Zirconium alloys are used as a material for the nuclear reactor fuel cladding. The mechanism of corrosion is influenced by the alloy used, on one hand, and by the corrosive medium to which the alloy is exposed, on the other hand. An assumption has been risen that the water medium with properties of a condensed phase at 360°C leads to formation of hydrated oxide with gel character, whereas steam medium behaving at 450°C as gas does not. The grant project is aimed a) at the differentiation of the influence of the exposure media on the corrosion nuclear fuel cladding, b) at the identification of transition form of hydrated zirconia of gel character in the reactor medium, when the gel form affects extent of the porous system and size of transport ways, and thus also input of water into the boundary of metal / corrosion layer controlling corrosion kinetics and thickness of the oxidic layer, c) at the description of the physical and physico-chemical properties characterizing the different structural forms of the corrosion layer of zirconia. According to literature research, the gel character of zirconia yet has not been studied separately in connection with reactor environment, and the quite new results obtained could further clarify mechanism of the corrosion process.