MICROSTRUCTURAL STUDY OF OXIDE LAYERS FORMED ON ZIRCALOY-4 IN AUTOCLAVE AND IN-REACTOR .2. IMPACT OF THE CHEMICAL EVOLUTION OF INTERMETALLIC PRECIPITATES ON THEIR ZIRCONIA ENVIRONMENT

A cause and effect relationship is likely established between the microstructural changes in zirconium alloys under irradiation and the acceleration of their oxidation process. In order to check this hypothesis, comparative microstructural analyses by analytical microscopy are carried out on oxide layers formed in autoclave and in reactor. This part of the study deals with the changes in chemical composition of the intermetallic precipitates, induced by irradiation and oxidation, and their impact on the zirconia microstructure. The main results are: (i) irradiation modifies the redistribution conditions of the iron contained in intermetallic precipitates, partly because it induces progressive amorphisation and iron-depletion of these precipitates, and partly because it may accelerate iron diffusion in zirconia, at least in a transient manner (ii) the oxide layer grown in reactor includes iron-rich areas, especially in its median part. A link between this enrichment and the presence of a significant proportion of tetragonal zirconia is established and discussed.