U.S. Department of Energy

Pacific Northwest National Laboratory

Outcomes: Modeling the Interfacial Effects, Partitioning, and Production Routes of Epsilon Particles in Uranium Oxide

Principal Investigator: Richard Clark

Completed: FY18

Outcomes: The project used molecular scale analyses and first principles modeling to study formation and properties of noble metal phase particles. These particles, referred to as epsilon phase or five metal phase (based upon historic descriptions of their composition), include molybdenum, technetium, ruthenium, rhodium and palladium. They form during irradiation of nuclear fuel and are the source of deleterious effects related to fuel integrity loss over time. As part of the project, researchers re-established a Radiochemical Processing Laboratory (RPL) capability that enabled a full cross-section of an irradiated uranium dioxide fuel pellet to be prepared and removed from hot cells. Other experimental work, which involved collaboration with the University of Missouri, provided the first viability of microscale irradiations. 

The project enabled important discoveries related to particle formation and behavior, including: 1) the existence of tellurium, the "sixth" element, in noble metal phase particles; 2) close association of the noble metal phase with important fission products, including xenon; 3) identification of noble metal phase particles and xenon within the outer zirconium fuel cladding; 4) elucidation of noble metal phase formation and behavior that implicates these species in the local production of high-pressure fission gas bubbles in fuel, ultimately leading to cladding corrosion at the fuel/cladding interface. These discoveries offer far-reaching implications for next generation fuel designs.

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