Effects of heat treatment on U–Mo fuel foils with a zirconium diffusion barrier

Abstract

A monolith fuel design based on U-Mo alloy has been selected as the fuel type for conversion of the United States' high performance research reactors (HPRRs) from highly enriched uranium (HEU) to low-enriched uranium (LEU). In this fuel design, a thin layer of zirconium is used to eliminate the direct interaction between the U-Mo fuel meat and the aluminum-alloy cladding during irradiation. The co-rolling process used to bond the Zr barrier layer to the U-Mo foil during fabrication alters the microstructure of both the U-10Mo fuel meat and the U-Mo/Zr interface. This work studied the effects of post-rolling annealing treatment on the microstructure of the co-rolled U-Mo fuel meat and the U-Mo/Zr interaction layer. Microscopic characterization shows that the grain size of U-Mo fuel meat increases with the annealing temperature, as expected. The grain sizes were ~9, ~13, and ~20 µm for annealing temperature of 650, 750, and 850°C, respectively. No abnormal grain growth was observed. The U-Mo/Zr interaction-layer thickness increased with the annealing temperature with an Arrhenius constant for growth of 184kJ/mole, consistent with a previous diffusion-couple study. The interaction layer thickness was 3.2 ± 0.5 µm, 11.1 ± 2.1 µm, 27.1 ± 0.9 µm for annealing temperature of 650, 750, to 850°C, respectively. The homogeneity of Mo improves with post rolling annealing temperature and with U-Mo coupon homogenization. The phases in the Zr/U-Mo interaction layer produced by co-rolling, however, differ from those reported in the previous diffusion couple studies.

Interdiffusion, Reactions, and Phase Transformations Observed during Fabrication of Low Enriched Uranium Monolithic Fuel System for Research and Test Reactors

“…However, excessive growth of the U10Mo/Zr interaction layer was observed with post-rolling heat treatment. Homogenization heat treatment at 1000°C prior to rolling also led to reduced variation in Mo content throughout the fuel, but was not reported to have any effect on grain size and morphology after rolling [15].…”

“…The effects of post-rolling heat treatments were analyzed by Jue et al [15], and it was found that the homogeneity of the U10Mo fuel increased with respect to composition as annealing temperature increased, i.e. the extent of chemical banding decreased.…”

“…Park et al [22] analyzed the growth of the U10Mo/Zr interaction layer with respect to changes in holding time and temperature during the HIP process, and found that the thickness did not significantly change with temperature up to 580°C up to 90 minutes or with hold time up to 345 minutes at 560°C. Jue et al [15] performed a series of post-rolling heat treatments (without…”

Observed Changes in As-Fabricated U-10Mo Monolithic Fuel Microstructures After Irradiation in the Advanced Test Reactor

“…The monolithic fuel plate microstructure that will be present in a fuel plate going into a reactor is intimately related to the fabrication processes employed to make it. 5,6 The characteristics of this microstructure must be well understood, since the ultimate fuel plate performance during irradiation will depend on the performance of the phases that compose this microstructure. Microstructural fea-tures are correlated to areas of interest within an as-fabricated fuel plate; the U-10Mo fuel foil, the Zr diffusion barrier, and the Al-6061 cladding are of interest, along with features at the U-10Mo/Zr, Zr/Al-6061 cladding, U-10Mo/ Al-6061 cladding, and Al-6061 cladding/Al-6061 cladding interfaces.…”