Irradiation growth occurs simultaneously with irradiation creep if there is an applied stress. The two processes are considered to be independent and additive, even though they compete for the same irradiation-produced defects mechanistically. Earlier ZIRAT reviews providing more detail can be found in the ZIRAT7 STR (Adamson & Rudling, 2002) and the Fuel Material Technology Report, Vol. 2 (Rudling et al., 2007).

Irradiation growth is a change in the dimensions of a zirconium alloy reactor component even though the applied stress is nominally zero. It is an approximately constant volume process, so if there is, for example, an increase in the length of a component, the width and/or thickness must decrease to maintain constant volume. Understanding of the detailed mechanism is still evolving; however a clear correlation of growth to microstructure evolu­tion exists, and many empirical observations have revealed key mechanistic aspects. The inherent anisotropy of the Zr crystallographic structure plays a strong role in the mechanism, as materials with isotropic crystallographic structure (like stainless steel, copper, Inconel, etc.) do not undergo irradia­tion growth. It should not be confused with irradiation swelling, which does not conserve volume and does not occur in zirconium alloys under normal reactor operating conditions.

Irradiation growth is strongly affected by fluence, CW, texture, irradiation temperature and material chemistry (alloying and impurity elements).