Microstructural characterization of a thin film ZrN diffusion barrier in an As-fabricated U–7Mo/Al matrix dispersion fuel plate

Abstract

The United States High Performance Research Reactor Fuel Development program is developing low enriched uranium fuels for application in research and test reactors. One concept utilizes U-7 wt,% Mo (U-7Mo) fuel particles dispersed in Al matrix, where the fuel particles are coated with a 1 m-thick ZrN coating. The ZrN serves as a diffusion barrier to eliminate a deleterious reaction that can occur between U-7Mo and Al when a dispersion fuel is irradiated under aggressive reactor conditions. To investigate the final microstructure of a physically-vapor-deposited ZrN coating in a dispersion fuel plate after it was fabricated using a rolling process, characterization samples were taken from a fuel plate that was fabricated at 500°C using ZrN-coated U-7Mo particles, Al matrix and AA6061 cladding. Scanning electron and transmission electron microscopy analysis were performed. Data from these analyses will be used to support future microstructural examinations of irradiated fuel plates, in terms of understanding the effects of irradiation on the ZrN microstructure, and to determine the role of diffusion barrier microstructure in eliminating fuel/matrix interactions during irradiation. The as-fabricated coating was determined to be cubic-ZrN (cF8) phase. It exhibited a columnar microstructure comprised of nanometer-sized grains and a region of relatively high porosity, mainly near the Al matrix. Small impurity-containing phases were observed at the U-7Mo/ZrN interface, and no interaction zone was observed at the ZrN/Al interface. The bonding between the U-7Mo and ZrN appeared to be mechanical in nature. A relatively high level of oxygen was observed in the ZrN coating, extending from the Al matrix in the ZrN coating in decreasing concentration. The above microstructural characteristics are discussed in terms of what may be most optimal for a diffusion barrier in a dispersion fuel plate application.

Swelling of U-7Mo/Al-Si dispersion fuel plates under irradiation – Non-destructive analysis of the AFIP-1 fuel plates

“…The second phase is mathematically similar to matrix shrinkage observed in curing concrete. It has been well established that localized stresses result in micro-cracks in the matrix during this process 49 (analogous to those observed in the ZrN-coated particle tests in SELENIUM 50 ). The density of micro-cracks increases with larger aggregate size, spherical aggregate (compared to angular aggregate) and when there is strong bonding between the particle and matrix (weak bonding allows crack blunting).…”

Characterization of fresh EMPIrE and SEMPER FIDELIS U(Mo)/Al fuel plates made with PVD-coated U(Mo) particles

“…EDS profiles presented in Figure 6d show that the two analyzed ZrN coatings are partly oxidized: their oxidation probably occurred during the storage of the coated powder before plate manufacturing, as proposed in [10]. In Figure 6d, two ZrN layers which cover two different particles can be seen: in the following, we will focus on the U(Mo) particle located on the left hand side.…”

“…The second one undoubtedly presents more radial cracks. This cracking is very probably due to deformation incompatibilities between the coating and its substrate, during plate manufacturing and in particular during temperature transients as proposed in literature [10].…”

Microstructural Changes and Chemical Analysis of Fission Products in Irradiated Uranium-7 wt.% Molybdenum Metallic Fuel Using Atom Probe Tomography

“…There have been several characterization studies carried out on U-Mo particles irradiated over a range of fission rates, temperatures, and fission densities [2,14,[16][17][18][19][20][21][22][23]. As demonstrated in previous work using transmission electron microscopy (TEM) and scanning electron microscopy (SEM), key features observed are the formation of a fission gas bubble superlattice along with decoration of grain boundaries by large fission gas bubbles [12,19].…”