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Development of an environmentally benign reprocessing technology—pyrometallurgical reprocessing technology
Progress
in Nuclear
Energy,
Vol.
32, No. 314. pp. 381-387,
1998
0 1997 Published
by Elsevier Science Ltd Printed in Great Britain 0149-1970/98 $19.00 + 0.00
Pergamon
PII:SO149-1970(97)00032-2
DEVELOPMENT OF AN ENVIRONMENTALLY BENIGN REPROCESSING TECHNOLOGY - PYROMETALLURGICAL REPROCESSING TECHNOLOGY -
T. NISHIMURA, T. KOYAMA, M. IIZUKA, and H. TANAKA Central Research Institute of Electric Power Industry 2- 1 1- 1 Iwato-kita, Komae-shi, Tokyo 201, JAPAN
ABSTRACT Present status of research and development on pyrometallurgical reprocessing technology in Central Research Institute of Electric Power Industry is described with emphasis on electrorefining and waste salt immobilization. As for the electrorefming, three different electrodes -- anode basket, solid cathode, and liquidcadmium cathode -- have been being investigated; prismatic anode basket with faster rotation was found to accelerate metal-fuel dissolution. Morphology and collection efficiency of electrodeposited uranium on solid cathode were found to vary with ratio of cathode to anode surface area. Liauid-cadmium cathode with a uaddleshape stirrer was developed and determined-maximum uranium concentration into the cathode without dendrite formation. As for waste salt immobilization, sodalite is proposed as waste form and synthesized by dry reaction without gas generation. Measured leachability of the synthesized sodalite is as low as those of vitrified waste form. 0 1997 Published by Elsevier Science Ltd INTRODUCTION Nuclear power does not generate greenhouse gases such as carbon dioxide gas, so it is considered to be one of the promising solutions for the global warming issue. Considering limitation of uranium-235 resource, however, fast-reactor and reprocessing are vital for a long-term stable supply of nuclear power. From the point of view of the global environment preservation it is also important in developing reprocessing technology to enhance nonproliferation of atomic weapons and lower release of radioactive wastes into the environment. Metal fuel cycle, which consists of metal-fuel fast reactor and pyrometallurgical reprocessing, has been proposed as such recycling technology by Argonne National Laboratory (ANL) (Chang, 1989). Metal fuel cycle possesses favorable proliferation-resistant, environment-protection, and inherent-safety attributes and also promises greatly improved economics relative to conventional aqueous reprocessing (Chang, 1989). Therefore Central Research Institute of Electric Power Industry (CRIEPI) started research and development on metal fuel cycle in 1986, including a joint study on pyrometallurgical technology with the United States Department of Energy. Major progress of the research and development in CRIEPI are described with emphasis on electrorefining and waste salt immobilization. PYROMETALLURGICAL REPROCESSING FLOWSHEET Figure 1 shows the schematic flowsheet of pyrometallurgical reprocessing of spent metal fuel. Spent metal fuel is disassembled and chopped into small pieces. In the electrorefining step, the chopped fuel is anodically dissolved, and uranium and plutonium are electrochemically separated from most. fission uroducts and collected at solid cathode and liauid-cadmium cathode with transuranic elements. Electrolvte salt and cadmium solvent that come out with the cathode products are removed by high-temperature retort l’n the cathode processing step. The purified cathode products, uranium and uranium-plutonium ingots are then mixed to adjust their composition and cast into fuel slugs by injection. 381
382
T. Nishimura et al.
Salt waste containing alkali-metal, alkaline-earth, and rare-earth fission products comes out with the electrolyte salt used in the electrorefining step. Metal waste, which consists of fuel assembly duct, cladding hull, and noble-metal fission products left in anode baskets, is generated in the disassembly, chopping, and electrorefining steps. Tritium and noble-gas fission products are discharged in the chopping and electrorefining steps.
%I;
Gas
(T,
New
Xe, Kr)
Fuel salt, Cd
I e
Electrorefming u-TRU
Zr
I
u-salt
Disassembly and Chopping
) +
Cathode Processing
Pin Casting
Crucible
Mold Crucible
TRU: Pu, Np, Am, Cm RE :RareEarth NM : Noble Metal &@JJ&&
Salt Waste (Cs, Sr, RF) Fig. 1. Schematic Flowsheet of Pyrometallurgical Reprocessing of Metal Fuel ELECTROREFINING OF METAL FUEL
Figure 2 shows typical operation sequence of the electrorefining process, which is the process for recovery of actinides (uranium and transuranic elements) by using electrochemical difference among elements fin molten LiCl-KC1 salt and liquid cadmium under high-purity argon atmosphere at 773 K as shown in Fig. 3 (Koyama, 1994).
