The iodine-induced strain rate sensitivity of Zircaloy fuel rod cladding

The strain rate sensitivities and failure times characteristic of iodine-induced stress corrosion cracking of Zircaloy fuel rod cladding are important because they can be related to certain power ramping rates which may increase fuel rod failure probabilities. The CCSCC model developed in this paper approximates these failure characteristics by simulating the transition from the slower (less observable) non-corrosive creep cracking (CC) regime to the faster (more observable) stress corrosion cracking (SCC) regime. Components of the CCSCC model include: ZrI4 production by chemical reaction between Zircaloy and iodine; diffusion of the ZrI4 to the crack tip; chemisorption, embrittlement, and damage accumulation at the crack tip; and crack initiation times and growth rates. Results indicate that the Zr-I2 SCC process is dominated by competition between the chemical reaction and material creep rate phenomena, rather than by stress thresholds or by the requirement that a complete monolayer of ZrI4 form on the exposed Zircaloy surface at the crack tip. Failure times are dominated by the time required to initiate active crack growth. The SCC process is apparently not limited by diffusion kinetics on the time scale of laboratory experiments. Conflicting results were found concerning the chemical reaction rate at the crack tip. (orig.)