Epsilon Metal Summary Report FY 2011

The Epsilon-metal ({var_epsilon}-metal) phase was selected in FY 2009 as a potential waste form to for immobilizing the noble metals found in the undissolved solids + aqueous stream, and the soluble Tc from ion-exchange process, each resulting from proposed aqueous reprocessing. {var_epsilon}-metal phase is observed in used nuclear fuel and the natural reactors of Oklobono in Gabon, where the long-term corrosion behavior was demonstrated. This makes {var_epsilon}-metal a very attractive waste form. Last fiscal year, {var_epsilon}-metal was successfully fabricated by combining the five-metals, Mo, Ru, Rh, Pd and Re (surrogate for Tc), into pellets followed by consolidation with an arc melter. The arc melter produced fully dense samples with the epsilon structure. However, some chemistry differences were observed in the microstructure that resulted in regions rich in Re and Mo, and others rich in Pd, while Ru and Rh remained fairly constant throughout. This year, thermal stability (air), and corrosion testing of the samples fabricated by arc melting were the main focus for experimental work. Thermal stability was measured with a differential scanning calorimeter - thermogravimetric analyzer, by both ramp heating as well as step heating. There is clear evidence during the ramp heating experiment of an exothermic event + a weight loss peak both beginning at {approx}700 C. Step heating showed an oxidation event at {approx}690 C with minimal weight gain that occurs just before the weight loss event at 700 C. The conclusion being that the e-metal begins to oxidize and then become volatile. These findings are useful for considering the effects of voloxidation process. Three different pellets were subjected to electrochemical testing to study the corrosion behavior of the epsilon-metal phase in various conditions, namely acidic, basic, saline, and inert. Test was done according to an interim procedure developed for the alloy metal waste form. First an open circuit potential was measured, followed by linear polarization sweeps. The linear polarization sweep range was the Tafel equation was fit to the linear polarization sweep data to determine the corrosion rate of each pellet in each test solution. The average calculated corrosion rates of the three pellets according to solution conditions were: -1.91 x 10{sup -4} mm/yr (0.001 M NaOH), -1.48 x 10{sup -3} mm/yr (0.01 M NaCl), -8.77 x 10{sup -4} mm/yr (0.001 M H{sub 2}SO{sub 4}), -2.09 x 10{sup -3} mm/yr (0.001 M NaOH + 0.01 M NaCl), and -1.54 x 10{sup -3} mm/yr (0.001 M H{sub 2}SO{sub 4} + 0.01 M NaCl). Three single-pass flow through (SPFT) test were conducted at a flow rate of 10 ml/day, at 90 C, and pH of 2.5, 7.0, and 9.0 for up to 322 days. Results of the tests indicate that dissolution rates were 5 x 10{sup -4} g m{sup 2} d{sup -1} at pH 9.0, 1.2 x 10{sup -4} g m{sup -2} d{sup -1} at pH 7.0, and 2 x 10{sup -4} g m{sup -2} d{sup -1} at pH 2.5. The sample used for the pH 7.0 SPFT test contains extra Re compared to samples used for the other two SPFT test, which came from a single pellet. The corrosion data measured this year indicate that the {var_epsilon}-metal phase is chemically durable. The two chemically different phases, but structurally the same, behave differently during dissolution according to the microstructure changes observed in both the electrochemical and in SPFT test. Characterization of the test specimens after testing suggests that the dissolution is complex and involves oxidative dissolution followed by precipitation of both oxide and metallic phases. These data suggest that the dissolution in the electrochemical and SPFT tests is different; a process that needs further investigation

