Calculations of irradiation growth in zirconium

Abstract

Reaction rate theory is used to calculate the dimensional changes in zirconium that result from the annihilation of irradiation-induced point defects at internal sinks. The available data on point defect parameters and the microstructures produced by irradiation are reviewed in an effort to provide a reasonable, justifiable basis for the calculations. In accordance with the experimental evidence, the microstructure for the calculations is assumed to contain two main classes of sinks: dislocations with (a)-type Burgers vectors and grain boundaries. The growth strain results from a net flux of interstitials annihilating at edge dislocations and a corresponding vacancy flux arriving at the grain boundaries. The study includes a discussion of the sensitivity of the growth calculation to the expressions chosen for the sink annihilation probabilities, to the sink concentrations and to anisotropy of the microstructure. Finally, a comparison is made between calculated growth rates and experimental data for cold-worked Zircaloy-2, irradiated at 550 K. Two models are used to describe the behaviour of screw dislocations, which receive a net flux of vacancies. In model 1, the screw dislocations act as perfect sinks, forming dislocation helices, while in model 2, the excess vacancies do not annihilate at the screw dislocations but migrate by pipe diffusion to the grain boundaries. The absolute magnitude of the growth rate in cold-worked Zircaloy at 550 K, calculated using model 2, is much closer to experimental values than that obtained with model 1, which greatly underestimates the growth rate.