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Decontamination of transuranic contaminated metals by melt refining
NUCLEAR AND CHFMICAL WASTE MANAGEMENT,Vol. 4, pp. 129-134,1983 Printed in the USA. Al1rights reserved.
0191-815X183 $3.00 + .oO Copyright 0 1983Pergamon Press Ltd.
DECONTAMINATION OF TRANSURANIC CONTAMINATED METALS BY MELT REFINING B. HeshmatpouP G. L. Copeìand R. L. Heestand Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA
ABSTRACT.
Melt refining of transuranic contaminated metals is a possible decontamination process with the potential advantages of producing metal for reuse and of simplifying chemical analyses. By routinely achieving the 10 nCi/g ( - 0.1 ppm) leve1 by melt refining, scrap metal can be removed from the transuranic waste category. To demonstrate the effectiveness of this melt refïning process, mild steel, stainless steel, nickel, and copper were contaminated with 500 ppm &g/g) PuO, and melted with various fluxes. The solidified slags and metals were analyzed for their plutonium contents, and corresponding partition ratios for plutonium were calculated. Some metals were double refined in order to study the effect of secondary slag treatment. The initial weight of the slags was also varied to investigate the effect of slag weight on the degree of plutonium removal. In general, al1 four metals could be decontaminated below 1 ppm @g/g) Pu (- 100 nCi/g) by a single slag treatment. Doubling the slag weight did not improve decontamination significantly; however, double slag treatment using 5 wt.% slag did decontaminate the metals to below 0.1 ppm (&g) Pu (10 nCi/g).
INTRODUCTION
glovebox enclosures in the nuclear industry. The high tost of retrievable storage and its ultimate disposal in a repository is a strong incentive to reduce the volume requiring storage. Furthermore, the wastage of scarce and strategie metals and alloys is also a concern. Smelting of surface contaminated scrap is attractive, because the generated ingots (or other forms) are of convenient shape for high density storage or grout infusion. In addition, radionuclides could be concentrated in the slag with possible recovery. Because of ingot homogeneity, the degree of residual contamination of the metal could easily be measured. The method of disposition or possible reuse of the metal, if it is sufficiently decontaminated, may then be identified. The thermodynamic principle underlying smelting behavior is that an alloying element wil1 partition between metal and slag toward an equilibrium in which its activity is the same in both phases. The strong oxide-forming tendency of plutonium wil1 favor its removal from the parent metal by an oxidizing slag to yield a clean metal. Other TRU elements are also strong oxide formers that would be expected to concentrate in slags. Thermodynamic calculations of slagmetal distribution of several contaminant elements in common metals at elevated temperatures have indicated their concentration in slags (1). Prior work in this area has shown that melting for
Nuclear operations that involve work in transuranic (TRU) elements have resulted in the accumulation of large quantities of contaminated waste. Metals constitute a large fraction of the volume of such waste and cannot be burned or otherwise treated as can combustible materials. Machines, tools, and structures associated with the nuclear fuel processing are subject to surface and crevice contamination. Airborne dusts of uranium and TRU compounds are common sources of low leve1 contamination. Furthermore, detection of low levels of contamination in complicated metallic forms is difficult. Particular precautions are taken in working with TRU contaminated materials due to their biological hazard. These materials are always handled within 1982; ACCEPTED 6 NOVEMBER1982. *Present address: Thermo Electron Corporation, Metals Division, 115 Eames Street, P.O. Box 132, Wilmington, MA 01887. Acknowledgements-The authors wish to thank W. B. Stines for his technical assistance during this study. This work was sponsored by the Office of Waste Operations and Technology, U.S. Department of Energy, under contract W-7405-eng-26 with the Union Carbide Corporation. Thanks are also extended to the many people who contributed to this project. We especially thank W. R. Laing and T. G. Scott from the Analytical Chemistry Division for analyzing the samples, R. G. Donnelly and F. J. Homan from the Metals and Ceramics Division for contributing substantial guidance and stimuiating discussions, E. L. Long, Jr. and J. M. Robbins for reviewing the manuscript, and P. T. Thornton for preparing the paper for publication. RECEIVED15 MAY
129
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HESHMATPOUR, C. L. COPELAND, AND R. L. HEESTAND
volume reduction is a technically feasible approach to the consolidating of metal waste and that melting may remove uranium contamination from many metals (1). The removal of TRU elements appears feasible also, but has not been extensively demonstrated. Melt refining of steels contaminated with plutonium has been suggested as a possible decontamination process with the potential advantages of producing steel for reuse and of simplifying analytical problems (2). Results of laboratory-scale investigation have indicated a high degree of decontamination by pyrochemical methods of plutonium- and americium-contaminated metals (3,4). In general, these studies were limited to mild steel, stainless steel, and nickel. The effects of slag weight and composition were not extensively investigated. In a previous study, the authors investigated the effect of slag type and composition, melting technique, and refractory materials on decontamination of uranium contaminated metals and alloys (5,6). Resistance furnace melting appeared to be a better melting technique for nonferrous scrap while induction melting was more suitable for ferrous metals. The degree of decontamination was not sensitive to slag type and composition. However, borosilicate and basic oxidizing slags were more effective on ferrous metals and copper; NaNOJ-NaCl-NaOH type slags were desirable for zinc, lead, and tin; fluoride type slags were effective for decontamination of aluminum. Recrystallized alumina proved to be the most compatible refractory for melt refining of both ferrous and nonferrous metals while graphite was suitable only for nonferrous metal processing. Results showed melt refining to be an effective technique for volume reduction and decontamination of contaminated metal scrap when proper slags, melting techniques, and refractories were used.
