NSUF 26-5655: TEM characterization of uranium secondary phases at the contact interface between UO2 fuel and Zircaloy-4 cladding under hydrothermal conditions

Hydrothermal corrosion experiments of UO2 in contact with Zircaloy-4 cladding have been conducted and SEM analysis shows uranium secondary phases forming at the interface of Zircaloy-4. To identify these phases, we propose high-resolution TEM analysis at the LAMDA facility at ORNL. Sample preparation for TEM compatibility will be completed via FIB through pre-established procedures in a past NEUP proposal. The identification of these uranium secondary phases will provide greater understanding on the redox chemistry of UO2 at the cladding/fuel surface interface during permanent disposal, thus closing knowledge gaps for the safety disposal of spent nuclear fuel which in return will assist in the continuation of currently existing nuclear power reactors. The planned samples for TEM analysis are ready to be delivered to ORNL upon notification of RTE acceptance. Sample delivery and preparation can be completed in the first month of the proposal period. TEM lamella preparation via FIB will be completed in the 3rd and 4th month of the proposal period, followed by STEM-EDS mapping and TEM/STEM imaging in the 5th and 6th month. Afterwards, data collection and analysis will be conducted for the in-process manuscript for publication by the 9th month of the project period. We anticipate that the uranium secondary phases are a mixture of uranium phases in differing oxidation states of U(IV) to U(VI), namely UO2, α-U3O8, and rutherfordine (UO2CO3), all of which can be differentiated by TEM analysis. The presence of these phases across our planned samples for analysis will allow for the elucidation of the redox behavior of UO2 at the UO2/Zircaloy-4 interface through phase-relationship analysis. Results derive here can then be used as input in multi-scale computational models for providing more robust predictions in SNF corrosion behavior to build confidence in the long-term disposal of SNF.

Additional Info

Field	Value
Awarded Institution	University of California-Irvine
DOI	10.46936/NSUF/60017660
Embargo End Date	2028-04-22
Facility Tech Lead	Kory Linton
NSUF Call	FY 2026 RTE 1st Call
PI	Steven Nguyen
PIE Facilities	Low Activation Materials Design and Analysis Laboratory
Prep Facilities	Low Activation Materials Design and Analysis Laboratory
Project Member	Dr. Jose Arregui-Mena - Oak Ridge National Laboratory (https://orcid.org/0000-0002-6131-3268)
Project Member	Professor Sarah Finkeldei, Assistant Professor - University of California-Irvine (https://orcid.org/0000-0002-1943-0604)
Project Member	Mr. Steven Nguyen, Graduate Student - University of California-Irvine (https://orcid.org/0000-0003-0860-3693)
Project Member	Dr. Juliane Weber, R&D Staff Member - Oak Ridge National Laboratory (https://orcid.org/0000-0001-7961-0220)
Project Type	RTE