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.