Mechanism of irradiation induced deformation of fuel assembly materials

To investigate the irradiation induced growth behaviours of zirconium-base alloys, the irradiation growth model has been developed on the basis of microscopic point of view. In the model, particular attention has been paid to the role of grain boundary and solute atoms. The adopted growth model assumes that the absorptions of point defects by internal sinks such as dislocations and grain boundaries is the direct cause of growth strain. The present growth model can evaluate the grain shape effects on growth strain and can predict the anomalous growth behaviour of Zr-Nb alloys. In the case of thin grain shape (∼0.2 x 3 x 2μm), the negative growth occurs stemming from the excess interstitial inflow into the grain boundaries. The model can also explain the temperature dependence of the growth rates of Zr-Nb alloys; the growth rate of the materials becomes lower when the irradiation temperature is raised. The solute atoms such as Sn, Fe, Ni, and Cr are expected to be nucleation sites of dislocation loops which contribute to the growth strains. Solute-interstitial interactions are possible mechanisms of temperature cycling effects; growth strain changes discontinually when the temperature is switched during irradiation. (author)