Chemical and Isotopic Characterization of Noble Metal Phase from

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Chemical and Isotopic Characterization of Noble Metal Phase from Commercial UO Fuel 2

Kristi L. Pellegrini, Chuck Z. Soderquist, Steve D. Shen, Eirik J. Krogstad, Camille J. Palmer, Kyzer Gerez, Edgar C Buck, Timothy G. Lach, Jon Michael Schwantes, and Richard Aaron Clark Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05549 • Publication Date (Web): 25 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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Analytical Chemistry

Chemical and Isotopic Characterization of Noble Metal Phase from Commercial UO2 Fuel Kristi L. Pellegrini a†, Chuck Z. Soderquist a, Steve D. Shen a, Eirik J. Krogstad a, Camille J. Palmer b, Kyzer R. Gerez b, Edgar C. Buck a, Timothy G. Lach a, Jon M. Schwantes a, Richard A. Clark a,* a b

Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, WA 99352 Oregon State University, 116 Radiation Center, Corvallis, OR

ABSTRACT: We report elemental and isotopic analysis for the noble metal fission product phase found in irradiated nuclear fuel. The noble metal phase was isolated from three commercial irradiated UO2 fuels by chemically dissolving the UO2 fuel matrix, leaving the noble metal phase as the undissolved residue. Macro amounts of this residue were dissolved using a KOH+KNO3 fusion and then chemically separated into individual elements for analysis by mass spectrometry. Though the composition of this phase has been previously reported, this work is the most comprehensive chemical analysis of the isolated noble metal phase to date. We report both elemental and isotopic abundances of the five major components of the noble metal phase (Mo, Tc, Ru, Rh, Pd). In addition, we report a sixth element present in high quantities in this phase, tellurium. Tellurium appears to be an integral component of noble metal particles.

Fission of U-235 and Pu-239 produces a little over 30 elements as fission products, including three of the six platinum metals (ruthenium, rhodium, and palladium). As the fuel burns, many of the fission products segregate in various places in the fuel, according to their chemical reactivity and mobility in the UO2 fuel. One of the segregated phases of fission products, well-known since the 1960s, is a metallic phase consisting largely of molybdenum and ruthenium, with smaller amounts of technetium, rhodium, and palladium.1-8 This phase has gone by several names in the literature, including white inclusions,12 fission product alloy,2 5-metal particles,9 epsilon particles,10-11 and noble metal phase.12-13 In this paper, we refer to it as the noble metal phase. When UO2 fuel is dissolved in nitric acid for chemical processing or for analysis, the noble metal phase remains largely undissolved as a chemically unreactive fine black sludge, and constitutes as much as several tenths of a percent of the irradiated fuel in commercial fuel reprocessing.14 Ruthenium is a major component of this phase and is virtually insoluble in nitric acid.15 In cut and polished UO2 fuel, the particles can appear to be up to a few microns across. However, once isolated and more closely inspected, these noble metal phase particles are shown to be aggregates of even smaller particles, a few nanometers in diameter.14 A phase this fine has large surface area which should make it more reactive and easier to dissolve, but part of it remains undissolved even after long treatment with strong, hot nitric acid. This non-reactivity is presumably caused by the ruthenium and rhodium within the phase.

Much of the early investigation of the noble metal phase was done by electron microprobe analysis on cut and polished fuel. Electron microprobe analysis has the advantage of looking directly at raw fuel, not attacked or altered by nitric acid. However, electron microprobe analysis is limited by the extremely high x-ray background from the intensely radioactive fuel and moderate sensitivity of the instrument. The presence and location of other minor fission products in the noble metal phase may not be apparent by microprobe analysis. Another approach to analysis of the noble metal phase is to analyze the material separately from the irradiated fuel matrix. The noble metal phase can be isolated by dissolving away the UO2 fuel in nitric acid. However, the undissolved solids probably have a different composition than they had in the raw spent fuel because some of the components of the noble metal phase are soluble in hot nitric acid (molybdenum, technetium, and palladium). The more soluble components originally present presumably leach out, partially or completely, and alter the measured composition. Previous work by Adachi et al14 and Cui et al (2004)16 dissolved spent UO2 fuel in 4M nitric acid at 100°C and 85% phosphoric acid at 113°C, respectively. Their results showed that the insoluble residue was largely ruthenium and molybdenum, with smaller amounts of technetium, rhodium, and palladium. Adachi et al also reported chromium, iron, and nickel, which are not fission products and could not have come from the fuel itself.14 Cui et al (2004) also reported smaller amounts of tellurium, but with large uncertainty.16

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The noble metal phase used in this work was obtained by dissolving the UO2 in a solution of ammonium carbonate and hydrogen peroxide.17 The dissolution operates at room temperature, one atmosphere pressure, and a pH around 10. This method is less aggressive than hot nitric acid or phosphoric acid used in previous studies. Under these mild conditions, the pulverized fuel and most of the actinides and fission products readily dissolve, leaving an extremely fine black suspension of the noble metal phase. The noble metal phase is presumably less attacked than it would have been if nitric acid or phosphoric acid had been used to dissolve the fuel. Ammonium carbonate solution with hydrogen peroxide is much less corrosive than hot nitric or phosphoric acid. In this paper, we report the chemical and isotopic composition of the noble metal phase isolated from irradiated fuels at three different burnups. Taking great effort to separate neighboring elements in order to avoid or minimize isobaric interferences prior to mass spectrometric analysis, we report the isotopic abundances of elements making up the noble metal phase in used nuclear fuel. These results are compared with depletion code predictions of the isotope inventories. Experimental Section Description of fuels. The noble metal material used in this work were from commercial zircaloy-clad UO2 fuel spanning a range of burnup. These fuels were extensively characterized at Pacific Northwest National Laboratory (PNNL) as part of an effort to make used fuel reference standards. The characterization reports are given in the far right column of Table 1. Table 1. Summary of Fuels Used for Analysis Fuel

