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 radiation is responsible for an important fraction of the radiation energy dissipated in these glasses. It has been reported previously that some borosilicate 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 calculations and diffraction experiments, the processes involved in more complex glasses could be more varied. In this work we irradiated an aluminum-borosilicate glass which is a candidate for the aforementioned purposes and which increases density during alpha irradiation from the 10B (n,α) 7Li reaction. We studied the effects of alpha irradiation on its microstructure, using several experimental techniques, and subsequently correlated the results. Small angle X-ray scattering (SAXS) measurements revealed the presence of inhomogeneities of about 10 Å in the untreated samples. After annealing these samples, TEM images displayed a contrast structure and helium pycnometry revealed density changes, both typical of glass phase separation. After irradiation, the glass density increased and the SAXS intensity decreased, indicating a compositional homogenization process in the samples subject to a higher dose of irradiation. Atomic displacements were calculated by means of the TRIM [1] computer code. The number of displacements produced by each 10B(n,α) 7Li reaction was estimated at 580 and involved distances of up to 15 Å. An increase in the density of the irradiated samples can be explained in terms of the atomic displacements produced by the nuclear reaction cascades of the reaction 10B (n,α) 7Li, in the scenario of pre-existing phase separation in the samples. In the case of the aluminum-borosilicate glasses studied here, which exhibit a fine phase separation, 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 phenomenon can lead to the design of new glasses which overcome radiation with a minimum of density change.