THERMODYNAMICS The thermodynamic treatment of the removal of TRU contaminants from a metal by melting assumes that the contaminant can combine with oxygen and enter a slag that acts as an inert solvent for the resulting oxides (1). It also assumes a complete thermodynamic equilibrium between the liquid metal and the slag. Oxidation of plutonium in infinite dilution with liquid iron can be expressed (5,7) as Pu + 2 FeO(slag) ?I PuO,(slag)
+ 2Fe(liq),
AG” = AG” (PuOJ - 2AG” (FeO) - RT ln[(2.307 x 10-3)yo(Pu)] =-RT In X(Pu02)X2(Fe) _V(FeO)(wt.% Pu) ’
(1)
(4
where AC” is the standard Gibbs free energy of reaction, AG”(PuOJ and AG”(Fe0) are the standard
Gibbs free energies of formation of Pu02 and FeO, respectively, R is the ideal gas constant, T is the temperature in ’K, y “(Pu) is the activity coefficient of plutonium in liquid iron, X represents the atomic (or mole) fraction of various species. The standard state of plutonium in liquid iron is taken as 1 wt.% solution. For slag composition of 50 Cao-44 SiO,FeO-1 Pu02 (wt.Vo), and a temperature of 1900 “K, Eq. 2 reduces to (7,8) wt.% Pu = 6.4 x 10-‘“/y”(Pu).
(3)
The partition ratio, X, is defined as the amount of contaminant in the slag, m(Pu)(slag), divided by the amount of contaminant in the melt, m(Pu)(melt); that is, x = m(Pu)(slag) m(Pu)(melt)
.
(4)
If the weight of slag is taken as 10 wt.% of the metal, Eqs. 3 and 4 wil1 result in (5) x = (1.4 x 106)y”(Pu).
(5)
We have not identified a reported value either for y “(Pu) or for y”(U) in liquid iron. Our results (7) indicate a value of about 3 x lO* for X in case of plutonium decontamination, which translates into about 2 x lom4 for yo from Eq. 5 and about 3.0 x 10V6for wt.% Pu (0.03 ppm) from Eq. 3 in liquid iron. Thus, theoretically 10 nCi/g (- 0.1 ppm) limit can be reached by melt refining. Furthermore, this analysis indicates that y “(Pu) is slightly larger than y’(U), which is consistent with the lower thermodynamic stability of PuOZ compared with that of uo,. The actual experimental melt refining test usually results in higher TRU or uranium contents in the metal than is predicted by thermodynamics. In practice, equilibrium conditions are not usually reached, and some slag particles are also entrapped in solidified ingots, giving rise to higher analytical results for plutonium content of the ingots. This problem can be overcome by improved liquid metal-slag separation. EXPERIMENTAL
METHODS
Previous melt refining studies on uranium contaminated metals have shown that a reasonable way to simulate melting of a contaminated metal is to premix slag and contaminant (PuO, in this case) prior to loading into a crucible that contains the metal of interest (1). The crucible assembly is then heated above the melting point of the metal and the slag and allowed to equilibrate. Since both UOZ and PuO, dissolve easily in slag, the transfer of uranium or plutonium to the melt wil1 be uniform. Theoretically, the result wil1 be the same regardless of how the sample is contaminated, because the equilibrium ac-
MJZLTREFINING OF TRU CONTAMINATED METALS
l31
tivity of contaminant is the same in both melt and slag. Compatibility studies for various crucible materials and several types of metallurgical slags have shown the best results were obtained with recrystallized alumina refractories (5). Effective slag types and compositions were identified for decontamination of uranium contaminated metals in a previous investigation (5). These slags were also used in melt refining studies of plutonium contaminated metals because of the similarities in chemical behavior of uranium and plutonium. Al1 experiments were conducted inside a glove box assembly to prevent any biological hazard from exposure to plutonium. To account for the background contamination of experimental samples from the glove box assembly previously used, several clean samples of mild steel and aluminum were melted inside the glove box assembly without adding any PuO, or slag. The reason for choosing these two metals for this particular experiment was the differente in their reactivity with respect to oxygen. Iron is more like steels, nickel, and other nonferrous metals of interest that have less tendency to retain contamination after melting; aluminum tends to react with contaminant oxides and thus retains higher concentration of contaminants after melting. The solidified ingots were sampled and analyzed for their plutonium contents. To study the solubility of contaminant or its degree of retention in the metal by melting, samples of several metals were contaminated by adding 500 ppm bg/g) PuO, to the metal sample and were melted without any fluxing agent. The solidified ingots were also sampled and analyzed for their plutonium contents. In a typical melt refining experiment, 500 ppm (pg/g) PuO, was thoroughly mixed with the flux and added to the metal in the crucible, intentionally contaminating the metal. Recrystallized alumina crucibles were used throughout this study because of their high slag resistance. The crucible assembly was placed inside a graphite crucible, and a graphite disk was placed on the top to avoid damage to heating elements of the furnace by accidental splash of liquid metal or slag. The mixture was melted in a tungsten-mesh resistance furnace under an argon atmosphere. After the slag-metal mixture melted, the temperature was raised further (- 100-150 “C), and sufficient time (0.5-1 h) was allowed for slag-metal equilibrium. It was then allowed to solidify in situ. Solidified slags were glassy and brittle and were separated easily from the metallic ingots and crushed into smal1 fragments for sampling. Metallic ingots were drilled in the middle to the depth of about 5 mm in two different locations, and the chips were discarded. This was done to remove any slag from the surface of the ingot. Samples of the ingot were
taken from the same two holes by further drilling. Care was taken to minimize recontamination of the metal samples; however, some cross contamination did occur and led to occasional scatter in the data. The samples of the metals and the slags were analyzed for their plutonium content by two different analyatical techniques. In the first technique, plutonium in the dissolved sample was extracted as a pure plutonium solution by thenoyltrifluoroacetone (TTA) extraction and the plutonium content of this solution was then measured by counting its CYactivity. This technique is sensitive to about 0.2 to 1 ppm (pg/g), depending on the sample. The second technique is sensitive to 0.01 ppm &g/g) and lower due to more sensitive counters, lower background, and purer materials. A portion of the dissolved sample was equilibrated with 242Putracer then the plutonium isotopes were coprecipitated with the metals as hydroxides. The hydroxides were dissolved, mixed with Al(NO,),, oxidized with KMn04, and extracted with methyl isobutyl ketone (hexone). Plutonium isotopes were back-extracted from the hexone, coprecipitated with praseodymium fluoride which was dissolved in dilute nitric acid, and extracted with TTA-xylene. The organic extract was evaporated on stainless steel disks and evaluated by low-background (Y counting and CX spectrometry. Proseodymium fluoride was used in preferente to the other rare earths because it is available in higher purity (lower background a). Metal refining was used to measure the partition ratio (defined as the amount of contaminant in the slag divided by the amount in the melt) for plutonium in several metals and slags. This investigation included mild steel, stainless steel, nickel, and copper. The slags used in the experimental work were: borosilicate glass (80 SiO, - 13 B203 - 4 Na,0 - 2 Al *03 - 1 K,O wt.%), blast furnace flux (40 Ca0 - 30 SiO, - 10 A1203 - 15 Fe20J - 5 CaF wt. Vo), high silica flux (60 SiO,- 30 Cao- 10 A1203 wt.Vo), and artificial basalt flux (42 SiO - 8 A1203 - 26 FesO, - 12 Cao-6 MgO-4 Na,O-2 K20 wt.%). The weight of metal samples was 200 to 300 g, and the corresponding slags were either 10% or 5% of the weight of metal sample. A few of the samples were double refined by separating the first slag from the ingot after solidification and remelting the ingot with a clean slag. This was done to examine whether double refining would produce a cleaner metal ingot.