Reactor Type

Approx. Burn-up (GWd/MTU)

Years Out of Reactor

Sources

ATM-105

BWR

30

34

18

ATM-106

PWR

45

36

19

ATM-109

BWR

65-70

24

20-21

Soderquist, et. al (2011) describes the handling and dissolution of the fuel.17 In brief summary, approximately 13 grams of each fuel was mechanically removed from its cladding, crushed, and sieved through a 212-µm mesh sieve. The crushed and sieved fuel was then dissolved in a solution of ammonium carbonate (200 g/liter) with periodic additions of 30% hydrogen peroxide, while stirring at room temperature. This solution had a pH of about 10. The UO2 fuel and most of the fission products dissolved readily, leaving a fine black residue of the noble metal phase. The black residue was centrifuged out of solution, rinsed with water to remove the ammonium carbonate solution, and then rinsed with 0.5M nitric acid to remove any precipitated carbonates. The black residue was then dried and weighed. At this point, the dose of the separated noble metal phase was sufficiently low to allow it to be removed from the hot cell and stored until further use. Dissolution of noble metal phase. The noble metal phase was dissolved for analysis by fusion with KOH+KNO3 in a nickel crucible. The mass of dry noble metal phase could not be accurately weighed; the sample containers had decomposed during storage and got plastic debris into the samples. The dry residue (approximately 50-100 mg, but contaminated with plastic debris) was placed into a 10-mL nickel crucible,

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previously fired to a shiny black finish. A solid flux of KOH + KNO3 was prepared in a ratio of 1.5 g KNO3: 2 g KOH. The dry flux, three times the weight of the sample, was added on top of the noble metal sample and stirred to mix. The amount of flux was chosen to minimize potassium in the final solution. The mixture was fused on a hot plate set on high heat. After cooling, the fused mass was dissolved in water and centrifuged to recover any undissolved noble metal phase. The undissolved noble metal phase was placed back into the crucible and dried there, and then it was fused again with fresh flux. Three successive fusions were required to completely dissolve the black noble metal phase. The final product was diluted to 250 mL in a glass volumetric flask in approximately 0.5M HCl. The product solution appeared deep orange and no black solids were visible or settled out of solution upon standing. Table 2: Masses of Raw UO2 Fuel and Undissolved Residue Fuel

Starting Mass (g)

Residue mass (mg)

ATM-105

13.087

26.6a

ATM-106

13.167

121.4

ATM-109

12.054

232.0

aRecent

analysis shows a transcription error was likely with ATM-105 mass of undissolved solids.

Radiochemical analysis of noble metal phase. The noble metal solutions were analyzed for gamma emitters, plutonium, strontium-90, and technetium-99. The elemental composition was measured by inductively coupled plasma-optical emission spectroscopy (ICP-OES). Gamma emitters were measured using a high-purity germanium gamma spectrometer. Plutonium Analysis: To measure plutonium, a subsample of each noble metal solution was transferred into a beaker. Several mL of concentrated nitric acid and a Pu-242 tracer were added, and the solution was evaporated to dryness. Each dry residue was re-dissolved in 2 mL of concentrated hydrochloric acid and 20 µL of concentrated nitric acid, then loaded onto an ion exchange column containing 1 mL of BioRad MP-1M resin (50100 mesh, chloride form, in concentrated hydrochloric acid). Plutonium holds up on the resin as an anionic chloride complex, while the majority of the other constituents pass through the column. Each column was washed with clean concentrated hydrochloric acid, then plutonium was then eluted off the resin using concentrated hydrochloric acid with several milligrams of ammonium iodide. The iodide ion reduces plutonium from (IV) to (III), which does not form a chloroanion and comes off the resin. Uranium and neptunium, which also form chloro-anionic complexes but are not reduced by the iodide solution, remain on the resin. The Pu, now free of other alpha emitting radionuclides, was mounted for alpha spectrometry by coprecipitation using 70 µg of NdF3 onto a 2-cm membrane filter. Samples were counted for 1000 minutes on a silicon diode alpha spectrometer. Plutonium-238 and Pu-239+240 were measured against the Pu-242 tracer. The concentration of each plutonium isotope was algebraically calculated from the alpha data and previously published isotope ratios.18-19, 22 Strontium Analysis: For the analysis of Sr-90, a small subsample of each noble metal solution was transferred into a beaker and evaporated gently to dryness. Each dry residue was dissolved in 2 mL of 8M nitric acid, then loaded onto a column containing 1.5 mL of Eichrom Sr spec resin in 8M nitric acid. The loaded column was then rinsed with 8 mL of 8M nitric acid