AL : Alkali metal ALE : Alkaline earth RE :Rareearth TRU : Pu,Np, Am, Cm Fig. 2. Typical Operation Sequence of Electrorefining Process The chopped spent fuel is loaded into anode baskets and anodically dissolved. Electronegative fission products (noble metals) are left in the anode baskets in metal form. The other elements are dissolved into the
Environmentally
benign reprocessing
technology
383
electrolyte salt in the form of their chlorides. Actinides are recovered by using two different types of cathodes. At first using solid cathode, uranium chloride is reduced and essentially pure uranium collected on the cathode (see Fig. 3; Since differences of Gibbs free energy changes between uranium and other elements are large on solid cathode, only uranium will be reduced). Then changing solid cathode for liquid-cadmium cathode, plutonium chloride is reduced and collected with uranium, other transuranic elements, and a small amount of rare-earth fission products (see Fig. 3; Gn liquid-cadmium cathode, differences of Gibbs free energy changes among actinides are so small that actinides are reduced together; Since separation factors for rare earths are large, most of these remain in the electrolyte salt). Chemically more active elements (alkali-metal, alkaline-earth, and rare-earth elements) remain in the electrolyte salt. kJ/mol -600 Solid Cathode
---- _____ _b
U-700 -
Cd Cathode
NP -
-800 -
Fig. 3. Gibbs Free Energy Change at 773 K Anodic Dissolution of Snent Metal Fuel Anode baskets should offer good contact between chopped fuel and bulk LiCl-KC1 salt to prevent accumulation of oxidized a&tides in the basket and to facilitate electrotransport of them to cathodes. For this purpose two shapes of anode baskets made of perforated stainless-steel plates were tested. One is cruciform and the other is cylindrical. Figure 4 presents dissolution rates of uranium segments charged in these baskets. Higher dissolution rates and current efficiencies are obtained with cruciform baskets than cylindrical ones. The results also show that dissolution rate increases with faster rotation of baskets. Small-scale tests with irradiated metal fuel were carried out in ANL. It is reported that over 99.9 % of uranium and over 99.99 % of plutonium are removed from cladding hulls by anodic dissolution (Benedict, 1993). 1.6 1 : long (0.050 m) piece, 120 rpm 2 : short(0.025 m) piece, 120 rpm 3 : long (0.050 m) piece, 60 r-pm
‘E J$
1.4
IO
1200
1400
1600
1800
2OcO
Anodic Current Density (A*mm2) Fig. 4. Uranium Dissolution Rate with Different Anode Baskets
T. Nishimura et al.
384
p Small-scale tests of uranium electrotransport from the liquid-cadmium anode to the solid cathode were carried out with changing concentrations of uranium in electrolyte salt (about 0.5 and about 2.0 weight %) and surface areas of solid cathodes. At low concentration no deposit was observed on the cathodes. At high concentration dendritic (thin needle shape) uranium deposits were obtained on all cathodes. Photographs and collection efficiencies of these deposits are shown in Fig. 5. With the smallest cathode surface area needles of the deposit were thin and long and it was expected that part of the deposit fell off. It seems to be the reason why the collection efficiency was extremely low in this case. In the medium case the deposit seemed to be much more adherent so that the collection efficiency was high. When cathode area was increased further, concentration polarization arose at the anode and the electrotmnsport was interrupted. The collection efficiency was however fairly good. These results indicate that ratio of cathode area to anode area is one of the important parameters of electrorefiner design.
Diameter of Cathode Cathode I Anode Area Ratio Collection Efficiency
a
b
15 mm
3omm
C
7omm
0.09
0.18
0.43
9.3 ‘45
77 %
65 %
Fig. 5. Uranium Deposits on Solid Cathode of Different Diameters Results of large prototype-scale (about 10 kg of uranium) tests are reported (Steindler, 1991). About 10 kg of uranium is routinely collected on solid cathode with the maximum collection rate of 0.35 kg-uranium per hour. The collection efficiency is about 50 8 in average because deposits are shaved into cylindrical shape by using blades around the cathode. Although there seems to be no crucial problem in recovery of uranium on solid cathode, improvement in processing rate is required.
