Transmission electron microscopy investigation of neutron irradiated Si and ZrN coated UMo particles prepared using FIB

Two fuel plates, containing Si and ZrN coated U-Mo fuel particles dispersed in an Al matrix, were irradiated in the BR2 reactor of SCK•CEN to a burn-up of ∼70% 235U. Five samples were prepared by INL using focused ion beam milling and transported to SCK•CEN for transmission electron microscopy (TEM) investigation. Two samples were taken from the Si coated U-Mo fuel particles at a burn-up of ∼42% and ∼66% 235U and three samples from the ZrN coated U-Mo at a burn-up of ∼42%, ∼52% and ∼66% 235U. The evolution of the coating, fuel structure, fission products and the formation of interaction layers are discussed. Both coatings appear to be an effective barrier against fuel matrix interaction and only on the samples having received the highest burn-up and power, the formation of an interaction between Al and U(Mo) can be observed on those locations where breaches in the coatings were formed during plate fabrication. •ZrN and Si coated U(Mo) fuel irradiated to high Burn-up was studied with TEM.•Both coatings are effective in preventing the interaction between U(Mo) and the Al matrix.•The Si coating is amorphous and intermixed with U(Mo) and Al.•The ZrN coating is polycrystalline and does not interact with U(Mo).•At the highest burnup, an Al-U(Mo) interaction layer is formed through cracks in the coating.