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Analytical Chemistry to wash out other radionuclides. Strontium was then eluted off the column with 8 mL of 0.05M nitric acid. The separated Sr90 was measured by liquid scintillation. Technetium Analysis: A subsample of each noble metal solution was transferred into a vial and diluted with several milliliters of water. The diluted solution was then passed through a hydrogen-form cation exchanger (Bio-Rad 50W-X8, 50-100 mesh). During the KOH-KNO3 fusion, the technetium was oxidized completely to the anion TcO4- and passes through this cation resin. Most of the other radionuclides are cations (Cs+, Sr2+, Y2+, Eu3+, Am3+) and load onto the resin. The column effluent was made basic with sodium hydroxide, then several milligrams of tetraphenylarsonium chloride were added. Technetium was extracted from this solution as [Ph4As][TcO4] into methyl isobutyl ketone. The methyl isobutyl ketone phase was evaporated nearly dry, then counted by liquid scintillation. Sequential separation of molybdenum, ruthenium, palladium, and tellurium. Of the components of the noble metal phase, technetium and rhodium are mono-isotopic and do not need to be measured by mass spectrometry. Molybdenum, ruthenium, palladium, and tellurium require chemical separation before mass spectral analysis, to eliminate isobaric interferences. A general scheme showing the dissolution process and elemental separations is shown in Figure 1. Black residue separated from UO2 fuel KOH-KNO3 fusion

Orange solution, 0.5M HCl Mo, Tc, Ru, Rh, Pd, Te, Pu, Zr, trace elements N2H4, Mg metal Solid s

Solution

Ru, Rh, Pd, Te metals

Mo, Tc, Pu, Am, Zr Extract Mo with 8hydroxyquinoline in CHCl3

HCl + H2O2

Ru, Rh, Pd, Te in HCl

Mo Extract RuO4 into CCl4

Ru Rh, Pd, Te in HCl

Tc, Pu, Am, Zr, contaminants

Extract Pd(DMG)2 into CH2Cl2

Pd Rh, Te in HCl N2H4

Rh, Te metals NaOH + H2O2 Solids

Rh, contaminants

Solution

Te

Figure 1. General scheme for dissolution and sequential separation of the noble metal phase

A subsample of each noble metal solution (between 20 to 50 mL) was transferred into a flask. The exact amount of solution of each sample transferred was chosen to yield a few hundred to a few thousand micrograms of each of the analytes (Mo, Tc, Ru, Rh, Pd and Te). Several milliliters of hydrazine hydrate was added and the solution was warmed, with stirring, to eliminate the nitrate ion. A glass funnel was placed in the mouth of each flask to contain spray. After the bubbling stopped and the nitrate was gone, each sample was transferred to a centrifuge tube and

0.1 g of magnesium metal turnings was added. Hydrazine will completely precipitate tellurium and will incompletely precipitate the noble metals. Magnesium ensures the complete precipitation of all of the noble metals. Solutions were then centrifuged to separate the precipitated metals (Ru, Rh, Pd, Te). The process was repeated until addition of magnesium turnings generated no precipitate, confirming that recovery of the noble metals was complete. The supernatant solution was set aside for separation of molybdenum. Ruthenium Separation: The black precipitate of Ru, Rh, Pd, and Te was dissolved in 1 mL of concentrated HCl with several drops of 30% H2O2. The metals dissolve quickly in this solution. Once dissolved, the solution was warmed to near boiling to expel Cl2, evaporate much of the HCl and decompose H2O2. The solution at this point was intense yellow-orange. The solution was rinsed into a 35-mL Teflon screw-cap centrifuge tube with 5 mL of 0.1M HCl. Approximately 0.10 g KIO4 was added, followed immediately by 5 mL CCl4. The tube was capped, mixed for about 30 minutes, and centrifuged to separate the phases. The KIO4 oxidizes the ruthenium under these conditions to RuO4, which extracts readily into CCl4. The upper, aqueous layer, which contained the Rh, Pd, and Te, was withdrawn and moved to a different container for subsequent separations. The CCl4 phase was washed a second time with 5 mL of clean 0.1M HCl, then centrifuged to separate the phases. The separated aqueous phase was discarded. A drop of methanol was added to the CCl4 to reduce RuO4 to RuO2, which slowly precipitated out of solution. About 5 mL of concentrated HCl was then added to extract the RuO2 into the aqueous phase. The tube was capped and shaken for a few minutes, then centrifuged to separate the Ru-containing aqueous phase, which now appeared pale yellow, from the CCl4. The ruthenium solution in HCl was wetashed with HNO3, re-dissolved in 10 mL of 0.5M nitric acid, then put into a clean storage container and set aside for mass spectrometry. Palladium Separation: In a 50-mL glass centrifuge tube, 20 mg of dimethylglyoxime dissolved in 1 mL of ethanol was added to the aqueous phase remaining after the ruthenium extraction. The mixture formed a bright yellow precipitate of Pd(DMG)2 almost immediately. The centrifuge tube was then filled with dichloromethane, capped, shaken for several minutes, then centrifuged to separate the phases. The Pd(DMG)2 extracted completely into the dichloromethane, leaving an essentially colorless aqueous solution. The aqueous phase containing the rhodium and tellurium was removed and transferred to a separate container for further processing. The dichloromethane solution was evaporated dry, then wet-ashed with several milliliters of concentrated nitric acid to convert the palladium to Pd(NO3)2. The pale brown residue of Pd(NO3)2 dissolved readily in 5 mL of 0.5M nitric acid and was set aside for mass spectrometry. Tellurium Separation: The rhodium-tellurium fraction, after palladium was removed, was evaporated nearly dry and wetashed with several milliliters of concentrated nitric acid to remove the excess DMG. The residue was taken up in several milliliters of 3M hydrochloric acid. Several milliliters of hydrazine and several milligrams of magnesium metal were added to reduce the tellurium and rhodium to metals. The precipitate was centrifuged out of solution, then suspended in 1 mL of 0.1M NaOH with several drops of hydrogen peroxide. This solution will dissolve elemental tellurium and oxidize telluride ion to Na2TeO3. The undissolved rhodium was



