfuric acid dissolution step over other methods for processing stainless steel fuels is that standard chemical engineering equipment and operations normally employed in the solvent extraction processing of fuels are used throughout. No special high temperature equipment is required and only one type of operation is needed to put the fuel in solution. The major equipment problem is the need of an exceedingly corrosion-resistant dissolver vessel.
Literature Cited (1) Adams, R. E., U. S. Atomic Energy Comm TID-7502, Part I, p. 146 (l$55). (2) Alter, H. W., Codding, J. W., .Jennings, A. S., Anal. Chem. 26, 1357 (1954). ( 3 ) Anderson. E. L.,Nucleonics 15, No. 10, 72 ‘(1957): (4) Bruce, F. R., Fletcher, J. M., Hyman, H. H., Katz, J. J . , eds., “Progress in Nuclear Energy,” Serles 111, “Process Chemistry,” McGrawHill, New York, 1956. (5) Dearing, B. E., Niedrach, L. W., U. S. Atomic Energy Comm., KAPL-1047 (1954) (Classified). (6) Lemon, R. B., Reed, D. C., “Experience with a Direct Maintenance Radiochemical Processing Plant,” International Conference on Peaceful Uses of Atomic Energy, vol. 9, 464, Paper 543, United Nations, New York, 1956. (7) Long, R. H., Lindroos, A. E., Randall, C. C., “Reactor Handbook,” vol. 4, “Fuel Processing,” 1st ed., Technical Information Service, U. S. Atomlc Energy Comm., 1953. (8) Miller, R. S., Smith, D. J., “Fifth Hot Laboratories and Equipment Conference,” p. 144, Am. SOC. Mech. Engrs., New York, 1957. (9) Pitzer, E. C., private communication. (IO) Savolainen, J. E., Blanco, R. E., Chem. Eng. Progr. 53, 78F (1957).
.,
RECEIVED for review August 22, 1957 ACCEPTED November 18, 1957
