Recent experimental evidence of creep and growth of Zr± 2.5% Nb CANDU{ pressure tubes under irradiation shows an increase in the growth rate with accumulated¯uence. Such a feature has been linked to a climb of c-type dislocation loops produced by a bias in the migration of vacancies and interstitials. In this work we link several material scales in order to relate the macroscopic response of the tubes with the physical mechanisms involving the point defects on the microscopic scale. Our model includes the in¯uence of internal stresses on defect di usivity, the dependence of creep and growth moduli of the single crystal on the evolving microstructure, and the intergranular stresses responsible for the bias. All these elements are combined in a self-consistent polycrystal model for creep and growth. We show that this approach can explain the observed growth acceleration and we derive physically meaningful values of some of the microstructural parameters.