separated and set aside. Tellurium was then precipitated from the solution with hydrazine, centrifuged out of solution, and dried. To confirm the presence of tellurium, the dry, black precipitate was dissolved in several drops of concentrated sulfuric acid. Elemental tellurium dissolves in concentrated sulfuric acid to produce a pink solution, and re-precipitates as black tellurium metal upon dilution with water. After converting back to tellurium metal by dilution with water, the black precipitate was separated by centrifugation and dissolved in a drop of concentrated nitric acid. The tellurium solution was then diluted to 10 mL with 0.5M nitric acid and set aside for mass spectrometry. Molybdenum separation: Molybdenum was separated from the reserved supernatant that remained after the noble metals were initially precipitated using hydrazine and magnesium metal turnings. In centrifuge tubes, 20 mg of Na2EDTA was added to each sample. After pH adjustment to 2 using ammonium hydroxide, 8 mL of 1% 8-hydroxyquinoline in chloroform was added. The chloroform layer immediately turned yellow with molybdenum. The tubes were capped and shaken for a few minutes, then centrifuged to separate phases. This process was repeated two additional times with fresh 8hydroxyquinoline solution in chloroform. After the third extraction, very little yellow appeared in the chloroform layer. The organic phases from each sample were combined and evaporated in a beaker to dryness, then wet-ashed with concentrated nitric acid to eliminate the 8-hydroxyquinoline. The residue was dissolved in 1 mL of 0.5M HCl in a 15-mL centrifuge tube. One-half mL of 10% NH4SCN solution was added to each tube followed by a drop of hydrazine hydrate to form the intensely red molybdenum thiocyanate. Five mL of diethyl ether was then added to extract the molybdenum. Tubes were capped, shaken, and centrifuged to separate the phases. This ether extraction step was repeated a second time with 5 mL of clean diethyl ether to ensure complete extraction of molybdenum. The combined ether phase from the two extractions was evaporated, then ashed with concentrated nitric acid to eliminate the thiocyanate. The residue was dissolved in 10 mL of 0.5M nitric acid and set aside for mass spectrometry. Isotopic analysis of each separated element. Isotopic analyses of the separated solutions were performed by static (simultaneous) multi-collection on a NuPlasma-II multicollector inductively coupled plasma-mass spectrometer (MC-ICP-MS) coupled with an Apex-Q sample introduction system. The NuPlasma-II is equipped with sixteen Faraday cups. The five ion counters on the instrument were not utilized in this study. Samples were first diluted with 2% nitric acid to a part per billion level concentrations to be safely analyzed on the instrument and to ensure consistency in the primary matrix. Prior to each element being analyzed, on-peak baselines were collected; these were subtracted from all relevant isotope ion beam intensities. A standard of natural isotopic composition was analyzed for both Ru and Te to determine the instrumental exponential mass fractionation factor, as well as approximate the sensitivity of the instrument. This correction factor was applied to all measured ratios and was solely dependent on the atomic mass of the isotopes relative to each other. After each sample was analyzed, a solution of 5% nitric acid and 0.1% hydrofluoric acid was aspirated for 20 seconds, followed by 2% nitric acid for 60 seconds while baselines were monitored to ensure complete washout between runs.

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Analyses on the NuPlasma-II were set up to simultaneously collect all isotopes of the element in question, as well as monitor signals of the mass channels that represented natural isobaric interferences. For Mo analyses, Zr and Ru mass channels were monitored. For Ru, Mo and Pd mass channels were monitored. For Pd, Ru and Cd mass channels were monitored. For Te, Sn and Sb channels were monitored. After baseline subtraction, all subsequent data analysis was conducted in an offline spreadsheet for flexibility in determining corrections to apply. It was determined that corrections for any isobaric interferences that would typically be applied to natural samples would be invalid due to the complexity of the system and the non-natural isotopic composition of the samples. Instead the efficacy of the chemical separations designed to eliminate isobaric interferences prior to analyses were confirmed by comparing the interference corrected and uncorrected ratios. This comparison satisfactorily yielded differences typically between 1-10%. Microscopic Characterization. The separated solids were initially characterized with Scanning Electron Microscopy (SEM) and Energy dispersive x-ray spectroscopy (EDS) using an FEI (Hillsboro, OR, USA) Quanta 250FEG™ field emission gun (FEG)-SEM. The SEM was used to locate regions of interest in the metal matrix microstructure. To minimize the effects of sample drift, a drift-correction mode was used during acquisition of the elemental maps. Lift-out samples for Scanning Transmission Electron Microscopy (STEM) were prepared using a FEI Helios 660 NanoLab™ field emission gun (FEG) dual beam focused ion beam/scanning electron microscope (FIB-SEM). The foils were on the order of 20 × 40 μm and could be analyzed safely in a JEOL (Japan) JEM ARM200C probe-corrected STEM operated at 200 kV. The microscope was equipped with a High Angle Annular Dark Field (HAADF) detector and EDS. Modeled Spent Fuel Inventories. For each of the fuel samples analyzed, nuclide inventories were simulated using ORIGEN-ARP. ORIGEN ARP performs point-depletion and decay calculations rapidly by utilizing pre-generated cross section libraries created in the SCALE code system.23 All fission products were predicted on the basis of the documented power histories and estimated decay time from reactor discharge to time of measurements were made in 2016. The assumed decay times are 34 years for ATM-105, 36 years for ATM-106, and 24 years for ATM-109. Table 3: General chemical composition of each sample, in milligrams of each element in the starting samples Analyte

105 (mg)