One of the problems in liquid-cadmium cathode is formation of uranium dendrite at the surface of the cathode. When uranium deposits faster than it is taken into bulk liquid cadmium, uranium dendrites form at the surface of the cathode. Once the dendrite grows up, electric current is concentrated on it and it works as a solid cathode where only uranium deposits. In order to avoid this problem we tested stirring of liquid cadmium using a paddle-shape stirrer, which is expected to enhance transport of deposited uranium from surface into bulk cadmium. Figure 6 presents the effect of stirring and cathode current density on dendrite formation (Koyama, 1995a). The height of each bar indicates how much uranium can be collected into the liquid-cadmium cathode before dendrite formation. It can be seen that more uranium is collected under impetuous stirring and low current density condition. Uranium was collected up to 10 weight 8 into the cathode with current efficiency of approximately 100 %.
Environmentally benign reprocessing
technology
385
Another type of liquid cadmium cathode called “ pounder type” has been studied in ANL. (Battles, 1994). A cylindrical ceramic block with a small opening (pounder) rotates and moves up and down over the surface of liquid-cadmium cathode at the same time. Uranium deposits are supposed to be pushed down into bulk cadmium by the motion of the block. It is reported that more than 10 weight % of plutonium is collected into the cathode at a rate of about 10 kg/m2/h in small-scale experiments with a current efficiency of about 100 %. But uranium dendrite formation was still observed at high uranium/plutonium ratios and presence of rare-earth elements in salt.
Uranium in LCC T 10 (
wt.% )
I-8 -6 -4
U Solubility + (2.35 wt.%)
2 r
Cathode Current Density ( kA*mm2)
Fig. 6. Maximum Amount of Uranium Taken in Liquid-Cd Cathode before Dendrite Formation Comouter Code Develooment for Design Outimization a) Simulation of Electrochemical Behavior. Composition of electrolyte salt is not kept constant durmg electrorefining operation. In order to find out the optimum conditions, a computational code, TRAIL, wan;developed for analysis and prediction of the electrotransport behavior of elements (Kobayashi, 1993). TRAIL employs a diffusion- limited electrochemical reaction model. A comparison between results of electrotransport tests with uranium and plutonium reported by Tomczuk (1992) and results calculated by TRAIL is shown in Fig. 7. The figure shows good agreement and it indicates that the behavior of each elements in the electrorefining step can be predicted by TRAIL. 0 I 0
TRAIL Calculation Experiment
20
40
60
Time(h) Fig. 7. Comparison of Electrochemical Behavior between Calculated Results and Experimental Data
T. Nishimura er al.
386
bL In order to develop a high-throughput electrorefmer, the electric resistance of the electrorefiner should be minimized and the electric current of it should be maximized. Since cell resistance depends on geometrical configuration of electrodes in the electrorefiner, a two-dimensional finite-element-method code, CAMBRIA, was developed for analysis of electric potential and current distribution by solving the Laplace equations (Kobayashi, 1995). Although this code calculates electric resistance without polarization effect, it predicts well as shown in Fig. 8. 30 ??
9
\ !
s25-
E
---O-
Experiment
TRAIL Calculation
820 3 7I 4 2 15 _ : 310
-
i ,---___ --__ --_
----.___
5 50
100
150
200
250
??
--__ 300
.-.a
350
400
Cathode Area (10m4 m2) Fig. 8. Cell Resistance of Electrorefiners TREATMENT OF WASTES Salt Waste Treatment After many batches of spent fuel are processed in the electronsfiner, a large amount of chemically active fission products (alkali-metal, alkaline-earth, and rare-earth fission products) are left in the form of chloride and accumulated in the electrolyte salt. These fission products generate problems such as decay heat removal, a rise in melting point of the salt, and an gradual increase in contamination of cathode products with fission products. The waste salt should be disposed of. Unfortunately the salt is water soluble and high-level radioactive, and alkali-metal and alkaline-earth chlorides are difficult to be dissolved into borosilicate glass waste form in high concentration, Therefore a method to immobilize the waste salt should be developed. At present two different immobilization methods are under study in CRIEPI. One is vitrification after electrolysis of salt waste into metallic waste and chlorine gas (Atomic Energy Society of Japan, 1995). Another is conversion to sodalite, N%[(AlO,),(SiO2, J*WaCl, which is a natural-occurring mineral containing chloride salts in its structure (Koyama, 1995b). In our study it is found that sodalite can be synthesized according to the following dry reaction without gas generation: 6NaA102 + 6SiOr + 2NaCl= N~[(AlO&oiO&]*2NaCl
(1)
We could also make sodalite-type waste form from a mixture of NaAlOr, SiO,, and simulated waste salt by pressing at 200 MPa and heating it at a temperature of 973 to 1173 K for 50 to 100 hours. The structures of the products were identified by X-ray diffraction. Leachability was measured for the synthesized specimen. As shown in Table I measured leachability of elements are as low as the values reported for zeolite and vitrified waste forms (Lewis, 1993). Other Wastes Stainless-steel duct and cladding are disposed of as metal wastes from disassembly and chopping step. The electronegative fission products (noble metals) and fuel-constituent zirconium remain in anode baskets and become metal waste. One possible way of immobilization of noble-metal fission products is to be dispersed in stainless-steel and zirconium alloy in containers. Tritium and noble gases (Xe and Kr) are released from spent fuel into pure argon atmosphere during chopping and electrorefining. Tritium will be converted to tritium-water and sorbed on molecular sieve. The noble gases will be separated from argon by cryogenic distillation, compressed and stored in high-pressure gas cylinder. So it would be expected that there is no release of fission gases in the pyrometallurgical reprocessing.
