Recovery of Noble Metals from Insoluble Residue of Spent Fuel

Journal of NUCLEAR SCIENCE and TECHNOLOGY, 23[6], pp. 540-549 (June 1986).

Keiji NAITO, Tsuneo MATSUI and Yuji TANAKAt Department of Nuclear Engineering, Faculty of Engineering, Nagoya University*

Received October 1, 1985

The recovery of Ru, Pd and Mo from the simulated insoluble residue produced in dissolver solution of spent fuel was studied by means of Pb extraction and the recovery was found to be more than 80% for Ru and Pd. It was also found that the recoveries of Ru, Pd and Mo were dependent on melting temperature and gas atmosphere. Observations by scanning electron microscopy and X-ray microanalysis indicated that the noble metals recovered in Pb button were contained in both forms of e-Ru(Pd, Mo) alloy and single metal. The solubility in nitric acid of Pd recovered in Pb was much higher than that of Pd in -Ru(Pd, Mo) alloy, suggesting the separation of Pd from e-Ru(Pd, Mo) alloy. The decon-e tamination factor for Ce as a stand-in of Pu was determined to be around 200 for one stage. KEYWORDS: cerium, decontamination factor, dissolver solution, fuel reprocess- ing, high level radioactive liquid wastes, insoluble residue, lead extraction, molybdenum, noble metals, palladium, recovery, ruthenium, spent fuels

I. INTRODUCTION The Pt-group metals are valuable and important as noble metals, but their natural resources are rather limited. Three of Pt-group metals, Pd, Rh and Ru, are produced as the fission products in nuclear reactors(1). As the number of the nuclear power plants increases, the amount of these noble metals produced in nuclear reactors increases, which is thought to become comparable to that of the natural resources in the world. The fission produced noble metals, thus, can be recognized as the important alternative resources to meet a part of the increasing need of noble metals. The various methods for separating the noble metals from other fission products have been proposed(2)~(7). Campbell(2) separated Rh and Pd from alkaline Purex waste by liquid- liquid extraction with tricapryl mono-methyl ammonium chloride. Panesko(3) recovered Pd selectively from alkaline Purex waste by activated charcoal. Liljenzin et al.(4) presented a method for separating noble metals from other fission products in nitric acid solution as a step of the process of removing the actinides from the high level radioactive liquid waste (HLLW). In their method, noble metals such as Pd, Ru and Tc were extracted from nitric acid solution by tributyl phosphate (TBP), then stripped by 9 M HNO3. Jensen et al.(5)~(7) reported lead oxide extraction as a method for recovering noble metals from fission product mixed oxides, which are obtained by calcining the HLLW. Some parts of the fission produced noble metals, however, still exist as the main constituents of the insoluble residue in dissolver solution of spent fuel(8)(13), which are to be separated from the dissolver solution by filtration as "sludge". The recovery of noble * Furo-cho, Chikusa-ku, Nagoya 464. t Present address : Japan Nuclear Fuel Service, Co. Ltd., Uchisaiwai-cho,Chiyoda-ku, Tokyo 100. 64 Vol. 23, No. 6 (June 1986) 541

metals from the insoluble residue is considered to be simpler and easier than that from HLLW. Smith & McDuffle(14) recently reported liquid metal extraction (separation method between two immiscible liquid metal systems such as Mg/U-576Cr, Mg/U-11%Fe, Al/Bi) for removing noble metals from insoluble residue, but their method is applicable to the mutual separation of noble metals rather than to the recovery from insoluble residue. In the present study, Pb metal is selected as a scavenger for noble metals and also a material to immobilize and store the radioactive noble metals till next mutual separation, and Na2B4O7 or B2O3 is used as a glass forming material to fix other constituents of insoluble residue. The recovery of noble metals is investigated by changing the operating temperature and time, gas atmosphere and bubbling time of molten Pb. The solubility in nitric acid of Pd recovered in Pb is also determined. The decontamination factor (DF) for Pu in this recovery process is very important. To evaluate the DF of Pu, the DF of Ce is measured as a stand-in of Pu, since the standard free energy of formation of CeO2 is similar to that of PuO2 over wide temperature range. It is, however, noted that the estimated value of DF for Pu obtained in this study should be verified by using Pu itself.