%RSD

106 (mg)

%RSD

109 (mg)

%RSD

Ru

6.00

1.0

15.53

0.2

22.28

1.2

Mo

2.60

1.5

13.23

0.3

17.58

0.4

Pd

1.06

2.0

8.60

1.4

10.08

0.4

Rh

1.35

0.6

2.44

2.0

3.03

3.3

Te

1.21

8.0

1.12

2.6

2.58

3.5

Tc

0.80

14.4

2.90

5.6

3.10

12.6

Fraction Sum

13.02

1.3

43.81

0.5

58.63

0.9


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Analytical Chemistry Results Bulk Elemental Analysis of Spent Fuel from Carbonate Dissolution. Selected results of the elemental analysis of the residuals of the spent nuclear fuel dissolved by the carbonateperoxide method are shown in Table 3. The sample solutions contained the noble metal phase, but also all other fuel components that incompletely dissolved or re-precipitated in the ammonium carbonate + hydrogen peroxide solution. A complete table of results from the elemental analysis showing the general chemical composition of the starting samples, by element, is found in the Supporting Information. The undissolved residue was substantially the noble metal phase; however, the residues had a significant amount of other fuel components, particularly zirconium and plutonium. The plutonium present was probably that which remained undissolved from the original ammonium carbonate, hydrogen peroxide dissolution of the fuel. Scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) analysis of previously prepared noble metal phase was able to confirm the presence of plutonium (Figure 2 and explained below). However, the association of plutonium with noble metal phase during their analysis in-situ within undissolved spent fuel crosssections has not been reported previously, suggesting the plutonium (and presumably other non-noble metal phase elements) found in the residue of the carbonate-peroxide spent fuel dissolution here likely resulted from incomplete dissolution or reprecipitation during dissolution process. Zirconium and the rare earths are probably chance fission product contaminants that were mostly, but incompletely removed during the fuel dissolution. Zirconium and the early rare earths are among the main fission products, by mass. Zirconium and the rare earths are highly electropositive and would form oxides in the fuel, not a metallic phase. Several other fission products were detected by ICP-OES in the sample solutions, but at trace concentration, with high uncertainty. While detectable in two of the three samples, uranium was nearly completely removed during the sample dissolution. The starting samples ranged from 13 to 59 milligrams of noble metal phase, ignoring the mass of other fission products and plutonium. The radiochemical composition of each sample is reported in Supporting Information. Plutonium isotopes 238, and 239+240, Am-241, and Cs-137 were the most significant radiological components in each of these samples. In Table 3, we have placed tellurium with the noble metal phase. Tellurium is a minor fission product, present in spent fuel at a much lower concentration than many other fission products. However, our analysis shows its concentration in the noble metal phase is high and comparable to or higher than the five fission product elements traditionally associated with this phase. Tellurium as a component of the noble metal phase. Previously prepared samples of the separated noble metal phase were reexamined using SEM. Samples were analyzed using a the FEI Quanta 250FEG SEM. Figure 2A shows an SEM image of a noble metal phase particle containing Te. The elemental distribution map of three elements; Pu, Ru and Pd, indicates that the vast majority of the solid material is associated with the noble metal phase (see Figure 2B). Plutonium, while detected, shows up on the map at locations discretely different from the noble metal phase. The prevalence of Pu as a discrete phase from the noble metal phase is consistent with the radiochemical analysis and the assumption that the carbonate-peroxide

dissolution incompletely dissolved the Pu or reprecipitated it as a secondary phase. EDS elemental analysis also did not detect the presence of oxygen, supporting that the contention that the noble metals exist as zero valent species or possibly telluride phases. If such a phase formed during the dissolution process (an oxidative leaching process), we would expect an oxidized form in the undissolved residue material. This result is in agreement with the findings of Cui et al (2012)24 using X-ray absorption spectroscopy and those of Buck et al (2015)25 using TEM-EDS that the separated material was indeed metallic.

Figure 2. (A) SEM image of ATM-105 separated solids, (B) RGB compositional EDS map showing separate particles of Pu (blue) within the matrix of the noble metal material which is dominated by Ru and Pd (yellow-green).

Tellurium is known to readily form tellurides with the less electropositive metals. It is easy to imagine that fission product tellurium would form tellurides with the fission product noble metals in irradiated fuel. To confirm that the tellurium is associated with the noble metal fraction and not formed during dissolution, samples of the noble metal phase were removed from the cladding section of a cross-section of ATM-109 and analyzed using a FIB-SEM. Figure 3 shows the analysis of one of these particles. The Te fraction is clearly associated with the metallic inclusions and separated from the Zr. In addition, iodine was observed to be associated with the phase agreeing with reports made by Buck, et al (2016).26 Additional analysis regarding the character and role of the Te association with noble metal phase particles is reported by Kessler et al (In Submission).

Figure 3. STEM-EDS mapping of a noble metal inclusion removed from the cladding liner of a cross section of ATM-109. Tellurium is associated with the noble metal fraction and incorporated as a 6th component of the phase.

Analysis of Mo, Tc, Ru, Rh, Pd, and Te in noble metal phase. Considering the (now six) major components of the noble metal phase, the concentration of each element for each fuel are given in Table 4 below Molybdenum, ruthenium, rhodium, palladium, and tellurium were determined by ICPOES. Technetium was measured by chemical separation followed by liquid scintillation counting.



