Local strain in cladding tube due to radial pellet cracking

A study was made to develop a method for evaluation of the local strain in a cladding tube of the Advanced Thermal Reactor due to radial cracking to a UO2 fuel pellet. Effects of the number of cracks, initial crack width and the friction coefficient of a pellet-clad interface on behaviors of the local strain in a cladding tube were evaluated with a modelized experiment. Analytical evaluation of a membrane strain was also carried out on the basis of a procedure similar to that proposed by J. H. Gittus, Nuclear Engineering and Design 18 (1972) 69-82, in order to follow the experimental results and to extend the model experiment to cladding tube. A Zircaloy-2 ring specimen with inner diameter of 95 mm, height of 25 mm and wall thickness of 5 mm was expanded at room temperature with equally divided peripheral dice of a tool steel set in a specimen. Strains on an external surface of the specimen were measured with 28 wire strain gages with gage length of 0.3 mm. An elastic-plastic analysis was carried out in order to evaluate the membrane hoop strain in the cladding tube on the basis of a simple procedure similar to that proposed by Gittus. The results of analysis showed that the maximum hoop strain occured at a location apart from the dice edge. This was caused by unloading in the crack opening portion. The strain concentration factor obtained from analysis is greater than that obtained from experiment. The difference of concentration factors between analysis and experiment is due to the bending strain. Therefore, the strain concentration factor at the inner surface is evaluated from the experimental concentration factor at the external surface and the analytical concentration factor of a membrane strain. (Auth.)