Irradiation stability of R7T7-type borosilicate glass

High-level nuclear waste containment glass is subjected to irradiation-induced stresses whose consequences must be assessed to guarantee the material behavior over time. Alpha decay from the minor actinides confined in the glass structure is responsible for most of the atom displacements. Minor actinide concentrations in the glass exceeding current levels are under consideration for future vitrification scenarios. Four R7T7-type borosilicate glass samples doped with 0.04, 0.4, 1.2 and 3.25 wt% (CmO2)-Cm-244 were fabricated to study not only the cumulative effect of high alpha decay doses (> 10(19) alpha/g)but also of the dose rate. The homogeneity of the materials and their chemical compositions were characterized to ensure they were suitably representative for the experimental program. The macroscopic behavior of the glasses was characterized for doses up to 3 x 1018 alpha/g. No significant effect on the initial alteration rate was detected which means that the chemical reactivity of the glass with pure water is not affected by such levels of alpha doses. The glass swelled slightly depending on the dose, saturating at about 0.5% after receiving a dose of about 2 x 1018 alpha disintegrations per gram of glass. The mechanical properties were observed to improve appreciably, with lower hardness but greater fracture toughness. A stabilization phenomenon comparable to that of the glass density was also observed. Comparing the experimental data with other findings obtained by atomistic modeling and external irradiation suggests that nuclear interactions caused by alpha decay recoil nuclei could be responsible for the experimental variations observed. The possible origins of the observed variations in macroscopic properties are also discussed. (c) 2006 Elsevier B.V. All rights reserved.