The effects of radiation on the density of an aluminoborosilicate glass

Glasses used for nuclear waste immobilization are subjected to high levels of radiation, and this may affect their physicochemical properties. Alpha r adiation is responsible for an important fraction of the radiation energy d issipated in these glasses. It has been reported previously that some boros ilicate glasses increase their density during irradiation while the density of other glasses decreases. Although the density increase of silica after irradiation has been understood, thanks mainly to molecular dynamics calcul ations and diffraction experiments, the processes involved in more complex glasses could be more varied. In this work we irradiated an aluminum-borosi licate glass which is a candidate for the aforementioned purposes and which increases density during alpha irradiation from the B-10 (n,alpha) Li-7 re action. We studied the effects of alpha irradiation on its microstructure, using several experimental techniques, and subsequently correlated the resu lts. Small angle X-ray scattering (SAXS) measurements revealed the presence of inhomogeneities of about 10 Angstrom in the untreated samples. After an nealing these samples, TEM images displayed a contrast structure and helium pycnometry revealed density changes, both typical of glass phase separatio n. After irradiation, the glass density increased and the SAXS intensity de creased, indicating a compositional homogenization process in the samples s ubject to a higher dose of irradiation. Atomic displacements were calculate d by means of the TRIM [1] computer code. The number of displacements produ ced by each 10B(n,alpha) Li-7 reaction was estimated at 580 and involved di stances of up to 15 Angstrom An increase in the density of the irradiated s amples can be explained in terms of the atomic displacements produced by th e nuclear reaction cascades of the reaction B-10 (n,alpha) Li-7, in the sce nario of pre-existing phase separation in the samples. In the case of the a luminum-borosilicate glasses studied here, which exhibit a fine phase separ ation, the density of the Si-rich phase increases with the incorporation of Na and B atoms. The B-rich phase also increases its density with the flow of Si atoms from the matrix. Vacancies created by irradiation in the glass structure, are responsible for a density decrease. The final effect is due to the sum of all contributions described, which in this case results in a net density increase of the irradiated samples. An understanding of this ph enomenon can lead to the design of new glasses which overcome radiation wit h a minimum of density change. (C) 2001 Elsevier Science BY. All rights res erved.