Corrosion Testing of a Simulated Five-Metal Epsilon Particle in Spent Nuclear Fuel

Abstract

The five-metal epsilon particle represents an important component with respect to the corrosion of spent nuclear fuel as it is the principal host for 99Tc. This radionuclide has a high solubility in oxidizing environments (as TcO4 -), a half-life of 213,000 years, and has been proposed as a monitor for the corrosion rate of spent fuel. As such, an understanding of the corrosion processes affecting epsilon particles may have important implications on our ability to accurately assess radionuclide release rates from spent nuclear fuel.Non-radioactive metal powders were mixed in an atomic percent ratio of 40% Mo, 30% Ru, 15% Pd, 10% Re, and 5% Rh (Re used as a surrogate for Tc), a composition that simulates epsilon particles in spent reactor fuel [1,2]. The powders were electric arc-melted in the presence of an Ar-purged atmosphere. Scanning electron microscopy images of the as-cast samples denote a primary sample heterogeneity, with two distinct zones, each composed of a mixture of very fine-grained Mo-, Mo-Ru-Re, and Mo-Ru-Pd rich crystals. Results from vapor hydration tests (200°C) for time periods up to 35 days indicate the formation of Mo- and Reenriched alteration phases. Phase growth was enhanced when air was periodically replenished in the test vessels suggesting that oxide and/or hydroxide alteration phases may play a role in the corrosion process. MCC-1 tests (90°C) were conducted for time periods up to 182 days in leachant solutions that were prepared with either a) deionized water or b) deionized water acidified with nitric acid to a pH of 3.0. Solutions from the deionized water tests progressively decreased in pH with increasing reaction time (5.5 to 4.0), while the pH values remained at a near-constant value of 3.0 in the acidified solutions. Solution results indicate a preferential release of Mo followed by Re. Normalized Re/Mo release ratios varied from 0.20 to 0.86, and increased as a function of reaction time and acidity. Release of the platinum group metals (Pd, Rh, and Ru) was relatively insignificant in comparison to Mo and Re.

Distribution of metallic fission-product particles in the cladding liner of spent nuclear fuel

“…[3][4][5][6][7][8] Some fission products are retained within the UO 2 matrix in solid solution, while at the same time the noble gases-xenon (Xe) and krypton (Kr)-and the 4d group metals-molybdenum (Mo), technetium (Tc), ruthenium (Ru), rhodium (Rh), and palladium (Pd)-are trapped as gas bubbles and partitioned into metallic phases, respectively. [9][10][11][12][13][14] This metallic phase has gone by several names in the literature, including white inclusions, 15,16 fission-product alloy, 16 5-metal particles, 17 epsilon particles, 18,19 and noble metal phase. 6,20 Knowledge of the distribution of radionuclides across the fuel matrix or partitioning as discrete phases within or outside the fuel grains is necessary to make predictions about potential release in the event of cladding failure during SNF storage, transportation, or long-term geologic disposal.…”

Chemical and Isotopic Characterization of Noble Metal Phase from Commercial UO2 Fuel

“…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 UO 2 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. − This phase has gone by several names in the literature, including white inclusions, , fission product alloy, 5-metal particles, epsilon particles, , and noble metal phase. , In this paper, we refer to it as the noble metal phase.…”

Isolation of long-lived technetium-99 in confinement matrices

“…The particles of the ε phase vary from 20 nm to 10 µ m in size at boundaries of UO 2 grains (Fig. 2) or in within-grain microfractures (Wronkiewicz et al, 2002). The chemical composition of the ε phase is as follows (at %): 41 Mo, 30 Ru, 14 Pd, O 4 -10 Tc, and 5 Rh (approximately the same as in SNF) (Bruno and Ewing, 2006).…”