II. CONCENTRATION AND SPECIFIC ACTIVITY OF NOBLE METALS

1. Amounts Contained in Spent Fuel The change in the concentrations (g/MTIHM (metric ton of initial heavy metal)) and that in the specific activities (Ci/g) of Ru, Rh and Pd produced as the fission products in 1t of PWR spent fuel at 33,000 MWd/t were calculated by the isotope generation and depletion computer code ORIGEN 2(15). The results are shown against cooling time in Fig. 1(a),~(c), where the solid and dotted lines indicate the concentration and the specific activity, respectively, and the number on each line indicates the mass number of each isotope. Chain line indicates a level of the specific activity of 2 x 10-3 mCi/g, below which the fission products are not regarded as radioactive materials in practice. It is seen from Fig. 1(a) that the amount of Ru reaches about 2.2 kg/MTIHM and Ru is produced by the decay of (99)Tc with a half-life of 2.14 x 105 yr. The specific activity99 of Ru decreases gradually with the decay of 106Ru and becomes to be below the level of 2 x 10-3 mCi/g after 37 yr from shut-down of the nuclear reactor. As seen in Fig. 1 (b), the yield of Rh is about 0.47 kg/MTIHM. Rhodium-106 (half-life =29.8 s) and 103mRh (half-life=56 .1 min) are in radioactive equilibria with 106Ru and 103Ru, respectively. Rhodium-102 and 102mRh are produced by the nuclear reactions : 103Rh(n, 2n) 102Rh and 102Rh(n , 2n) 102mRh, respectively. The circular and triangular points of 102Rh and Rh, respectively, shown in Fig. 1 (b) are the experimental data obtained from the activity102m analysis of Rh in spent fuels with burn-up 33,000 MWd/t by Roberts(16) from which the decay curves of 102Rh and 102mRh are estimated as shown in the chain lines in Fig. 1 (b). The specific activity of Rh is finally predominated by 102Rh with a half-life of 2.9 yr and decreases below the level of 2 x 10-3 pCi/g after 49 yr from shut-down of the nuclear reactor. Therefore it is concluded that the radioactivities of the fission produced Ru and Rh will reach a sufficiently low level to permit unrestricted commercial use after the storage of about 50 yr. The concentration of Pd is seen to be about 1.4 kg/MTIHM from Fig. 1(c). Since a radioactive isotope 107Pd with an extremely long half-life of 6.5 x 106 yr exists in the fission produced Pd, cooling effect cannot be expected for the case of Pd. However, since 107Pd decays with soft b-emission (33 keV), some uses with radiation shield may be available. 65 542 J. Nucl. Sci. Technol.,

(a) Ru

(b) Rh

66 Vol. 23, No. 6 (June 1986) 543

2. Amounts to be Recovered from Insoluble Residue In this study, the recovery of fission produced noble metals is intended from the insoluble residue produced when a spent fuel is dissolved in nitric acid. The fractions of the fission produced noble metals included in insoluble residue to the total yield of spent fuel are estimated to be about 50 w/o for Ru, about 25 w/ofor Rh and about 20 w/ofor Pd(8)~(2). The other parts of noble metals are present in the dissolver solution as a form of ion or colloid, and go to the HLLW.

III. EXPERIMENTAL 1. Recovery Process Concept The Pt-group metals occur together naturally as alloys in mineral deposits. The natural Pt-group metals are recovered as the concentrates by electrolytic refining(17). The concentrates is dissolved in aqua regia and then noble metals in the residue are separated by means of lead oxide extraction. Since the constituent of the residue in aqua regia is similar to that of the insoluble residue in dissolver solution of spent fuel, the recovery process for natural noble metals may be similarly applicable to the fission produced noble metals. The recovery process of fission produced noble metals used in the present study is schematically shown in Fig. 2. Insoluble residue is mixed with Pb metal (scavenger) and glass-forming materials. This mixture is melted at high temperature and then splits into two phases by difference of the density : one is a Pb button phase containing the noble metals and the other is a glass phase containing actinide oxides and crud components. The glass phase is to be treated and disposed as solid radioactive waste. The radioactive noble metals in Pb metal are stored for about 50 yr, which is necessary for the decay of 106Ru and 102Rh, and then separated mutually by the method commonly adopted for the natural noble metals. Since the radioactive noble metals are immobilized in Pb metals, radiation shield and removal of decay heat can easily be done. This method, therefore, has the merit that both the fixation of sludge containing insoluble residue and the recovery of noble metals can be carried out simultaneously. However, all of noble metals produced in a spent fuel can not be recovered by this method, but 20 to 50 w/o of the total amounts are recovered as mentioned already. The recovery process shown in a dotted line square in Fig. 2 was Fig. 2 Block diagram for recovery process of fission produced noble metals investigated in this study. The principle of the recovery process tested in this study is similar to that recently developed for the recovery of noble metals from HLLW by Jensen et al. at Pacific North- west Lab. (PLN) (5)~(7)which also resembles basically to that usually adopted for the recovery of natural noble metals(17). But the present method described above is different from that of PNL group in the following points : 67 544 J. Nucl. Sci. Technol.,

