The proposed research will study the fuel clad chemical interaction (FCCI) between UO2 fuel and FeCrAl cladding material. Two selected alloys from the ATF-1 irradiation experiment ATF-73 (led by ORNL) will be studied. These materials are part of a test assembly of several H-cups (thermal diffusion couple experiments). The two allows are C35M (similar to C26M, a GE candidate cladding alloy) and APMT (a GE candidate cladding alloy). This proposed work fills a large knowledge gap in the understanding of FCCI between UO2 fuel and FeCrAl alloys as there are no studies to date of FCCI in this system. There are two aspects of FCCI that will be focused on in the proposed work. The first is increased understanding of the potential impact of fission products on the integrity of FeCrAl cladding. Previous studies have shown that austenitic steels and 9CrODS ferritic steels may be susceptible to attack on the inner diameter of the cladding due to Cs and Te. This is due to the formation of Cr, Cs, and Te oxides and tellurides especially within the grain boundaries. Little is known, however, about how Al may play a role in the interaction of fission products such as Cs and Te with the fuel cladding. Additionally, it is proposed that some O from the UO2 fuel may diffuse into the inner diameter of the FeCrAl cladding forming an aluminum oxide film. This oxide film could play a large role in preventing tritium permeation and fission product transport through the cladding. While predicted through thermodynamic models, no experimental evidence for the formation of this aluminum oxide film is available. Therefore the proposed work scope will be the first FeCrAl FCCI study, and will be able to provide high impact and valuable information to guide future studies and product development for industry.

Because the goal of the proposed work is understanding FCCI chemistry and mechanisms, the work will focus heavily on advanced microstructural characterization. This characterization includes SEM/EDS work in order to gain qualitative chemical information about the interface between the fuel and the FeCrAl cladding, as well as helping pinpoint areas of interest for FIB lift out of TEM samples. TEM characterization will include using diffraction and high resolution TEM to get structural information of the oxide film and any fission product containing secondary phases, EDS maps to get qualitative chemical information, EDS line scans/point scans to get quantitative chemical phase information, and precession electron diffraction (PED) in order to understand how grain boundary character may impact fission product transport within the cladding. The proposed work scope will include 3 days of SEM/EDS, 3 days of SEM/FIB lift out (2 FIB lift outs/day), and 8 days of TEM/STEM/EDS/PED. At least one full day of analysis is set aside for each TEM sample in order to accomplish the full suite of characterization techniques. Because this is a collaboration between industry and several national labs the proposed work is anticipated to generate high impact publications and guide industry development of commercial ATF cladding.

The proposed work is the first study on fuel clad chemical interaction between UO2 fuel and FeCrAl cladding material. Because this FCCI may impact the integrity of the cladding under normal service conditions, it is imperative to the development of FeCrAl cladding to understand the mechanism and impact of FCCI on the cladding. This work scope, therefore, is directly in line with the objectives of the accident tolerant fuels campaign and the advanced fuels campaign. Because this research is also focused on a cladding material which shows promise in having safer and more reliable operation in the current BWR commercial fleet, it is also highly relevant to the DOE-NE light water reactor sustainability program. Additionally, a large advantage of the proposed work is that it is a collaboration between the national labs and industry. Therefore, the impact of the work is both scientific and will aid in the development of a potential commercial product which is in line with the objectives for the Gateway to Advanced Innovation in Nuclear.