RESULTS
AND DISCUSSION
Background Measures To measure the background contamination of experimental samples from that of the glove box assembly, five clean samples of mild steel (each
132
B. HJWRWATPOUR, G. L. COPELAND, AND R. L. HBESTAND
weighing 300 g) and three clean samples of aluminum (each weighing 150 g) were melted by resistance furnace heating inside the glove box assembly. The mild steel experiments were conducted at 1600 “C and those of aluminum at 1000 “C. Each sample was held for about 1 h at these temperatures after melting. Recrystallized alumina crucibles (placed inside a larger graphite crucible with a graphite disk on top) were used in al1 cases. NO fluxing agents and no Pu02 were added to these samples. Solidified ingots were sampled and analyzed for their plutonium contents. The results of these tests indicated an average plutonium content of 0.012 ppm @g/g) for mild steel and an average plutonium content of 0.48 ppm bg/g) for aluminum samples. These two quantities represent the average background contamination of the test samples from the glove box assembly and were used to make the necessary corrections of plutonium contents of melt refined samples; that is, the background contamination value was deducted from the plutonium contents of melt refined samples obtained directly from chemical analysis of the samples. The background contamination, especially in the case of the steel samples, was very small. Measurement of Bulk and Surface DrStribution of Plutonium To study the solubility of contaminant and its degree of retention in the metals, samples of mild steel, stainless steel, nickel, cbpper, and aluminum were melted in a similar way, except that 500 ppm &g/g) PuOZ was added to each metal-crucible assembly prior to melting. NO fluxing agent was added to any sample in this experiment. The samples from the surface were taken by very shallow drilling of the ingots. These results (Table 1) indicate that nickel and stainless steel have a stronger tendency to retain and to dissolve plutonium as compared with mild steel, aluminum, and copper. Still, a large portion of the contaminant tends to collect or absorb on the surface because of its limited solubility in the metal and the differente in surface free energy compared with that of metal. The results show that the use of proper
slags is necessary for efficient removal of plutonium from contaminated metals. Plutonium Removal Measurements To study the feasibility of removal of plutonium by melt refining and to measure the partition ratio for plutonium in metal and slag, several samples of mild steel, stainless steel, copper, and nickel were contaminated with plutonium and melted with various fluxes. Some samples were double refined by separating the first slag from the ingot after solidification and remelting the ingot with a clean slag. This was done to study the effect of secondary slag treatment on plutonium decontamination. The initial weight of added fluxes was also varied in the case of mild steel and stainless steel to study the effect of slag weight on the degree of plutonium decontamination. Table 2 represents the results of melt refining experiments on four types of metals. In general, al1 three slags (borosilicate, blast furnace, and high silica) were effective fluxing agents for removal of plutonium from mild steel, stainless steel, copper, and nickel. Table 2 shows that values less than 1 ppm &g/g) Pu (< 100 nCi/g) can be obtained by a single slag treatment in the smal1 scale samples. It appears that plutonium concentration for double refined samples is lower than 10 nCi/g ( - 0.1 ppm, which is the goal). The plutonium content of the secondary slags as shown in Table 2 indicates further reduction in plutonium content of remelted and reslagged ingots. Smaller slag weights (5 wt.Vo) were as effective in decontaminating mild steel and stainless steel. This indicates that the use of smal1 slag weight and double refining is more effective than a single treatment with large amount of slag. Seitz and co-workers (3,4) used a similar technique and studied the decontamination of plutonium- and americium-contaminated metals. They contaminated their samples with about 400 ppm bg/g) PuOl, and in some cases used larger slag weights and different slag types than those used in our study. Their investigation included mild steel, stainless steel, and nickel. However, the effects of slag weight and com-
TABLE 1 Distribution of Plutonium in the Bulk and Surfnee of Metnls Coataminated with 500 ppm (cg/@
Metal
Metal Weight (g)
Mild steel Copper Aluminum Stainless steel Nickel
300 300 100 200 300
Temperature (“C) 1600 1150 800 1550 1500
aRatio of plutonium content of the surface to bulk.