Environmentally benign reprocessing technology
381
Table I. Comparison of Normalized Release Rates (g/m*d) Element Li K Na Cs Sr Ba Al Si Cl I
Synthesized sodalite 0.0047 0.0061 0.0023 0.0013 0.00035 0.000069 0.0015 0.0017 0.0022 0.0029
Zeolite *’ 0.12 0.031 0.018 0.0066 <1.0x lo-5 < 1.0 x 10” 0.0048 0.0041 ::
Borosilicate glass *I 0.29
0.22 0.28 NM NM 0”: 0.23 ::
NM : not measured *’reported values (Lewis, 1993) SUMMARY
Major progress and present status of our research and development are summarized on electrorefining and waste treatment processes. Although it is still in an early stage of development comparing to the conventional aqueous reprocessing, any crucial problems regarding actual application of pyrometaJlurgica1 reprocessing technology are not found yet. We are planning to continue research and development to establish technical bases necessary for practical application of this technology as advanced reprocessing technology together with research and development of metal-fuel fast reactor technology. ACKNOWLEDGMENT The authors wish to acknowledge contributions from joint studies with Toshiba Corp. on electrorefining and Hitachi Ltd. on salt waste treatment. We also acknowledge information exchange with Argonne National Laboratory under the joint study with the United States Department of Energy. REFERENCES Atomic Energy Society of Japan (1995) Metal Fuel Cvcle Technologv - Present Status and Future Persnective, Atomic Energy Society of Japan, p. 18 1. Battles J. E. et al. (1994) Chemical Technolopv Division Annual Technical Renort 1993, Argonne National Laboratory Report ANL-94115. Benedict R. W. et al. (1993) Small-scale Irradiated Fuel Electrorefining, GLOBAL ‘93, p. 1331, Seattle, 12-I 7 Sentember. Chang Y. I. (1989) The Integral Fast Reactor, Nucl. Technol.. 88,129. Kobayasbi T. and Tokiwai M. (1993) Development of TRAIL, a Simulation Code for the Molten Salt Electrorefining of Spent Nuclear Fuel, J. Allovs and Comnounds. 197,7. Kobayashi T. et al. (1995) Investigation of Cell Resistance for Molten Salt Electrorefming of Spenr Nuclear Fuel, J. Nucl. Sci. Technol.. 32,68. Koyama T. et al. (1994) Pvrometallurav Data Book, CRIEPI Report T93033. Koyama T. et al. (1995a) Development of Molten Salt Electrorefining - Studv of Uranium Denosiiion Liauid Cad ‘urn Cathode without Formation of Dendrite, CRIEPI Report T95004. Koyama T. et% (1995b) Immobilization of Halide Salt Waste from Pyrochemical Reprocessing by Forming Natural Occurring Mineral; SODALITE, GLOBAL ‘95, p. 1744, Versailles, 1l-14 September. Lewis M. A. et al. (1993) Salt-occluded Zeolites as an Immobilization Matrix for Chloride Waste Salt ,iJ Am. Ce am Sac .76 2826. Steinf;ler ‘M. J: et A. (1991) Chemical Technoloev Division Annual Technical Report 1990, Argonne National Laboratorv Renort ANL-91/18. Tomczuk Z.-et al: (1992) Uranium Transport to Solid Electrodes in Pyrochemical Reprocessing of Nuclear Fuel, J. Electrochem. Sot.. 139,3523.
in Nuclear
Energy,
Vol.