(1) Operating temperature : When dealing with the high radioactive materials by dry process, the contamination of the surroundings by vapors and fine particles of radioactive materials should be minimized. At high temperature of 1,373 K adopted by the PNL group, Ru and Tc cannot be recovered sufficiently due to volatile loss in the forms of Tc2O7(g) and RuO4(g), as was written in their report(7). Furthermore higher operating temperature imposes high thermal load on the apparatus. The operating temperature in the present study is attempted to be in the range of 823~1,173K by selecting the glass forming materials with low melting temperatures, such as B2O3 (m. p.=723 K) and Na,2B4O7(m. p.=—1,016K), and Pb metal (m. p. =600 K) as a scavenger. (2) Chemical form: The noble metal elements exist mainly in various forms of ion, complex or colloid in HLLW, although a mixture composed of RuO2, PdO and Rh2O, was used as the simulated fission product calcined mixture in the PNL experiments(7). On the other hand, the noble metals in insoluble residue exist as the Mo-Tc-Ru-Rh-Pd alloy(8)~(12). In the present study, Mo-Ru-Pd alloy is used as the simulated fission produced noble metals of the insoluble residue. (3) Decontamination of Pu: Plutonium is usually included in insoluble residue. In the present study, the fixation of PuO2, in glass is evaluated by CeO2 as a simulation of Pu02. 2. Sample Preparation The composition of the simulated residue used in the present study is shown in Table 1 in comparison with the averaged composition of insoluble residue(8)~(3). It is assumed, as seen from Table 1, that the insoluble residue is composed of 70 w/o Mo-Tc-Ru-Rh-Pd alloy with a hexagonal close-packed (hcp) structure, 4w/o actinide oxides such as UO2 and PuO2, and 26 w/o cladding and crud components(8)~(13). The chemical properties of Tc and Ru are considered to be similar and those of Rh and Table 1 Compositions of insoluble residue Pd, too, since both Tc and Ru possess the and simulated residue same hcp structures and are completely miscible each other(18), and both Rh and Pd have the same face-centered cubic (fcc) structures and are completely miscible each other.(19). The ternary Mo-Ru-Pd alloy, therefore, was used for this experiment in- stead of the Mo-Tc-Ru-Rh-Pd alloy. The mixture of Ru, Mo and Pd powders (all are of 99.9% purity) was pressed into pellets. The pellets were heated in a tungsten Knudsen cell by the high-frequency induction coil in a vacuum of 4 x 10-5 Pa at 1,473 K for 6 h and then 1,573 K for 2 h. The X-ray diffraction analysis indicated that each sample was of single e-Ru (Pd, Mo) phase with hcp structure. t Average composition at burn up 33,000 MWd/t The pellets were pulverized into powder form, is estimated from the composition data in Refs. (8)~(13). and then used for the recovery experiment. The mixed powder of 0.070 g e-Ru (Pd, Mo), 0.004 g CeO2 and 0.74 g glass forming materials (B2O3 or Na2B4O7), and 3.7 g granular Pb metal were placed in a porcelain crucible and then lifted up by a magnet to the hot zone of a preheated furnace and melted at 68 Vol. 23, No. 6 (June 1986) 545 temperatures (823<=T/K<=1,173) for the time from 10 min to 10 h in a stream of air or Ar gas (Po2~1 Pa). After melting, the specimen was quenched from the furnace into the cold zone in a dryice-ethyl alcohol trap. The effect of stirring of the molten Pb by gas bubbling on the recovery of noble metals was also studied : After melting the sample for 15 min, air or Ar gas was introduced into the molten sample in the porcelain cruicible through an alumina or a stainless steel tube to bubble with a constant gas flow rate (~1 cm3/s) and then quenched. 3. Analysis The distribution of noble metals in Pb buttons was observed by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The concentration of noble metals recovered in Pb button was determined by inductive coupled plasma (ICP) spectrometry. Cerium in Pb button was also analyzed to evaluate the decontamination factor of Pu.