Plutonium, ppm &g/g) Bulk
Surface
Plutonium Ratio0
0.25 0.40 5.40 46.30 80.00
535 16,000 18,300 6160 115
2130 39,900 3390 133 1.5
MELT REFINING OF TRU CONTAMINATED METALS
133
TABLE 2
Resultsof Melt Refining Tests on 200 g Samplesof Four Met& Coataminatedwith 500 ppm &g/g) PuO, Slag
Sample
Amount (wt. olo)
1 2 3 4 5 6 7 3c
Plutonium Content, ppm bg/g) Metal
Slag
Partition Ratiob
10 10 10 5 5 5 10 5
Mild Steel (1600 “C) Borosilicate 0.20 0.11 Blast furnace High silica 0.20 Borosilicate 0.05 Blast furnace 0.30 High silica 0.20 2.00 Basalt High silica 0.06
1470 2790 2360 2280 2670 3970 942 30
735 2536 1180 2280 445 993 47 25
1 2 3 4 5 6 1C
10 10 10 5 5 5 5
Stainless steel (1600 “C) 0.60 Borosilicate Blast furnace 0.30 0.42 High silica 0.75 Borosilicate 0.21 High silica Blast furnace 0.50 0.04 Borosilicate
1120 1700 2240 3680 2640 4880 130
187 567 533 245 629 488 163
1 2 3
10 10 10
Nickel (1550 “C) 0.53 Borosilicate 0.63 Blast furnace High silica 0.23
3750 4250 1490
708 675 648
1 2 3
10 10 10
Copper (1400 “C) Borosilicate 0.20 Blast furnace 0.60 High silica 0.60
6450 940 640
3225 157 107
Vpea
BNominal composition of slags are as follows: borosilicate slag = 80 - SiOl 13- B1O, 4 - Na,0 2 -Al lO, 1 - K,O (wt.%); blast furnace slag = 40 - Ca0 30- SiOl lO- AlzO1 15 - Fe,O, 5 - CaF, (wt.%); highsilica slag = 60- SiO, 30- Ca0 lO- Al,O, (wt.%); basalt slag = 42 - SiOl 8 - AlzO, 26 - Fe,O, 12-Ca0 6-MgO4-NazO 2-K,O (wt.%). bRatio of total plutonium in slag to total plutonium in melt. CDouble refined with fresh slag.
position were not studied extensively. They indicated a somewhat higher degree of decontamination for plutoniumand americium-contaminated metals than obtained in our investigation. Comparison of the results of this investigation and a previous study on uranium decontamination by the authors (5) shows that the removal of plutonium is as effective as the removal of uranium in melt refining of contaminated metals. This was also predicted theoretically in a previous section of this report. These smal1 scale tests have been made by allowing the slag and metal to solidify in situ in the furnace because of equipment limitations. The remaining plutonium contamination in the metal samples is due to smal1 slag inclusions. Under normal smelting operations, the molten metal is poured beneath the slag or the slag is removed prior to casting. Further work is needed to obtain improved metal-slag separation to determine if the 10 nCi/g (-0.1 ppm) leve1 can be achieved routinely without double slag treatment and thereby remove scrap metal from the TRU waste classification.
SUMMARY AND CONCLUSIONS Feasibility of the decontamination of TRU contaminated metals by melt refining was demonstrated in this investigation. Several samples of metals (mild steel, stainless steel, nickel, and copper) were contaminated with 500 ppm PuOz and melted with various fluxes (blast furnace, high silica, borosilicate glass, and artifical basalt) by resistance heating in a glove box assembly. The partition ratios for plutonium were calculated from the analytical results for each sample. Some samples were double refined to study the effect of secondary slag treatment on plutonium decontamination. The initial weight of slags was also varied to investigate its effect on plutonium removal. The results indicated that the use of proper slags wil1 be necessary for efficient removal of plutonium from contaminated metals. It was found that the three slags, borosilicate, blast furnace, and high silica, were effective fl* agents for removal of plutonium from the metals investigated. Values of less than 1 ppm @g/g) Pu (- 100 nCi/g) could be obtained by a
134
B. HESHMATPOUR, G. L. COPELAND, AND R. L. HEESTAND
single slag treatment in the smal1 scale samples. The plutonium concentration for double refined samples was lower than 0.1 ppm (or < 10 nCi/g, which is the goal). Changing the slag weight did not substantially affect the plutonium removal. The removal of TRU contaminants from metals by melt refining is as effective as the removal of uranium from metals. Establishment of a better equilibrium condition, use of more fluid slags, double refining, and improved metal-slag separation are necessary to routinely achieve the 10 nCi/g (-0.1 ppm) leve1 so that scrap metal may be removed from the TRU waste classification.
3.
4.
5.
6.
7.
REFERENCES 1. Copeland, G. L., Heestand, R. L., and Mateer, R. S. Volume reduction of low-leve1 contaminated selection of method and conceptual Oak Ridge National Laboratory Oak 2. Levitz, N. et af. Decontamination of
metal waste by meltingplan. ORNL/TM-6388, Ridge, Tennessee (1978). plutonium-contaminated
8.
glove boxes. ANL-8128, Argonne National Laboratory, Argonne, Illinois (1974). Gerding, T. J., Seitz, M. G., and Steinder, M. J. Salvage of plutonium and americium contaminated metals, 70th AICHE Annual Meeting, Session 109, Paper No. 1096, New York, November 13-17 (1977). Seitz, M. G., Gerding, T. J., and Steinder, M. J. Decontamination of metals containing plutonium and americium. ANL-7813, Argonne National Laboratory, Argonne, Illinois (1979). Heshmatpour, B. and Copeland, G. L. The effect of slag composition and process variables on decontamination of metallic wastes by melt refining. ORNL/TM-7501, Oak Ridge National Laboratory, Oak Ridge, Tennessee (1981). Heshmatpour, B., Copeland, G. L., and Heestand, R. L. Disintegration and size reduction of slags and metals after melt refining of contaminated metallic wastes. ORNL/TM-767 1, Oak Ridge National Laboratory, Oak Ridge, Tennessee (1981). Heshmatpour, B., Copeland, G. L., and Heestand, R. L. Decontamination of transuranic waste metal by melt refining. ORNL/TM-7951, Oak Ridge National Laboratory, Oak Ridge, Tennessee (1981). Glassner, A. The thermochemical properties of the oxides, fluorides, and chlorides to 2500 K. ANL-5750, Argonne National Laboratory, Argonne, Illinois (1957).
