SC VNIINM methods have been applied for studying oxide films formed on fuel assemblies during operation in NPP reactors, using the Research Institute of Atomic Reactors' hot cells. Transmission electron microscopy, along with energy-dispersive X-ray spectroscopy, have been applied to study the microstructure, phase, and elements composition of oxide films formed on fuel element cladding samples made of alloys E110 (Zr-1 %Nb) and E635 (Zr-1 %Nb-0.35 %Fe-1.2 %Sn) after the autoclave tests in distilled water, and also after four-year operation (for E110 alloys) and six-year operation (for E635 alloys) in the VVER-1000 reactor.

Oxide films formed on the surface of different materials have a different microstructure (grain size, tetragonal phase proportion, presence of defects, etc.) and a different phase composition. Primarily, the crystal structure of oxide films is a monoclinic crystal, but a considerable proportion of the tetragonal phase appears near the “metal-oxide” interface. Alloys of both types indicate the amorphization of second phase precipitates as the oxide film grows. The study of redistribution of alloying elements between the particles and the oxide film in the E635 alloy shows that iron and niobium are concentrated in pores. The research allows determination of the correlation between the concentration of micropores in the oxide film and an increase in the distance from the “metal-oxide” interface.

The study of the structure-phase state of irradiated oxide films with a thickness ranging from 10 μm (for E110 alloy fuel elements claddings) to 80 μm (formed on the E635 alloy fuel elements claddings) shows the absence of second-phase precipitates within their volume. The research data show that the concentration of micropores in oxide films after irradiation corresponds to concentration of micropores after long-term autoclave tests. Therefore, the reactor irradiation does not result in a change in porosity within the oxide film structure.