[en] The phenomena of carburization in the cladding of absorbing elements in fast breeder reactors was studied by the techniques of X-ray microanalysis and micro-hardness. A comparison of the results obtained for different types of pins showed that the intensity and depth of the carburization were closely dependent on the amount of free carbon present in the boron carbide. An assessment was made of the effects of the stresses produced within the cladding. Mechanical properties were in general little affected; however, in the case of the most severely carburized pin a distinct loss of ductility was observed