0191-815X183 $3.00 + .oO Copyright 0 1983Pergamon Press Ltd.
DECONTAMINATION OF TRANSURANIC CONTAMINATED METALS BY MELT REFINING B. HeshmatpouP G. L. Copeìand R. L. Heestand Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA
ABSTRACT.
Melt refining of transuranic contaminated metals is a possible decontamination process with the potential advantages of producing metal for reuse and of simplifying chemical analyses. By routinely achieving the 10 nCi/g ( - 0.1 ppm) leve1 by melt refining, scrap metal can be removed from the transuranic waste category. To demonstrate the effectiveness of this melt refïning process, mild steel, stainless steel, nickel, and copper were contaminated with 500 ppm &g/g) PuO, and melted with various fluxes. The solidified slags and metals were analyzed for their plutonium contents, and corresponding partition ratios for plutonium were calculated. Some metals were double refined in order to study the effect of secondary slag treatment. The initial weight of the slags was also varied to investigate the effect of slag weight on the degree of plutonium removal. In general, al1 four metals could be decontaminated below 1 ppm @g/g) Pu (- 100 nCi/g) by a single slag treatment. Doubling the slag weight did not improve decontamination significantly; however, double slag treatment using 5 wt.% slag did decontaminate the metals to below 0.1 ppm (&g) Pu (10 nCi/g).
INTRODUCTION
glovebox enclosures in the nuclear industry. The high tost of retrievable storage and its ultimate disposal in a repository is a strong incentive to reduce the volume requiring storage. Furthermore, the wastage of scarce and strategie metals and alloys is also a concern. Smelting of surface contaminated scrap is attractive, because the generated ingots (or other forms) are of convenient shape for high density storage or grout infusion. In addition, radionuclides could be concentrated in the slag with possible recovery. Because of ingot homogeneity, the degree of residual contamination of the metal could easily be measured. The method of disposition or possible reuse of the metal, if it is sufficiently decontaminated, may then be identified. The thermodynamic principle underlying smelting behavior is that an alloying element wil1 partition between metal and slag toward an equilibrium in which its activity is the same in both phases. The strong oxide-forming tendency of plutonium wil1 favor its removal from the parent metal by an oxidizing slag to yield a clean metal. Other TRU elements are also strong oxide formers that would be expected to concentrate in slags. Thermodynamic calculations of slagmetal distribution of several contaminant elements in common metals at elevated temperatures have indicated their concentration in slags (1). Prior work in this area has shown that melting for
Nuclear operations that involve work in transuranic (TRU) elements have resulted in the accumulation of large quantities of contaminated waste. Metals constitute a large fraction of the volume of such waste and cannot be burned or otherwise treated as can combustible materials. Machines, tools, and structures associated with the nuclear fuel processing are subject to surface and crevice contamination. Airborne dusts of uranium and TRU compounds are common sources of low leve1 contamination. Furthermore, detection of low levels of contamination in complicated metallic forms is difficult. Particular precautions are taken in working with TRU contaminated materials due to their biological hazard. These materials are always handled within 1982; ACCEPTED 6 NOVEMBER1982. *Present address: Thermo Electron Corporation, Metals Division, 115 Eames Street, P.O. Box 132, Wilmington, MA 01887. Acknowledgements-The authors wish to thank W. B. Stines for his technical assistance during this study. This work was sponsored by the Office of Waste Operations and Technology, U.S. Department of Energy, under contract W-7405-eng-26 with the Union Carbide Corporation. Thanks are also extended to the many people who contributed to this project. We especially thank W. R. Laing and T. G. Scott from the Analytical Chemistry Division for analyzing the samples, R. G. Donnelly and F. J. Homan from the Metals and Ceramics Division for contributing substantial guidance and stimuiating discussions, E. L. Long, Jr. and J. M. Robbins for reviewing the manuscript, and P. T. Thornton for preparing the paper for publication. RECEIVED15 MAY
129
B.
130
HESHMATPOUR, C. L. COPELAND, AND R. L. HEESTAND
volume reduction is a technically feasible approach to the consolidating of metal waste and that melting may remove uranium contamination from many metals (1). The removal of TRU elements appears feasible also, but has not been extensively demonstrated. Melt refining of steels contaminated with plutonium has been suggested as a possible decontamination process with the potential advantages of producing steel for reuse and of simplifying analytical problems (2). Results of laboratory-scale investigation have indicated a high degree of decontamination by pyrochemical methods of plutonium- and americium-contaminated metals (3,4). In general, these studies were limited to mild steel, stainless steel, and nickel. The effects of slag weight and composition were not extensively investigated. In a previous study, the authors investigated the effect of slag type and composition, melting technique, and refractory materials on decontamination of uranium contaminated metals and alloys (5,6). Resistance furnace melting appeared to be a better melting technique for nonferrous scrap while induction melting was more suitable for ferrous metals. The degree of decontamination was not sensitive to slag type and composition. However, borosilicate and basic oxidizing slags were more effective on ferrous metals and copper; NaNOJ-NaCl-NaOH type slags were desirable for zinc, lead, and tin; fluoride type slags were effective for decontamination of aluminum. Recrystallized alumina proved to be the most compatible refractory for melt refining of both ferrous and nonferrous metals while graphite was suitable only for nonferrous metal processing. Results showed melt refining to be an effective technique for volume reduction and decontamination of contaminated metal scrap when proper slags, melting techniques, and refractories were used.
