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Irradiation growth of recrystallized annealed (RXA) Zircaloy is divided into four stages and a model is presented to account for each stage. Stage I is a short-time, low-strain transient caused by the accumulation of point defects, small interstitial loops, and vacancy clusters. Stage II is a quasi-steady-state region of relatively low strain rate during which the loops grow and intrinsic dislocations climb. Stage III is a transient during which the strain rate increases due to the production and motion of irradiation-induced dislocation lines. Stage IV is a high-strain-rate, steady-state region during which nonrecoverable strain is caused predominantly by glide of the irradiation-induced dislocations.
The proposed model is based on two new mechanisms: (1) direct production of an interstitial dislocation loop accompanied by a vacancy cluster in the primary damage event, and (2) production of dislocations due to the activation of Frank-Read sources by internal stresses caused by interaction of the loops with themselves and with intrinsic (cold work) dislocations. Nonconservative, recoverable strain is due to climb of all dislocations, whereas conservative, nonrecoverable strain is caused by glide of irradiation-induced and intrinsic dislocations under the action of the internal stress. The conservative strain follows a (1–3f) texture dependence.
The model provides a reasonable representation of growth behavior of longitudinally oriented RXA Zircaloy, the effect of texture and cold work on growth, and the temperature dependence of growth. Equations used to represent the model were solved in their transient forms at each time step of irradiation histories which permitted variations in flux and temperature. Calculations revealed that a nonlinear flux dependence can exist in growth behavior under various conditions of temperature and material structure and that this dependence can change as the irradiation history accumulates. Thus, the current practice of using fluence as the independent variable in data analysis and model development is questionable. The results of using the model to examine recent data obtained in the Experimental Breeder Reactor II at high temperatures and fast fluxes suggest that growth under these conditions is strongly sensitive to variations in flux and rather weakly dependent upon temperature.
zirconium, zirconium alloys, irradiation growth, light-water breeder reactor recrystallized, cold work, neutron irradiation, temperature, fluence, texture, dislocations, internal stress, volume change, strain
Fellow scientist, LWBR Reactor Engineering, Bettis Atomic Power Laboratory, Westinghouse Electric Corp., Bethel Park, PA