Thermal diffusion behavior of ternary UMoX alloys with Al coating

Uranium-molybdenum (UMo) alloys are characterized by 7–10 wt.% Mo content and offer assurance in achieving high densities of U assemblies in fuel compositions, making them a compelling option for research reactors in the quest for high neutron fluxes. However, the use of these materials in fuel plates faces numerous challenges, mostly reflected in the formation of an amorphous interdiffusion layer (IDL) with poor thermal conductivity between the UMo kernel and the Al matrix or cladding. In addition, swelling of the fuel plate and accumulation of gaseous fission products can be observed. These effects can be suppressed by introducing an interdiffusion barrier layer (IDB) between the fuel and the Al matrix/cladding or using ternary alloys (UMoX) instead of binary UMo. This experiment focuses on the second approach, the investigation of ternary UMoX alloys. Zr, Pt, Nb, and Ti are selected as the third candidate element (X) because of their relevant material properties. The experimental work involved the production of UMoX alloys, with a subset of samples undergoing heat treatment at 1173 K for homogenization. All samples were coated with Al using physical vapor deposition (PVD) to mimic the Al matrix and/or cladding in fuel plates. Both the ternary alloys and binary UMo reference samples were annealed to simulate the diffusion processes expected during in-pile operation and to study γ-phase stability. The results revealed that UMoTi and UMoZr are not suitable because of irregular interdiffusion. UMoPt formed a relatively thin and regular IDL compared to that of pure UMo. UMoNb did not show interdiffusion and was chemically stable under long heat treatment. In conclusion, UMoNb and UMoPt are promising candidates for high-density fuels, offering a simpler alternative to UMo fuels, as they do not require an additional IDB layer.