32, No. 314. pp. 381-387,
1998
0 1997 Published
by Elsevier Science Ltd Printed in Great Britain 0149-1970/98 $19.00 + 0.00
Pergamon
PII:SO149-1970(97)00032-2
DEVELOPMENT OF AN ENVIRONMENTALLY BENIGN REPROCESSING TECHNOLOGY - PYROMETALLURGICAL REPROCESSING TECHNOLOGY -
T. NISHIMURA, T. KOYAMA, M. IIZUKA, and H. TANAKA Central Research Institute of Electric Power Industry 2- 1 1- 1 Iwato-kita, Komae-shi, Tokyo 201, JAPAN
ABSTRACT Present status of research and development on pyrometallurgical reprocessing technology in Central Research Institute of Electric Power Industry is described with emphasis on electrorefining and waste salt immobilization. As for the electrorefming, three different electrodes -- anode basket, solid cathode, and liquidcadmium cathode -- have been being investigated; prismatic anode basket with faster rotation was found to accelerate metal-fuel dissolution. Morphology and collection efficiency of electrodeposited uranium on solid cathode were found to vary with ratio of cathode to anode surface area. Liauid-cadmium cathode with a uaddleshape stirrer was developed and determined-maximum uranium concentration into the cathode without dendrite formation. As for waste salt immobilization, sodalite is proposed as waste form and synthesized by dry reaction without gas generation. Measured leachability of the synthesized sodalite is as low as those of vitrified waste form. 0 1997 Published by Elsevier Science Ltd INTRODUCTION Nuclear power does not generate greenhouse gases such as carbon dioxide gas, so it is considered to be one of the promising solutions for the global warming issue. Considering limitation of uranium-235 resource, however, fast-reactor and reprocessing are vital for a long-term stable supply of nuclear power. From the point of view of the global environment preservation it is also important in developing reprocessing technology to enhance nonproliferation of atomic weapons and lower release of radioactive wastes into the environment. Metal fuel cycle, which consists of metal-fuel fast reactor and pyrometallurgical reprocessing, has been proposed as such recycling technology by Argonne National Laboratory (ANL) (Chang, 1989). Metal fuel cycle possesses favorable proliferation-resistant, environment-protection, and inherent-safety attributes and also promises greatly improved economics relative to conventional aqueous reprocessing (Chang, 1989). Therefore Central Research Institute of Electric Power Industry (CRIEPI) started research and development on metal fuel cycle in 1986, including a joint study on pyrometallurgical technology with the United States Department of Energy. Major progress of the research and development in CRIEPI are described with emphasis on electrorefining and waste salt immobilization. PYROMETALLURGICAL REPROCESSING FLOWSHEET Figure 1 shows the schematic flowsheet of pyrometallurgical reprocessing of spent metal fuel. Spent metal fuel is disassembled and chopped into small pieces. In the electrorefining step, the chopped fuel is anodically dissolved, and uranium and plutonium are electrochemically separated from most. fission uroducts and collected at solid cathode and liauid-cadmium cathode with transuranic elements. Electrolvte salt and cadmium solvent that come out with the cathode products are removed by high-temperature retort l’n the cathode processing step. The purified cathode products, uranium and uranium-plutonium ingots are then mixed to adjust their composition and cast into fuel slugs by injection. 381
382
T. Nishimura et al.
Salt waste containing alkali-metal, alkaline-earth, and rare-earth fission products comes out with the electrolyte salt used in the electrorefining step. Metal waste, which consists of fuel assembly duct, cladding hull, and noble-metal fission products left in anode baskets, is generated in the disassembly, chopping, and electrorefining steps. Tritium and noble-gas fission products are discharged in the chopping and electrorefining steps.
%I;
Gas
(T,
New
Xe, Kr)
Fuel salt, Cd
I e
Electrorefming u-TRU
Zr
I
u-salt
Disassembly and Chopping
) +
Cathode Processing
Pin Casting
Crucible
Mold Crucible
TRU: Pu, Np, Am, Cm RE :RareEarth NM : Noble Metal &@JJ&&
Salt Waste (Cs, Sr, RF) Fig. 1. Schematic Flowsheet of Pyrometallurgical Reprocessing of Metal Fuel ELECTROREFINING OF METAL FUEL
Figure 2 shows typical operation sequence of the electrorefining process, which is the process for recovery of actinides (uranium and transuranic elements) by using electrochemical difference among elements fin molten LiCl-KC1 salt and liquid cadmium under high-purity argon atmosphere at 773 K as shown in Fig. 3 (Koyama, 1994).