THERMODYNAMICS The thermodynamic treatment of the removal of TRU contaminants from a metal by melting assumes that the contaminant can combine with oxygen and enter a slag that acts as an inert solvent for the resulting oxides (1). It also assumes a complete thermodynamic equilibrium between the liquid metal and the slag. Oxidation of plutonium in infinite dilution with liquid iron can be expressed (5,7) as Pu + 2 FeO(slag) ?I PuO,(slag)
+ 2Fe(liq),
AG” = AG” (PuOJ - 2AG” (FeO) - RT ln[(2.307 x 10-3)yo(Pu)] =-RT In X(Pu02)X2(Fe) _V(FeO)(wt.% Pu) ’
(1)
(4
where AC” is the standard Gibbs free energy of reaction, AG”(PuOJ and AG”(Fe0) are the standard
Gibbs free energies of formation of Pu02 and FeO, respectively, R is the ideal gas constant, T is the temperature in ’K, y “(Pu) is the activity coefficient of plutonium in liquid iron, X represents the atomic (or mole) fraction of various species. The standard state of plutonium in liquid iron is taken as 1 wt.% solution. For slag composition of 50 Cao-44 SiO,FeO-1 Pu02 (wt.Vo), and a temperature of 1900 “K, Eq. 2 reduces to (7,8) wt.% Pu = 6.4 x 10-‘“/y”(Pu).
(3)
The partition ratio, X, is defined as the amount of contaminant in the slag, m(Pu)(slag), divided by the amount of contaminant in the melt, m(Pu)(melt); that is, x = m(Pu)(slag) m(Pu)(melt)
.
(4)
If the weight of slag is taken as 10 wt.% of the metal, Eqs. 3 and 4 wil1 result in (5) x = (1.4 x 106)y”(Pu).
(5)
We have not identified a reported value either for y “(Pu) or for y”(U) in liquid iron. Our results (7) indicate a value of about 3 x lO* for X in case of plutonium decontamination, which translates into about 2 x lom4 for yo from Eq. 5 and about 3.0 x 10V6for wt.% Pu (0.03 ppm) from Eq. 3 in liquid iron. Thus, theoretically 10 nCi/g (- 0.1 ppm) limit can be reached by melt refining. Furthermore, this analysis indicates that y “(Pu) is slightly larger than y’(U), which is consistent with the lower thermodynamic stability of PuOZ compared with that of uo,. The actual experimental melt refining test usually results in higher TRU or uranium contents in the metal than is predicted by thermodynamics. In practice, equilibrium conditions are not usually reached, and some slag particles are also entrapped in solidified ingots, giving rise to higher analytical results for plutonium content of the ingots. This problem can be overcome by improved liquid metal-slag separation. EXPERIMENTAL
METHODS
Previous melt refining studies on uranium contaminated metals have shown that a reasonable way to simulate melting of a contaminated metal is to premix slag and contaminant (PuO, in this case) prior to loading into a crucible that contains the metal of interest (1). The crucible assembly is then heated above the melting point of the metal and the slag and allowed to equilibrate. Since both UOZ and PuO, dissolve easily in slag, the transfer of uranium or plutonium to the melt wil1 be uniform. Theoretically, the result wil1 be the same regardless of how the sample is contaminated, because the equilibrium ac-
MJZLTREFINING OF TRU CONTAMINATED METALS
l31
tivity of contaminant is the same in both melt and slag. Compatibility studies for various crucible materials and several types of metallurgical slags have shown the best results were obtained with recrystallized alumina refractories (5). Effective slag types and compositions were identified for decontamination of uranium contaminated metals in a previous investigation (5). These slags were also used in melt refining studies of plutonium contaminated metals because of the similarities in chemical behavior of uranium and plutonium. Al1 experiments were conducted inside a glove box assembly to prevent any biological hazard from exposure to plutonium. To account for the background contamination of experimental samples from the glove box assembly previously used, several clean samples of mild steel and aluminum were melted inside the glove box assembly without adding any PuO, or slag. The reason for choosing these two metals for this particular experiment was the differente in their reactivity with respect to oxygen. Iron is more like steels, nickel, and other nonferrous metals of interest that have less tendency to retain contamination after melting; aluminum tends to react with contaminant oxides and thus retains higher concentration of contaminants after melting. The solidified ingots were sampled and analyzed for their plutonium contents. To study the solubility of contaminant or its degree of retention in the metal by melting, samples of several metals were contaminated by adding 500 ppm bg/g) PuO, to the metal sample and were melted without any fluxing agent. The solidified ingots were also sampled and analyzed for their plutonium contents. In a typical melt refining experiment, 500 ppm (pg/g) PuO, was thoroughly mixed with the flux and added to the metal in the crucible, intentionally contaminating the metal. Recrystallized alumina crucibles were used throughout this study because of their high slag resistance. The crucible assembly was placed inside a graphite crucible, and a graphite disk was placed on the top to avoid damage to heating elements of the furnace by accidental splash of liquid metal or slag. The mixture was melted in a tungsten-mesh resistance furnace under an argon atmosphere. After the slag-metal mixture melted, the temperature was raised further (- 100-150 “C), and sufficient time (0.5-1 h) was allowed for slag-metal equilibrium. It was then allowed to solidify in situ. Solidified slags were glassy and brittle and were separated easily from the metallic ingots and crushed into smal1 fragments for sampling. Metallic ingots were drilled in the middle to the depth of about 5 mm in two different locations, and the chips were discarded. This was done to remove any slag from the surface of the ingot. Samples of the ingot were
taken from the same two holes by further drilling. Care was taken to minimize recontamination of the metal samples; however, some cross contamination did occur and led to occasional scatter in the data. The samples of the metals and the slags were analyzed for their plutonium content by two different analyatical techniques. In the first technique, plutonium in the dissolved sample was extracted as a pure plutonium solution by thenoyltrifluoroacetone (TTA) extraction and the plutonium content of this solution was then measured by counting its CYactivity. This technique is sensitive to about 0.2 to 1 ppm (pg/g), depending on the sample. The second technique is sensitive to 0.01 ppm &g/g) and lower due to more sensitive counters, lower background, and purer materials. A portion of the dissolved sample was equilibrated with 242Putracer then the plutonium isotopes were coprecipitated with the metals as hydroxides. The hydroxides were dissolved, mixed with Al(NO,),, oxidized with KMn04, and extracted with methyl isobutyl ketone (hexone). Plutonium isotopes were back-extracted from the hexone, coprecipitated with praseodymium fluoride which was dissolved in dilute nitric acid, and extracted with TTA-xylene. The organic extract was evaporated on stainless steel disks and evaluated by low-background (Y counting and CX spectrometry. Proseodymium fluoride was used in preferente to the other rare earths because it is available in higher purity (lower background a). Metal refining was used to measure the partition ratio (defined as the amount of contaminant in the slag divided by the amount in the melt) for plutonium in several metals and slags. This investigation included mild steel, stainless steel, nickel, and copper. The slags used in the experimental work were: borosilicate glass (80 SiO, - 13 B203 - 4 Na,0 - 2 Al *03 - 1 K,O wt.%), blast furnace flux (40 Ca0 - 30 SiO, - 10 A1203 - 15 Fe20J - 5 CaF wt. Vo), high silica flux (60 SiO,- 30 Cao- 10 A1203 wt.Vo), and artificial basalt flux (42 SiO - 8 A1203 - 26 FesO, - 12 Cao-6 MgO-4 Na,O-2 K20 wt.%). The weight of metal samples was 200 to 300 g, and the corresponding slags were either 10% or 5% of the weight of metal sample. A few of the samples were double refined by separating the first slag from the ingot after solidification and remelting the ingot with a clean slag. This was done to examine whether double refining would produce a cleaner metal ingot.