Initial Evaluation of Processing Methods for an Epsilon Metal Waste Form

During irradiation of nuclear fuel in a reactor, the five metals, Mo, Pd, Rh, Ru, and Tc, migrate to the fuel grain boundaries and form small metal particles of an alloy known as epsilon metal ({var_epsilon}-metal). When the fuel is dissolved in a reprocessing plant, these metal particles remain behind with a residue - the undissolved solids (UDS). Some of these same metals that comprise this alloy that have not formed the alloy are dissolved into the aqueous stream. These metals limit the waste loading for a borosilicate glass that is being developed for the reprocessing wastes. Epsilon metal is being developed as a waste form for the noble metals from a number of waste streams in the aqueous reprocessing of used nuclear fuel (UNF) - (1) the {var_epsilon}-metal from the UDS, (2) soluble Tc (ion-exchanged), and (3) soluble noble metals (TRUEX raffinate). Separate immobilization of these metals has benefits other than allowing an increase in the glass waste loading. These materials are quite resistant to dissolution (corrosion) as evidenced by the fact that they survive the chemically aggressive conditions in the fuel dissolver. Remnants of {var_epsilon}-metal particles have survived in the geologically natural reactors found in Gabon, Africa, indicating that they have sufficient durability to survive for {approx} 2.5 billion years in a reducing geologic environment. Additionally, the {var_epsilon}-metal can be made without additives and incorporate sufficient foreign material (oxides) that are also present in the UDS. Although {var_epsilon}-metal is found in fuel and Gabon as small particles ({approx}10 {micro}m in diameter) and has survived intact, an ideal waste form is one in which the surface area is minimized. Therefore, the main effort in developing {var_epsilon}-metal as a waste form is to develop a process to consolidate the particles into a monolith. Individually, these metals have high melting points (2617 C for Mo to 1552 C for Pd) and the alloy is expected to have a high melting point as well, perhaps exceeding 1500 C. The purpose of the work reported here is to find a potential commercial process with which {var_epsilon}-metal plus other components of UDS can be consolidated into a solid with minimum surface area and high strength Here, we report the results from the preliminary evaluation of spark-plasma sintering (SPS), hot-isostatic pressing (HIP), and microwave sintering (MS). Since bulk {var_epsilon}-metal is not available and companies could not handle radioactive materials, we prepared mixtures of the five individual metal powders (Mo, Ru, Rh, Pd, and Re) and baddeleyite (ZrO{sub 2}) to send the vendors of SPS, HIP, and MS. The processed samples were then evaluated at the Pacific Northwest National Laboratory (PNNL) for bulk density and phase assemblage with X-ray diffraction (XRD) and phase composition with scanning electron microscopy (SEM). Physical strength was evaluated qualitatively. Results of these scoping tests showed that fully dense cermet (ceramic-metal composite) materials with up to 35 mass% of ZrO{sub 2} were produced with SPS and HIP. Bulk density of the SPS samples ranged from 87 to 98% of theoretical density, while HIP samples ranged from 96 to 100% of theoretical density. Microwave sintered samples containing ZrO{sub 2} had low densities of 55 to 60% of theoretical density. Structurally, the cermet samples showed that the individual metals alloyed in to {var_epsilon}-phase - hexagonal-close-packed (HCP) alloy (4-95 mass %), the {alpha}-phase - face-centered-cubic (FCC) alloy structure (3-86 mass %), while ZrO{sub 2} remained in the monoclinic structure of baddeleyite. Elementally, the samples appeared to have nearly uniform composition, but with some areas rich in Mo and Re, the two components with the highest melting points. The homogeneity in distribution of the elements in the alloy is significantly improved in the presence of ZrO{sub 2}. However, ZrO{sub 2} does not appear to react with the alloy, nor was Zr found in the alloy

Epsilon Metal Summary Report FY 2011

The Epsilon-metal ({var_epsilon}-metal) phase was selected in FY 2009 as a potential waste form to for immobilizing the noble metals found in the undissolved solids + aqueous stream, and the soluble Tc from ion-exchange process, each resulting from proposed aqueous reprocessing. {var_epsilon}-metal phase is observed in used nuclear fuel and the natural reactors of Oklobono in Gabon, where the long-term corrosion behavior was demonstrated. This makes {var_epsilon}-metal a very attractive waste form. Last fiscal year, {var_epsilon}-metal was successfully fabricated by combining the five-metals, Mo, Ru, Rh, Pd and Re (surrogate for Tc), into pellets followed by consolidation with an arc melter. The arc melter produced fully dense samples with the epsilon structure. However, some chemistry differences were observed in the microstructure that resulted in regions rich in Re and Mo, and others rich in Pd, while Ru and Rh remained fairly constant throughout. This year, thermal stability (air), and corrosion testing of the samples fabricated by arc melting were the main focus for experimental work. Thermal stability was measured with a differential scanning calorimeter - thermogravimetric analyzer, by both ramp heating as well as step heating. There is clear evidence during the ramp heating experiment of an exothermic event + a weight loss peak both beginning at {approx}700 C. Step heating showed an oxidation event at {approx}690 C with minimal weight gain that occurs just before the weight loss event at 700 C. The conclusion being that the e-metal begins to oxidize and then become volatile. These findings are useful for considering the effects of voloxidation process. Three different pellets were subjected to electrochemical testing to study the corrosion behavior of the epsilon-metal phase in various conditions, namely acidic, basic, saline, and inert. Test was done according to an interim procedure developed for the alloy metal waste form. First an open circuit potential was measured, followed by linear polarization sweeps. The linear polarization sweep range was the Tafel equation was fit to the linear polarization sweep data to determine the corrosion rate of each pellet in each test solution. The average calculated corrosion rates of the three pellets according to solution conditions were: -1.91 x 10{sup -4} mm/yr (0.001 M NaOH), -1.48 x 10{sup -3} mm/yr (0.01 M NaCl), -8.77 x 10{sup -4} mm/yr (0.001 M H{sub 2}SO{sub 4}), -2.09 x 10{sup -3} mm/yr (0.001 M NaOH + 0.01 M NaCl), and -1.54 x 10{sup -3} mm/yr (0.001 M H{sub 2}SO{sub 4} + 0.01 M NaCl). Three single-pass flow through (SPFT) test were conducted at a flow rate of 10 ml/day, at 90 C, and pH of 2.5, 7.0, and 9.0 for up to 322 days. Results of the tests indicate that dissolution rates were 5 x 10{sup -4} g m{sup 2} d{sup -1} at pH 9.0, 1.2 x 10{sup -4} g m{sup -2} d{sup -1} at pH 7.0, and 2 x 10{sup -4} g m{sup -2} d{sup -1} at pH 2.5. The sample used for the pH 7.0 SPFT test contains extra Re compared to samples used for the other two SPFT test, which came from a single pellet. The corrosion data measured this year indicate that the {var_epsilon}-metal phase is chemically durable. The two chemically different phases, but structurally the same, behave differently during dissolution according to the microstructure changes observed in both the electrochemical and in SPFT test. Characterization of the test specimens after testing suggests that the dissolution is complex and involves oxidative dissolution followed by precipitation of both oxide and metallic phases. These data suggest that the dissolution in the electrochemical and SPFT tests is different; a process that needs further investigation