Ion beam-induced amorphisation of freudenbergite

Using the HVEM-tandem facility at Argonne National Laboratory, the critical dose of 1.5 MeV Kr+ ions for amorphisation of freudenbergite (D-c (freudenbergite)) at room temperature was found to be 1.6 +/- 0.3 x 10(18) ions m(-2) D-c (freudenbergite) is lower than D-c (zirconolite) and D-c (perovskite) (3.5-5.5 and 3.9-9.2x10(18) ion m(-2), respectively). Freudenbergite can occur in Synroc-C, a titanate wasteform designed for immobilising high level radioactive waste (HLW). In Synroc-C, zirconolite and perovskite will contain the majority of the actinides in HLW. Freudenbergite will contain less than 0.2 wt% actinides but will experience displacement damage due to the alpha decay of actinides in surrounding phases. In agreement with the experimental findings of previous authors, our calculations show that in Synroc-C, freudenbergite will remain crystalline after zirconolite and perovskite have become amorphous. Neither of the two current parameters (structural freedom, for susceptibility to amorphisation, S) for estimating the relative radiation resistance of different phases is capable of predicting the relative radiation resistance of freudenbergite, zirconolite and perovskite. The low D-c of freudenbergite may result from Na+ ions having significantly lower E-d values compared to the other elements in freudenbergite, zirconolite and perovskite, in the electric fields induced by heavy ion irradiation of TEM specimens. If this hypothesis is true, it challenges the assumption that heavy ion irradiation can be used to compare the relative radiation resistance of different phases and will have serious implications for the predictive parameters of radiation resistance. (C) 2000 Elsevier Science B.V. All rights reserved.