AL : Alkali metal ALE : Alkaline earth RE :Rareearth TRU : Pu,Np, Am, Cm Fig. 2. Typical Operation Sequence of Electrorefining Process The chopped spent fuel is loaded into anode baskets and anodically dissolved. Electronegative fission products (noble metals) are left in the anode baskets in metal form. The other elements are dissolved into the
Environmentally
benign reprocessing
technology
383
electrolyte salt in the form of their chlorides. Actinides are recovered by using two different types of cathodes. At first using solid cathode, uranium chloride is reduced and essentially pure uranium collected on the cathode (see Fig. 3; Since differences of Gibbs free energy changes between uranium and other elements are large on solid cathode, only uranium will be reduced). Then changing solid cathode for liquid-cadmium cathode, plutonium chloride is reduced and collected with uranium, other transuranic elements, and a small amount of rare-earth fission products (see Fig. 3; Gn liquid-cadmium cathode, differences of Gibbs free energy changes among actinides are so small that actinides are reduced together; Since separation factors for rare earths are large, most of these remain in the electrolyte salt). Chemically more active elements (alkali-metal, alkaline-earth, and rare-earth elements) remain in the electrolyte salt. kJ/mol -600 Solid Cathode
---- _____ _b
U-700 -
Cd Cathode
NP -
-800 -
Fig. 3. Gibbs Free Energy Change at 773 K Anodic Dissolution of Snent Metal Fuel Anode baskets should offer good contact between chopped fuel and bulk LiCl-KC1 salt to prevent accumulation of oxidized a&tides in the basket and to facilitate electrotransport of them to cathodes. For this purpose two shapes of anode baskets made of perforated stainless-steel plates were tested. One is cruciform and the other is cylindrical. Figure 4 presents dissolution rates of uranium segments charged in these baskets. Higher dissolution rates and current efficiencies are obtained with cruciform baskets than cylindrical ones. The results also show that dissolution rate increases with faster rotation of baskets. Small-scale tests with irradiated metal fuel were carried out in ANL. It is reported that over 99.9 % of uranium and over 99.99 % of plutonium are removed from cladding hulls by anodic dissolution (Benedict, 1993). 1.6 1 : long (0.050 m) piece, 120 rpm 2 : short(0.025 m) piece, 120 rpm 3 : long (0.050 m) piece, 60 r-pm
‘E J$
1.4
IO
1200
1400
1600
1800
2OcO
Anodic Current Density (A*mm2) Fig. 4. Uranium Dissolution Rate with Different Anode Baskets
T. Nishimura et al.
384
p Small-scale tests of uranium electrotransport from the liquid-cadmium anode to the solid cathode were carried out with changing concentrations of uranium in electrolyte salt (about 0.5 and about 2.0 weight %) and surface areas of solid cathodes. At low concentration no deposit was observed on the cathodes. At high concentration dendritic (thin needle shape) uranium deposits were obtained on all cathodes. Photographs and collection efficiencies of these deposits are shown in Fig. 5. With the smallest cathode surface area needles of the deposit were thin and long and it was expected that part of the deposit fell off. It seems to be the reason why the collection efficiency was extremely low in this case. In the medium case the deposit seemed to be much more adherent so that the collection efficiency was high. When cathode area was increased further, concentration polarization arose at the anode and the electrotmnsport was interrupted. The collection efficiency was however fairly good. These results indicate that ratio of cathode area to anode area is one of the important parameters of electrorefiner design.
Diameter of Cathode Cathode I Anode Area Ratio Collection Efficiency
a
b
15 mm
3omm
C
7omm
0.09
0.18
0.43
9.3 ‘45
77 %
65 %
Fig. 5. Uranium Deposits on Solid Cathode of Different Diameters Results of large prototype-scale (about 10 kg of uranium) tests are reported (Steindler, 1991). About 10 kg of uranium is routinely collected on solid cathode with the maximum collection rate of 0.35 kg-uranium per hour. The collection efficiency is about 50 8 in average because deposits are shaved into cylindrical shape by using blades around the cathode. Although there seems to be no crucial problem in recovery of uranium on solid cathode, improvement in processing rate is required.
