Gel structure of the corrosion layer on cladding pipes of nuclear fuel

The fuel material in a nuclear reactor is protected by cladding pipes made of alloyed zirconium. In contact with water near its critical temperature, a corrosion layer of hydrated zirconium dioxide ZrO2·nH2O, probably with an amorphous gel structure under given conditions, is formed on the cladding. To verify the presence of the gel structure, an analysis was made by water vapour desorption of the original corrosion oxide layers stored in a given autoclave liquid, their dehydrated modifications, and modifications rehydrated in an aqueous medium. This analysis enabled the varying water content to be determined as a characteristic quantity reflecting the nature of its binding. Microhardness values as a measure of plastic deformation of the crystalline and amorphous forms of zirconium dioxide were also determined. Unambiguous agreement of the results obtained by sorption analysis and by microhardness measurement allows us to conclude that the corrosion layer in situ has properties corresponding to a reversible xerogel.