Modelling of the evolution of vacancy clusters in neutron-irradiated iron

There is experimental evidence for the formation of stable nm-sized vacancy clusters (VC) in low-copper iron alloys irradiated with fast neutrons under in-service conditions of reactor pressure vessels (RPVs) (BÖHMERT et al. 2001, CUMBLIDGE et al. 2003). Modelling of the evolution of VCs in pure iron under neutron irradiation is both of fundamental interest and of practical importance, because vacancies and VCs (even if not present after long-term irradiation) may play an intermediary role in the formation of Cu-rich clusters in RPV steels resulting in an irradiation-induced degradation of mechanical properties. In the present investigation we apply rate theory (RT) to simulate the evolution of VCs in pure bcc iron. Similar work was reported by ODETTE (1998) and HARDOUIN DUPARC et al. (2002). ODETTE (1998) directly solved the master equation with clustering of self-interstitial atoms (SIAs) neglected. HARDOUIN DUPARC et al. (2002) considered both planar SIA clusters and planar VCs and transformed the master equation into a Fokker-Planck equation, which may be a problem at small cluster sizes. The objective of the present investigation is to introduce the effect of lattice relaxation into the framework of ODETTE’s approach. The model can be adjusted in such a way that both volume fraction and mean size of irradiation-induced vacancy clusters measured by means of SANS are reproduced. The values of the specific surface energy of clusters and the dislocation density needed to adjust the model are reasonable estimates for the alloy under investigation. The expectation that the surface energies for the two dose rates considered should agree is fulfilled more closely, if cluster-matrix interaction is taken into account.