One of the problems in liquid-cadmium cathode is formation of uranium dendrite at the surface of the cathode. When uranium deposits faster than it is taken into bulk liquid cadmium, uranium dendrites form at the surface of the cathode. Once the dendrite grows up, electric current is concentrated on it and it works as a solid cathode where only uranium deposits. In order to avoid this problem we tested stirring of liquid cadmium using a paddle-shape stirrer, which is expected to enhance transport of deposited uranium from surface into bulk cadmium. Figure 6 presents the effect of stirring and cathode current density on dendrite formation (Koyama, 1995a). The height of each bar indicates how much uranium can be collected into the liquid-cadmium cathode before dendrite formation. It can be seen that more uranium is collected under impetuous stirring and low current density condition. Uranium was collected up to 10 weight 8 into the cathode with current efficiency of approximately 100 %.
Environmentally benign reprocessing
technology
385
Another type of liquid cadmium cathode called “ pounder type” has been studied in ANL. (Battles, 1994). A cylindrical ceramic block with a small opening (pounder) rotates and moves up and down over the surface of liquid-cadmium cathode at the same time. Uranium deposits are supposed to be pushed down into bulk cadmium by the motion of the block. It is reported that more than 10 weight % of plutonium is collected into the cathode at a rate of about 10 kg/m2/h in small-scale experiments with a current efficiency of about 100 %. But uranium dendrite formation was still observed at high uranium/plutonium ratios and presence of rare-earth elements in salt.
Uranium in LCC T 10 (
wt.% )
I-8 -6 -4
U Solubility + (2.35 wt.%)
2 r
Cathode Current Density ( kA*mm2)
Fig. 6. Maximum Amount of Uranium Taken in Liquid-Cd Cathode before Dendrite Formation Comouter Code Develooment for Design Outimization a) Simulation of Electrochemical Behavior. Composition of electrolyte salt is not kept constant durmg electrorefining operation. In order to find out the optimum conditions, a computational code, TRAIL, wan;developed for analysis and prediction of the electrotransport behavior of elements (Kobayashi, 1993). TRAIL employs a diffusion- limited electrochemical reaction model. A comparison between results of electrotransport tests with uranium and plutonium reported by Tomczuk (1992) and results calculated by TRAIL is shown in Fig. 7. The figure shows good agreement and it indicates that the behavior of each elements in the electrorefining step can be predicted by TRAIL. 0 I 0
TRAIL Calculation Experiment
20
40
60
Time(h) Fig. 7. Comparison of Electrochemical Behavior between Calculated Results and Experimental Data
T. Nishimura er al.
386
bL In order to develop a high-throughput electrorefmer, the electric resistance of the electrorefiner should be minimized and the electric current of it should be maximized. Since cell resistance depends on geometrical configuration of electrodes in the electrorefiner, a two-dimensional finite-element-method code, CAMBRIA, was developed for analysis of electric potential and current distribution by solving the Laplace equations (Kobayashi, 1995). Although this code calculates electric resistance without polarization effect, it predicts well as shown in Fig. 8. 30 ??
9
\ !
s25-
E
---O-
Experiment
TRAIL Calculation
820 3 7I 4 2 15 _ : 310
-
i ,---___ --__ --_
----.___
5 50
100
150
200
250
??
--__ 300
.-.a
350
400
Cathode Area (10m4 m2) Fig. 8. Cell Resistance of Electrorefiners TREATMENT OF WASTES Salt Waste Treatment After many batches of spent fuel are processed in the electronsfiner, a large amount of chemically active fission products (alkali-metal, alkaline-earth, and rare-earth fission products) are left in the form of chloride and accumulated in the electrolyte salt. These fission products generate problems such as decay heat removal, a rise in melting point of the salt, and an gradual increase in contamination of cathode products with fission products. The waste salt should be disposed of. Unfortunately the salt is water soluble and high-level radioactive, and alkali-metal and alkaline-earth chlorides are difficult to be dissolved into borosilicate glass waste form in high concentration, Therefore a method to immobilize the waste salt should be developed. At present two different immobilization methods are under study in CRIEPI. One is vitrification after electrolysis of salt waste into metallic waste and chlorine gas (Atomic Energy Society of Japan, 1995). Another is conversion to sodalite, N%[(AlO,),(SiO2, J*WaCl, which is a natural-occurring mineral containing chloride salts in its structure (Koyama, 1995b). In our study it is found that sodalite can be synthesized according to the following dry reaction without gas generation: 6NaA102 + 6SiOr + 2NaCl= N~[(AlO&oiO&]*2NaCl
(1)
We could also make sodalite-type waste form from a mixture of NaAlOr, SiO,, and simulated waste salt by pressing at 200 MPa and heating it at a temperature of 973 to 1173 K for 50 to 100 hours. The structures of the products were identified by X-ray diffraction. Leachability was measured for the synthesized specimen. As shown in Table I measured leachability of elements are as low as the values reported for zeolite and vitrified waste forms (Lewis, 1993). Other Wastes Stainless-steel duct and cladding are disposed of as metal wastes from disassembly and chopping step. The electronegative fission products (noble metals) and fuel-constituent zirconium remain in anode baskets and become metal waste. One possible way of immobilization of noble-metal fission products is to be dispersed in stainless-steel and zirconium alloy in containers. Tritium and noble gases (Xe and Kr) are released from spent fuel into pure argon atmosphere during chopping and electrorefining. Tritium will be converted to tritium-water and sorbed on molecular sieve. The noble gases will be separated from argon by cryogenic distillation, compressed and stored in high-pressure gas cylinder. So it would be expected that there is no release of fission gases in the pyrometallurgical reprocessing.