Table 4: Concentrations of Major Components in the Noble Metal Phase Analyte

ATM-105 (%)

ATM-106 (%)

ATM-109 (%)

Mo

20.0 ± 0.4

30.2 ± 0.2

30.0 ± 0.3

Tc

6.2 ± 0.9

6.6 ± 0.4

5.3 ± 0.7

Ru

46.1 ± 0.7

35.4 ± 0.2

38.0 ± 0.6

Rh

10.3 ± 0.1

5.6 ± 0.1

5.2 ± 0.2

Pd

8.1 ± 0.2

19.6 ± 0.3

17.2 ± 0.2

Te

9.3 ± 0.8

2.6 ± 0.1

4.4 ± 0.2

Consistent with prior reports,14, 16, 24 the ruthenium and molybdenum concentrations are much greater than the palladium, rhodium, and technetium in the noble metal phase for all three burnups. In general and based upon the few data points we have measured here, as the burnup of the fuel increases, the concentration of each element in the fuel increases. However, the extent and trend of the changes in those ratios with burnup is difficult to quantify considering measurement uncertainty and the limited data set reported here. Some of the components (particularly technetium and tellurium) are more chemically reactive than the others, and the measured concentration may depend heavily on the exact conditions of the fuel dissolution. Nonetheless, a few trends are evident. First, the relative abundance of both ruthenium and rhodium decreases in the noble metal phases with burnup, while the opposite trend is observed for molybdenum and palladium. The relative abundance of technetium in noble metal phase was found to be essentially independent of burnup. While the ruthenium and molybdenum content individually changed with burnup, the sum of these two elements in the noble metal phase was found to account for approximately 2/3 of the total mass independent of burnup. Isotopic Analysis of Individual Element Streams. The chemically separated molybdenum, ruthenium, palladium, and tellurium were analyzed by MC-ICP-MS as described above. The isotopic abundances of the analyzed streams are reported in Table 5. During analysis of each element, mass channels representing isobaric interferences were also monitored. A review of the possible interfering isobars was performed and compared to the collected data. From this work, we determined that the samples were well separated, and the major isotopes (not starred) in Table 5 are free of interference. A review of the expected isobaric interferences and comparison to the presented data is given in the Supporting Information. Discussion Dissolution Process. Results presented here represent the first elemental and isotopic characterization of noble metal phase residue from the dissolution of spent nuclear fuel by the carbonate-peroxide method and the first isotopic analysis of the noble metal phase following elemental separations. These results confirm that the carbonate-peroxide dissolution process completely dissolves the UO2 and the majority of fission products. The separated noble metal phase residue was substantially pure, according to the ICP-OES data. Analytical results of the noble metal phase are generally consistent with previous observations of the elemental makeup

Page 6 of 9

of this phase, with one exception. Here, tellurium was detected in all three samples of noble metal phase analyzed, and at an abundance similar to the five major elements previously known to be part of the noble metal phase. This finding suggests that tellurium may be an integral component of noble metal phase particles in used nuclear fuel, rather than simply a contaminant. Elemental mapping using SEM on a cross-section of undissolved used nuclear fuel containing noble metal phase was used to test this assertion. Combining the large abundance of tellurium found in these samples and technetium leaching results previously reported by Soderquist and Hanson27 suggests the carbonate-peroxide process also attacks the noble metal phase less than hot nitric acid. (Tellurium is soluble in hot nitric acid, and tellurium can be leached from metal tellurides under alkaline, mildly oxidizing conditions.28) Chemical Separations. The chemistry required to resolve the insoluble residue from fuel into its components is complicated, but essential to obtain the mass spectral data. The noble metal phase from used fuel is an unusual mixture of uncommon elements, a mixture found nowhere else. The sequential separation of its six components has not been previously published. The chemistry must recover each component of the noble metal phase, in reasonably high yield, free of isobaric interferences. The yield does not need to be 100% for mass spectrometry, but it does need to be high enough to make the blank negligible. All potential isobaric interferences must be considered in the chemistry, not just interference between the six target elements. This work used milligram-scale separations, which has the advantage the analyst can see the colors and precipitates. If something goes wrong with the chemistry, it is immediately apparent, a distinct advantage in chemical separations as complex as these. A thorough discussion on the development of the sequential separation of the noble metal phase is found in the Supporting Information. Comparison to Expected Calculated Inventory. Results from elemental and isotopic analyses were compared to predictions of isotope inventories within the bulk fuel using ORIGEN-ARP (Table S-3 in the Supporting Information). The majority of the measured ratios were in line with the calculated ratios. In general, the calculated-to-experimental ratio were consistent for isotopes with sufficient isotopic abundance. Exceptions include Pd-104 and 106, which are enriched relative to predictions. All other isotopes of Pd are fed from fission within their fission chain by relatively short-lived radionuclides. The formation pathway of both Pd-104 and 106 is through other components of the noble metal phase (ruthenium and rhodium). The earlier formation of the noble metal phase could thus enrich these isotopes. Using the inventories calculated using ORIGEN-ARP, an expected elemental inventory per gram of fuel was calculated for each analyzed fuel. These values were compared to the amount of each element collected in this work and corrected using Table 2. A minimum percent of the six main components bound in the noble metal phase of fuel is shown in Table 6. These values are a minimum as some could have been leached during the fuel dissolution process, though the carbonateperoxide dissolution would dissolve the phase less than a nitric acid dissolution.27 Some of the noble metal phase was also not recovered. The starting mass of undissolved solids for ATM105 is suspect as this work has revealed a probable transcription error in the original collection. Comparing Tables 2 and 3 shows


Page 7 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Analytical Chemistry Table 5: Observed isotopic abundances of molybdenum, ruthenium, palladium, and tellurium from three commercial UO2 fuels. Nuclide

