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An understanding of fuel rod cladding characteristics, that determine thelimiting stress level for crack initiation and propagation, is a fundamental requirement forthe design and development of water reactor clad materials. Conventional mechanical testsused at present to evaluate fracture properties are of limited use. The reason is that thefracture toughness procedure, meeting ASTM standards, is adapted for thick flat-platematerials that differ significantly from fuel cladding geometry (0.57 mm thick). Therefore,results that can be related to clad structure and performance need to be obtained byappropriately characterised tests outside the purview of current ASTM standards. TheInternal Conical Mandrel (ICM) test is designed to simulate the effect of fuel pelletdiametrical increase on a clad with an existing crack. It consists in forcing a cone, havinga tapered increase in diameter, inside the Zircaloy cladding with an initial axial notch orpre-cracked notch. The purpose of this test is to quantify the crack initiation andpropagation criteria. The experimental system has been designed and is operating in ourlaboratory. The axial load measured during the test can only be indicative of the crackprogress, and cannot be used for a quantitative evaluation of toughness due to frictioncoefficient. From a fracture mechanics point of view, these considerations make theevaluation of toughness using the Rice's integral (J) very difficult to achieve. In order toovercome these problems and evaluate the J integral, a finite element simulation of crackbehavior (or characteristics) during this test is used.