IV. RESULTS AND DISCUSSION

1. Distribution of Noble Metals in Pb Secondary electron micrographs of Pb buttons and characteristic X-ray photographs of Ru, Pd and Mo in the Pb buttons by EPMA are shown in Photo. 1(a)~(c) as a typical ex- ample for the distribution of the noble metals in Pb, where the Pb buttons were obtained at 1,023 K for 5 h of melting. In this figure, three kinds of samples are shown : (a) the upper part of a Pb button close to the boundary surface between glass and Pb in air flow, (b) a middle part of a Pb button in air flow and (c) the upper part of a Pb button close to the boundary surface between glass and Pb in Ar flow. It is seen from Photo. 1 that the noble metals recovered in a Pb button are not homogeneous. Ruthenium is seen to be

Recovery condition : Pb : Na213,07: residue =50 : 10 : 1 (weight ratio), T=1,023 K, 5 h, air flow for (a) and (b), Ar flow for (c) Photo. 1(a)~(c) Secondary electron micrographs and characteristic X-ray photographs 69 546 J. Nucl. Sci. Technol.,

segregated in the form of particles and simply occluded in a Pb button. Palladium seems to be located not only with Ru at the same place but also to exist separately and/or distribute rather uniformly in Pb button. Some of the Pd are, thus, thought to be separated from e-Ru (Pd, Mo) alloy initially mixed with Pb metal during the recovery process and form a solid solution with Pb. The separation of Pd will be discussed again in the section of dissolution of Pd in nitric acid in this paper. Trace amount of Mo is observed in Photo. 1 (a) but no Mo is seen in Photo. 1 (b). Considering these facts mentioned above, together with the fact that Mo recovered in Ar gas flow is located with Ru at the same place in Pb buttons as seen in Photo. 1 (c), a part of Mo may be separated from e-Ru (Pd, Mo) alloy by being oxidized in air and then recovered in glass. 2. Recovery of Noble Metals The temperature dependence of the recovery of noble metals in Pb button obtained in this study is shown in Fig. 3. The recoveries of Ru, Pd and Mo are seen to increase with increasing temperature in the case of B2O5 used as a glass forming material but to be nearly constant in the case of Na2B4O7 irrespective of temperatures from 1,023 to 1,173 K. The recovery with Na2B4O7 was higher than that with B2O3 at 1,023 K. Since viscosities of Na2B4O7 and B2O03glasses change with temperature(20), the recovery is plotted in Fig. 4 as a function of viscosity at each experimental temperature. The decrease of the recovery with increasing viscosity of glass may be explained by two reasons : First, the fine powder of the simulated noble metal is easily occluded in high-viscosity glass. Second, high- viscosity glass does not easily separate from Pb button.

•› Ru, •¢Pd, • Mo (Na2B4O7, Ar) •› Ru, •¢Pd, • Mo (Na2B4O7, Ar) Ru, •£Pd, •¡Mo (B203, Ar) •œ Ru, •£Pd, •¡Mo (B2O3, Ar) •œ Pb : glass : residue =50 : 10 : 1, melting for 5 h Pb : glass : residue =50 : 10 : 1, melting for 5 h

Fig. 3 Recovery vs. melting temperature Fig. 4 Recovery vs. viscosity of glass

The effects of melting time and gas atmosphere (air flow or Ar flow) on the recovery of noble metals were also investigated. The effects are likely to be small as seen from Fig. 5. Nearly equal recovery of Ru and Pd after 10 min to that after 10 h indicates that recovery with Pb almost completes once molten Pb occludes noble metal powders and then separates from glass. On the other hand, the recovery of Mo in air flow decreases with time probably due to the removal of oxidized Mo from Pb button by vaporization and/or transference to glass from Pb.

70 Vol. 23, No. 6 (June 1986) 547

•› Ru, •¢ Pd, • Mo (Ar); •œ Ru, •£ Pd, •¡ Mo (air) Pb : Na2B4O7: residue=50 : 10 : 1, T=1,023 K •› Ru, •¢ Pd, • Mo (bubbling with Ar, 0.6 cm3/s) Fig. 5 Recovery vs. melting time Ru, •£ Pd, •¡ Mo (bubbling with air, 0.7 •œcm3/s) Pb : Na2B4O7: residue =50 : 10 : 1, T=1,023 K