ATM-105

%RSD

ATM-106

%RSD

ATM-109

%RSD

Mo-92

0.00088*

2.33

0.0028*

5.00

0.00157*

1.13

Mo-94

0.00105*

1.43

0.00413*

1.90

0.00189*

1.35

Mo-95

0.2157

0.25

0.193

0.87

0.2018

0.25

Mo-96

0.01486

0.20

0.0251

1.12

0.02955

0.22

Mo-97

0.2420

0.12

0.2385

0.24

0.2362

0.17

Mo-98

0.2503

0.08

0.2532

0.22

0.2508

0.15

Mo-100

0.2753

0.10

0.2828

0.30

0.2781

0.14

Ru-96

0.0327

2.22

0.00110*

5.72

0.00126*

3.13

Ru-98

0.0122*

2.65

0.00132*

2.19

0.00055*

6.17

Ru-99

0.068

2.35

0.0049

3.50

0.00151*

5.33

Ru-100

0.082

1.81

0.0647

0.26

0.07760

0.07

Ru-101

0.261

1.13

0.3107

0.06

0.30928

0.02

Ru-102

0.330

0.46

0.3420

0.05

0.35442

0.03

Ru-104

0.214

0.55

0.2753

0.04

0.25537

0.02

Pd-102

0.0007*

20.9

0.00022*

29.3

0.00023*

30.2

Pd-104

0.1669

0.17

0.2026

0.10

0.2331

0.18

Pd-105

0.2827

0.15

0.2499

0.09

0.2418

0.17

Pd-106

0.3008

0.13

0.2752

0.08

0.2683

0.16

Pd-107

0.1338

0.12

0.1422

0.07

0.1331

0.15

Pd-108

0.0881

0.13

0.09851

0.07

0.0936

0.15

Pd-110

0.02695

0.14

0.03138

0.08

0.02993

0.23

Te-120

0.00096*

2.25

0.00097*

3.39

0.00161*

3.77

Te-122

0.00055*

3.24

0.00136*

0.84

0.00138*

2.32

0.00080*

0.94

0.00080*

8.85

Te-123

Not Detected

Te-124

0.00055*

8.71

0.00178*

0.77

0.00156*

3.84

Te-125

0.02131

0.20

0.02743

0.04

0.02255

0.42

Te-126

0.00347

1.02

0.00799

0.32

0.00652

1.28

Te-128

0.19121

0.03

0.20343

0.02

0.2003

0.07

Te-130

0.7819

0.02

0.75625

0.01

0.7652

0.03

*Values are from ratios with isotope signal intensities measuring between 0.1mV and 1mV. This is only 2-20x higher than the average detector noise of 0.0485mV, and is reflected in the elevated uncertainty.

Table 6: Percent of total expected (ORIGEN) inventory accounted (bound) in analyzed noble metal phase. ATM-105 (%)a

ATM-106 (%)

ATM-109 (%)

Mo

5.6 ± 0.1

52.4 ± 0.3

60.9 ± 0.4

Tc

7.1 ± 1.5

50.2 ± 4.2

51.5 ± 9.8

Ru

20.4 ± 0.3

83.7 ± 0.3

97.5 ± 1.8

Rh

20.5 ± 0.2

70.4 ± 2.1

108.4 ± 5.3

Pd

6.3 ± 0.2

61.1 ± 1.3

51.3 ± 0.3

Te

18.9 ± 2.3

29.8 ± 1.1

58.5 ± 3.0

Element

aRecent

analysis shows a transcription error was likely for the ATM-105 mass of undissolved solids.

more recovery in this analysis (without taking into account some species as oxides) than initial material. Some of this material had also been used for other analyses at PNNL. Though

this value still represents a minimum, the true value is undoubtedly much higher. Review of the results of ATM-106 and 109 show that a majority of these fission products are bound in the noble metal phase. Conclusion The noble metal phase from three commercial irradiated fuels was chemically dissolved and characterized including elemental separations and isotopic analysis. Though traditionally thought of as a five-metal phase composed of Mo, Tc, Ru, Rh, and Pd, we found significant quantities of Te on the order of the other components. Further analysis revealed that the Te is associated and bound in the phase making it a “sixth” element. The Te could be important for formation of the phase. In this work, separations were used to develop a sequential analysis scheme for the noble metal particles from irradiated fuel. Using this scheme, we report the first isotopic analysis of the noble metal phase using elemental separations to minimize



isobaric interferences. The isotopic abundances of the noble metal phase components were compared to the expected values of the full fuel inventory from ORIGEN-ARP calculations. This analysis revealed that the measured isotopic abundances, with few exceptions, of the six elements in the phase match well with the expected bulk fuel inventory. Exceptions include Pd-104 and 106, which appear to be enriched relative to predictions compared to the other Pd isotopes. The noble metal phase analyzed in this work was recovered from fuel dissolved using the carbonate-peroxide method. This method is less harsh than traditional dissolution methods which leaves the noble metal phase even more intact. In this work, we showed that the noble metal phase particles represent a sink for a significant portion of the total inventory of the elements that make up this phase suggesting that the majority of the inventory of these elements are actually bound up in this refractory phase.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Present Addresses †IB3 Global Solutions, Oak Ridge, TN

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was funded by Pacific Northwest National Laboratory under Laboratory Directed Research and Development (LDRD) funds with support from the Nuclear Process Science Initiative. Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the United States Department of Energy under contract DE-AC05-76RL0-1830.