Environmentally benign reprocessing technology
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Table I. Comparison of Normalized Release Rates (g/m*d) Element Li K Na Cs Sr Ba Al Si Cl I
Synthesized sodalite 0.0047 0.0061 0.0023 0.0013 0.00035 0.000069 0.0015 0.0017 0.0022 0.0029
Zeolite *’ 0.12 0.031 0.018 0.0066 <1.0x lo-5 < 1.0 x 10” 0.0048 0.0041 ::
Borosilicate glass *I 0.29
0.22 0.28 NM NM 0”: 0.23 ::
NM : not measured *’reported values (Lewis, 1993) SUMMARY
Major progress and present status of our research and development are summarized on electrorefining and waste treatment processes. Although it is still in an early stage of development comparing to the conventional aqueous reprocessing, any crucial problems regarding actual application of pyrometaJlurgica1 reprocessing technology are not found yet. We are planning to continue research and development to establish technical bases necessary for practical application of this technology as advanced reprocessing technology together with research and development of metal-fuel fast reactor technology. ACKNOWLEDGMENT The authors wish to acknowledge contributions from joint studies with Toshiba Corp. on electrorefining and Hitachi Ltd. on salt waste treatment. We also acknowledge information exchange with Argonne National Laboratory under the joint study with the United States Department of Energy. REFERENCES Atomic Energy Society of Japan (1995) Metal Fuel Cvcle Technologv - Present Status and Future Persnective, Atomic Energy Society of Japan, p. 18 1. Battles J. E. et al. (1994) Chemical Technolopv Division Annual Technical Renort 1993, Argonne National Laboratory Report ANL-94115. Benedict R. W. et al. (1993) Small-scale Irradiated Fuel Electrorefining, GLOBAL ‘93, p. 1331, Seattle, 12-I 7 Sentember. Chang Y. I. (1989) The Integral Fast Reactor, Nucl. Technol.. 88,129. Kobayasbi T. and Tokiwai M. (1993) Development of TRAIL, a Simulation Code for the Molten Salt Electrorefining of Spent Nuclear Fuel, J. Allovs and Comnounds. 197,7. Kobayashi T. et al. (1995) Investigation of Cell Resistance for Molten Salt Electrorefming of Spenr Nuclear Fuel, J. Nucl. Sci. Technol.. 32,68. Koyama T. et al. (1994) Pvrometallurav Data Book, CRIEPI Report T93033. Koyama T. et al. (1995a) Development of Molten Salt Electrorefining - Studv of Uranium Denosiiion Liauid Cad ‘urn Cathode without Formation of Dendrite, CRIEPI Report T95004. Koyama T. et% (1995b) Immobilization of Halide Salt Waste from Pyrochemical Reprocessing by Forming Natural Occurring Mineral; SODALITE, GLOBAL ‘95, p. 1744, Versailles, 1l-14 September. Lewis M. A. et al. (1993) Salt-occluded Zeolites as an Immobilization Matrix for Chloride Waste Salt ,iJ Am. Ce am Sac .76 2826. Steinf;ler ‘M. J: et A. (1991) Chemical Technoloev Division Annual Technical Report 1990, Argonne National Laboratorv Renort ANL-91/18. Tomczuk Z.-et al: (1992) Uranium Transport to Solid Electrodes in Pyrochemical Reprocessing of Nuclear Fuel, J. Electrochem. Sot.. 139,3523.