The recovery is also measured by bub- Fig. 6 Recovery vs. bubbling time bling molten Pb with Ar or air gas flow. It is seen from Fig. 6 that the stirring effect on the recovery of Ru and Pd is not significant but that of Mo is very notable. Therefore gas bubbling is effective to remove Mo from molten Pb (i. e. from noble metals). In Fig. 7, the recovery is plotted against the weight ratio of Pb metal to simulated residue. The recoveries of Ru and Pd seem to be constant independently of Pb/residue weight ratio. It may be concluded that the Pb/residue weight ratio 5 or 10 is enough to obtain a high recovery. The reason for the large decrease of the recovery of Mo in air at high Pb/residue ratio is not clear at present. 3. Solubility of Pd in Nitric Acid To examine the mutual separability of •› Bu, •¢ Pd, • Mo (Ar); •œ Ru, •£ Pd, •¡ Mo (air) Na2B4O7 : residue =10 : 1, T=1,023 K, t=5 h noble metal elements recovered in Pb but- Fig. 7 Recovery us. Pb/residue weight ratio tons, a dissolution test was done. When Pb buttons are dissolved in nitric acid, some part of Pd is dissolved but Ru is almost not. The weight fraction of Pd dissolved in hot 3 M nitric acid (37 ml for 3.7 g of Pb) to total of Pd recovered in Pb button is plotted against melting time during the Pb extraction in Fig. 8. The solubility of Pd recovered in Ar flow at 1,023 K seems to be constant independently of the melting time, but to increase with increase of melting temperature probably due to the high probability of separation of Pd from e-Ru (Pd, Mo) alloy to form 71 548 J. Nucl. Sci. Technol., a solid solution with Pb. The solubility of Pd recovered in air flow is seen to be higher than that recovered in Ar flow. The high solubility of Pd recovered in air flow, especially after 1 h of melting time may be explained by the oxidation effect of Mo which causes separation of Pd and Mo from e-Ru (Pd, Mo) alloy. The separation of some part of the Pd was observed in Photo. 1 and the oxidation of Mo was discussed in relation to the low recovery of Mo in Fig. 5. Therefore, air flow is more suitable for the recovery •› Ar, •œ air : T=1,023 K AT T=1,123 K•¢ p Pb Na2B3O7 : residue process than Ar gas flow from the point of ^Ar T=1,1731( =50 : 10 : 1 • view of mutual separability of Ru and Pd. Fig. 8 Solubility of Pd in hot nitric The solubility in nitric acid of Pd in acid us. melting time e-Ru (Pd, Mo) alloy at pressent composition was determined to be around 3% by a separate experiment. It is therefore concluded that the solubility of Pd in e-Ru (Pd, Mo) alloy in nitric acid is increased by smelting e-Ru (Pd, Mo) alloy with molten Pb and by air flow during the Pb extraction process. 4. Decontamination Factor for Ce The decontamination factor for Ce, as a stand-in of Pu, was determined by measuring the concentration of Ce in a Pb button by ICP spectrometry. The amount of Ce was found to be 17+-10 mg in Pb button melted at the following experimental condition : Pb : Na2B4O7: residue=50 : 10 : 1 in weight ratio (Pb 3.7 g, CeO2~4mg), 1,023 K for 5 h and air flow. The decontamination factor for Ce was thus roughly calculated to be about 200 for one stage.

V. CONCLUSION Concentration and specific activity of fission produced noble metals (Ru, Rh and Pd) calculated with the computer code ORIGEN 2 showed that the radioactivities of the fission produced Ru and Rh will reach a sufficiently low level to permit unrestricted commercial use after the storage of about 50 yr, while the fission produced Pd will still remain as radioactive. The recovery of fission produced noble metals from the insoluble residue produced in dissolver solution of spent fuel was studied. The recovery of Ru and Pd from the simulated residue was obtained to be more than 80% by means of Pb extraction. The recovery of Ru, Pd and Mo was found to depend on melting temperature (the viscosity of glass) and atmosphere (oxidation of Mo in simulated residue). The noble metals (Ru and Pd) recovered in Pb buttons were observed to be contained in a form of fine particles of e-Ru (Pd, Mo) alloy or separately from the alloy. Molybdenum recovered in Pb buttons in air flow se- emed to exist separately from e-Ru (Pd, Mo) alloy. Some fractions of Mo were oxidized in air and removed from Pb button. The solubility of Pd recovered in Pb buttons in nitric acid was very high in comparison with that of Pd in e-Ru (Pd, Mo) alloy. The decontamination factor for Ce as a stand-in of Pu was determined to be around 200 for one stage.

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ACKNOWLEDGMENT The authors are indebted to Prof. H. Sakao of Nagoya University for the use of SEIVI and EPMA in this study. —REFERENCES

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