SUPPORTING INFORMATION Supporting information includes: detailed elemental and radiochemical analysis of undissolved solids, a review of the expected isobaric interferences, a discussion on the development of the sequential separation, and comparison of analyzed isotopic ratios with bulk fuel predictions using ORIGEN-ARP.

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8. Jeffery, B. M., Microanalysis of inclusions in irradiated UO2. J. Nucl. Mater. 1967, 22, 33-40. 9. Scheele, R.; McNamara, B.; Casella, A. M.; Kozelisky, A., On the use of thermal NF3 as the fluorination and oxidation agent in treatment of used nuclear fuels. J. Nucl. Mater. 2012, 424, 224-236. 10. Utsunomiya, S.; Ewing, R. C., The fate of the epsilon phase (Mo-Ru-Pd-Tc-Rh) in the UO2 of the Oklo natural fission reactors. Radiochim. Acta 2006, 94, 749-753. 11. Wronkiewicz, D. J.; Watkins, C. S.; Baughman, A. C.; Miller, F. S.; Wolf, S. F. Corrosion testing of a simulated five-metal epsilon particles in spent nuclear fuel, In Materials Research Society, Boston, MA, 2002; pp 625-632. 12. Kleykamp, H., The chemical state of the fission products in oxide fuels. J. Nucl. Mater. 1985, 131, 221-246. 13. Kaye, M. H.; Lewis, B. J.; Thompson, W. T., Thermodynamic treatment of noble metal fission products in nuclear fuel. J. Nucl. Mater. 2007, 366, 8-27. 14. Adachi, T.; Ohnuki, M.; Yoshida, N.; Sonobe, T.; Kawamura, W.; Takeishi, H.; Gunzi, K.; Kimura, T.; Sizuki, T.; Nakahara, Y.; Muromura, T.; Kobayashi, Y.; Okashita, H.; Yamamoto, T., Dissolution study of spent PWR fuel: Dissolution behavior and chemical properties of insoluble residues. J. Nucl. Mater. 1990, 174, 60-71. 15. Mousset, F.; Eysseric, C.; Bedioui, F. Studies of dissolution solutions of ruthenium metal, oxide and mixed compounds in nitric acid, In ATALANTE 2004, Nîmes (France), 2004. 16. Cui, D.; Low, J.; Sjostedt, C. J.; Spahiu, K., On Mo-Ru-Tc-PdRh-Te alloy particles exctracted from spent fuel and their leaching behavior under Ar and H2 atmospheres. Radiochim. Acta 2004. 17. Soderquist, C. Z.; Johnsen, A. M.; McNamara, B. K.; Hanson, B. D.; Chenault, J. W.; Carson, K., J.; Peper, S. M., Dissolution of irradiated commercial UO2 fuels in ammonium carbonate and hydrogen peroxide. Ind. Eng. Chem. Res. 2011, 50, 1813-1818. 18. Guenther, R. J.; Blahnik, D. E.; Campbell, T. K.; Jenquin, U. P.; Mendel, J. E.; Thomas, L. E.; Thornhill, C. K. Characterization of spent fuel approved testing material - ATM-105; Pacific Northwest National Laboratory: Richland, Washington, 1991. 19. Guenther, R. J.; Blahnik, D. E.; Campbell, T. K.; Jenquin, U. P.; Mendel, J. E.; Thornhill, C. K. Characterization of spent fuel approved testing material - ATM-106; Pacific Northwest National Laboratory: Richland, Washington, 1988. 20. Vaidyanathan, S.; Reager, R. D.; Shirai, Y.; Iwano, Y., High burnup BWR fuel pellet performance. In 1997 International Topical Meeting on Light Water Fuel Performance, American Nuclear Society: Portland, OR, 1997. 21. Orton, C. The Multi-Isotope Process Monitor: Non-destructive, Near-real-time Nuclear Safeguards Monitoring at a Reprocessing Facility. The Ohio State University, 2009. 22. Hanson, B. D.; Casella, A. M.; Soderquist, C. Z.; Johnsen, A. M.; Casella, A. J. Analysis of High-Burnup and MOx Fuel for Inclusion in International Spent Fuel Databases; Pacific Northwest National Laboratory: Richland, WA, 2012. 23. Bowman, S. M.; Gauld, I. C. OrigenArp Primer: How to Perform Isotopic Depletion and Decay Calculations with SCALE/ORIGEN; Tech. Rep. ORNL/TM-2010/43, Oak Ridge National Laboratory, 2010. 24. Cui, D.; Rondinella, V. V.; Fortner, J. A.; Kropf, A. J.; Eriksson, L.; Wronkiewicz, D. J.; Spahiu, K., Characterization of alloy particles extracted from spent nuclear fuel. J. Nucl. Mater. 2012, 420, 328-333. 25. Buck, E. C.; Mausolf, E. J.; McNamara, B. K.; Soderquist, C. Z.; Schwantes, J. M., Nanostructure of metallic particles in light water reactor used nuclear fuel. J. Nucl. Mater. 2015, 461, 236-243. 26. Buck, E. C.; Mausolf, E. J.; McNamara, B. K.; Soderquist, C. Z.; Schwantes, J. M., Sequestration of radioactive iodine in silverpalladium phases in commercial spent nuclear fuel. J. Nucl. Mater. 2016, 482, 229-235. 27. Soderquist, C. Z.; Hanson, B., Dissolution of spent nuclear fuel in carbonate-peroxide solution. J. Nucl. Mater. 2010, 396, 159-162. 28. Cooper, W. C., Tellurium. Van Nostrand Reinhold Co: New York, 1971; p 437.


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