RESULTS
AND DISCUSSION
Background Measures To measure the background contamination of experimental samples from that of the glove box assembly, five clean samples of mild steel (each
132
B. HJWRWATPOUR, G. L. COPELAND, AND R. L. HBESTAND
weighing 300 g) and three clean samples of aluminum (each weighing 150 g) were melted by resistance furnace heating inside the glove box assembly. The mild steel experiments were conducted at 1600 “C and those of aluminum at 1000 “C. Each sample was held for about 1 h at these temperatures after melting. Recrystallized alumina crucibles (placed inside a larger graphite crucible with a graphite disk on top) were used in al1 cases. NO fluxing agents and no Pu02 were added to these samples. Solidified ingots were sampled and analyzed for their plutonium contents. The results of these tests indicated an average plutonium content of 0.012 ppm @g/g) for mild steel and an average plutonium content of 0.48 ppm bg/g) for aluminum samples. These two quantities represent the average background contamination of the test samples from the glove box assembly and were used to make the necessary corrections of plutonium contents of melt refined samples; that is, the background contamination value was deducted from the plutonium contents of melt refined samples obtained directly from chemical analysis of the samples. The background contamination, especially in the case of the steel samples, was very small. Measurement of Bulk and Surface DrStribution of Plutonium To study the solubility of contaminant and its degree of retention in the metals, samples of mild steel, stainless steel, nickel, cbpper, and aluminum were melted in a similar way, except that 500 ppm &g/g) PuOZ was added to each metal-crucible assembly prior to melting. NO fluxing agent was added to any sample in this experiment. The samples from the surface were taken by very shallow drilling of the ingots. These results (Table 1) indicate that nickel and stainless steel have a stronger tendency to retain and to dissolve plutonium as compared with mild steel, aluminum, and copper. Still, a large portion of the contaminant tends to collect or absorb on the surface because of its limited solubility in the metal and the differente in surface free energy compared with that of metal. The results show that the use of proper
slags is necessary for efficient removal of plutonium from contaminated metals. Plutonium Removal Measurements To study the feasibility of removal of plutonium by melt refining and to measure the partition ratio for plutonium in metal and slag, several samples of mild steel, stainless steel, copper, and nickel were contaminated with plutonium and melted with various fluxes. Some samples were double refined by separating the first slag from the ingot after solidification and remelting the ingot with a clean slag. This was done to study the effect of secondary slag treatment on plutonium decontamination. The initial weight of added fluxes was also varied in the case of mild steel and stainless steel to study the effect of slag weight on the degree of plutonium decontamination. Table 2 represents the results of melt refining experiments on four types of metals. In general, al1 three slags (borosilicate, blast furnace, and high silica) were effective fluxing agents for removal of plutonium from mild steel, stainless steel, copper, and nickel. Table 2 shows that values less than 1 ppm &g/g) Pu (< 100 nCi/g) can be obtained by a single slag treatment in the smal1 scale samples. It appears that plutonium concentration for double refined samples is lower than 10 nCi/g ( - 0.1 ppm, which is the goal). The plutonium content of the secondary slags as shown in Table 2 indicates further reduction in plutonium content of remelted and reslagged ingots. Smaller slag weights (5 wt.Vo) were as effective in decontaminating mild steel and stainless steel. This indicates that the use of smal1 slag weight and double refining is more effective than a single treatment with large amount of slag. Seitz and co-workers (3,4) used a similar technique and studied the decontamination of plutonium- and americium-contaminated metals. They contaminated their samples with about 400 ppm bg/g) PuOl, and in some cases used larger slag weights and different slag types than those used in our study. Their investigation included mild steel, stainless steel, and nickel. However, the effects of slag weight and com-
TABLE 1 Distribution of Plutonium in the Bulk and Surfnee of Metnls Coataminated with 500 ppm (cg/@
Metal
Metal Weight (g)
Mild steel Copper Aluminum Stainless steel Nickel
300 300 100 200 300
Temperature (“C) 1600 1150 800 1550 1500
aRatio of plutonium content of the surface to bulk.
