Analysis of the structural integrity of the fuel rod cladding based on ring compression tests

Due to the reduced amount of material involved and the relatively simple test set-up, Ring Compression Tests (RCT) on fuel rod cladding specimens has become a well-accepted test to determine the conditions resulting in a brittle response on the cladding. Indeed, from its application under LOCA conditions, also it is used under the Spent Fuel Storage and Transportation conditions. Although the RCT may run the involved material through three stages: elastic, elasto-plastic and damage propagation and relevant information on material properties may be obtained, the non-homogenous stress and strain conditions makes the analysis of the test results, difficult. Even though, some efforts have successfully provided key cladding performance parameters such as the fracture toughness. Others approaches use the RCT as a screening test to determine conditions resulting in a Ductile-to-Brittle transition based on a selected criterion. This paper proposes a criterion from the RCT results based on first principles to address cladding ductility under the pinch loads that occurs during the transportation accident of the cask horizontal drop. The insights gained from a mechanical analysis of the RCT are applied on a number of RCT performed on unirradiated pre-hydrided specimens. Besides, RCT results performed on BWR irradiated cladding with several degrees of radial reorientation of the hydrides, imposed by a previous creep test, are also analyzed following the same approach. Based on this analysis and the expected diametric displacement, allowed by the end of irradiation pellet to clad gap and the outward cladding creep during drying and storage in a dry cask, a criterion is determined. (author)