Plutonium, ppm &g/g) Bulk
Surface
Plutonium Ratio0
0.25 0.40 5.40 46.30 80.00
535 16,000 18,300 6160 115
2130 39,900 3390 133 1.5
MELT REFINING OF TRU CONTAMINATED METALS
133
TABLE 2
Resultsof Melt Refining Tests on 200 g Samplesof Four Met& Coataminatedwith 500 ppm &g/g) PuO, Slag
Sample
Amount (wt. olo)
1 2 3 4 5 6 7 3c
Plutonium Content, ppm bg/g) Metal
Slag
Partition Ratiob
10 10 10 5 5 5 10 5
Mild Steel (1600 “C) Borosilicate 0.20 0.11 Blast furnace High silica 0.20 Borosilicate 0.05 Blast furnace 0.30 High silica 0.20 2.00 Basalt High silica 0.06
1470 2790 2360 2280 2670 3970 942 30
735 2536 1180 2280 445 993 47 25
1 2 3 4 5 6 1C
10 10 10 5 5 5 5
Stainless steel (1600 “C) 0.60 Borosilicate Blast furnace 0.30 0.42 High silica 0.75 Borosilicate 0.21 High silica Blast furnace 0.50 0.04 Borosilicate
1120 1700 2240 3680 2640 4880 130
187 567 533 245 629 488 163
1 2 3
10 10 10
Nickel (1550 “C) 0.53 Borosilicate 0.63 Blast furnace High silica 0.23
3750 4250 1490
708 675 648
1 2 3
10 10 10
Copper (1400 “C) Borosilicate 0.20 Blast furnace 0.60 High silica 0.60
6450 940 640
3225 157 107
Vpea
BNominal composition of slags are as follows: borosilicate slag = 80 - SiOl 13- B1O, 4 - Na,0 2 -Al lO, 1 - K,O (wt.%); blast furnace slag = 40 - Ca0 30- SiOl lO- AlzO1 15 - Fe,O, 5 - CaF, (wt.%); highsilica slag = 60- SiO, 30- Ca0 lO- Al,O, (wt.%); basalt slag = 42 - SiOl 8 - AlzO, 26 - Fe,O, 12-Ca0 6-MgO4-NazO 2-K,O (wt.%). bRatio of total plutonium in slag to total plutonium in melt. CDouble refined with fresh slag.
position were not studied extensively. They indicated a somewhat higher degree of decontamination for plutoniumand americium-contaminated metals than obtained in our investigation. Comparison of the results of this investigation and a previous study on uranium decontamination by the authors (5) shows that the removal of plutonium is as effective as the removal of uranium in melt refining of contaminated metals. This was also predicted theoretically in a previous section of this report. These smal1 scale tests have been made by allowing the slag and metal to solidify in situ in the furnace because of equipment limitations. The remaining plutonium contamination in the metal samples is due to smal1 slag inclusions. Under normal smelting operations, the molten metal is poured beneath the slag or the slag is removed prior to casting. Further work is needed to obtain improved metal-slag separation to determine if the 10 nCi/g (-0.1 ppm) leve1 can be achieved routinely without double slag treatment and thereby remove scrap metal from the TRU waste classification.
SUMMARY AND CONCLUSIONS Feasibility of the decontamination of TRU contaminated metals by melt refining was demonstrated in this investigation. Several samples of metals (mild steel, stainless steel, nickel, and copper) were contaminated with 500 ppm PuOz and melted with various fluxes (blast furnace, high silica, borosilicate glass, and artifical basalt) by resistance heating in a glove box assembly. The partition ratios for plutonium were calculated from the analytical results for each sample. Some samples were double refined to study the effect of secondary slag treatment on plutonium decontamination. The initial weight of slags was also varied to investigate its effect on plutonium removal. The results indicated that the use of proper slags wil1 be necessary for efficient removal of plutonium from contaminated metals. It was found that the three slags, borosilicate, blast furnace, and high silica, were effective fl* agents for removal of plutonium from the metals investigated. Values of less than 1 ppm @g/g) Pu (- 100 nCi/g) could be obtained by a
134
B. HESHMATPOUR, G. L. COPELAND, AND R. L. HEESTAND
single slag treatment in the smal1 scale samples. The plutonium concentration for double refined samples was lower than 0.1 ppm (or < 10 nCi/g, which is the goal). Changing the slag weight did not substantially affect the plutonium removal. The removal of TRU contaminants from metals by melt refining is as effective as the removal of uranium from metals. Establishment of a better equilibrium condition, use of more fluid slags, double refining, and improved metal-slag separation are necessary to routinely achieve the 10 nCi/g (-0.1 ppm) leve1 so that scrap metal may be removed from the TRU waste classification.
3.
4.
5.
6.
7.
REFERENCES 1. Copeland, G. L., Heestand, R. L., and Mateer, R. S. Volume reduction of low-leve1 contaminated selection of method and conceptual Oak Ridge National Laboratory Oak 2. Levitz, N. et af. Decontamination of
metal waste by meltingplan. ORNL/TM-6388, Ridge, Tennessee (1978). plutonium-contaminated
8.
glove boxes. ANL-8128, Argonne National Laboratory, Argonne, Illinois (1974). Gerding, T. J., Seitz, M. G., and Steinder, M. J. Salvage of plutonium and americium contaminated metals, 70th AICHE Annual Meeting, Session 109, Paper No. 1096, New York, November 13-17 (1977). Seitz, M. G., Gerding, T. J., and Steinder, M. J. Decontamination of metals containing plutonium and americium. ANL-7813, Argonne National Laboratory, Argonne, Illinois (1979). Heshmatpour, B. and Copeland, G. L. The effect of slag composition and process variables on decontamination of metallic wastes by melt refining. ORNL/TM-7501, Oak Ridge National Laboratory, Oak Ridge, Tennessee (1981). Heshmatpour, B., Copeland, G. L., and Heestand, R. L. Disintegration and size reduction of slags and metals after melt refining of contaminated metallic wastes. ORNL/TM-767 1, Oak Ridge National Laboratory, Oak Ridge, Tennessee (1981). Heshmatpour, B., Copeland, G. L., and Heestand, R. L. Decontamination of transuranic waste metal by melt refining. ORNL/TM-7951, Oak Ridge National Laboratory, Oak Ridge, Tennessee (1981). Glassner, A. The thermochemical properties of the oxides, fluorides, and chlorides to 2500 K. ANL-5750, Argonne National Laboratory, Argonne, Illinois (1957).