Radiation damage effects on the structure and properties of radioactive waste glasses

The performance assessment of glass matrices currently being used in different nuclear energy producing countries to contain and immobilise high-level waste (HLW) waste is crucial for safe and economic disposal. During the first ~500 years of geological disposal fission products will me the main source or radiation and they decay by beta-gamma emission. We studied different borosilicate glasses used in different countries to immobilise HLW, such as Indian glass (NaBaBSi), UK glasses (LiNaBSi), also called MW- Mixture Windscale, UK-CaZn a modified version of MW, French glass (SON68) and a glass proposed by six collaborating nations, called the International Simple Glass (ISG), to see whether / how irradiation defects are dependent on glass composition. Glasses were externally irradiated using a 60Co gamma source to study the effects of beta-gamma radiation (a gamma emitter loses it energy to atomic electrons they then further interact via coulombic interactions); and by He2+ ion implantation to study the effects of alpha particles. A multi-spectroscopic approach was used to characterise glass specimens before and after irradiation. NaBaBSi and LiNaBSi glasses were irradiated using 60Co gamma photons and we found boron-oxygen hole centres (BOHC), electrons trapped at alkali cations or ET centres and peroxy-radicals (PORs) as common defect in these glasses. In addition, E- or polaron centres which may be related to formation of elemental / metallic sodium colloids formed in NaBaBSi glass. Time-dependent thermal annealing of the irradiated glasses revealed that POR’s are the most thermally stable of the defect centres. BOHC and ET centres were common to SON68, ISG and CaZn glasses. In NaBaBSi, LiNaBSi and SON68 glasses loaded with non-active simulated HLW no sharp and intense signals indicative of radiation-induced paramagnetic defect centres was observed. In the study of NaBaBSi and LiNaBSi glasses doped with 0.19. 0.99, and 4.76 mol% Fe2O3 prepared in an oxidising melting environment, electron paramagnetic resonance (EPR) spectroscopy showed that gamma irradiation induced sharp and intense signals exist for only the 0 and 0.19 mol% Fe2O3 doped samples and disappeared for samples containing higher molar concentrations of Fe2O3. It is postulated that, upon gamma irradiation, in LiNaBSi glass Fe2+ is oxidised to Fe3+ by the capture of holes, whereas in NaBaBSi glass Fe3+ is reduced to Fe2+ due to capture of electrons. Further research is needed to understand the reasons behind these different behaviours

X-Ray fluorescence analysis of feldspars and silicate glass: effects of melting time on fused bead consistency and volatilisation

Reproducible preparation of lithium tetraborate fused beads for XRF analysis of glass and mineral samples is of paramount importance for analytical repeatability. However, as with all glass melting processes, losses due to volatilization must be taken into account and their effects are not negligible. Here the effects of fused bead melting time have been studied for four Certified Reference Materials (CRM’s-three feldspars, one silicate glass), in terms of their effects on analytical variability and volatilization losses arising from fused bead preparation. At melting temperatures of 1065 °C, and for feldspar samples, fused bead melting times shorter than approximately 25 minutes generally gave rise to greater deviation of XRF-analyzed composition from certified composition. This variation might be due to incomplete fusion and / or fused bead inhomogeneity but further research is needed. In contrast, the shortest fused bead melting time for the silicate glass CRM gave an XRF-analyzed composition closer to the certified values than longer melting times. This may suggest a faster rate of glass-in-glass dissolution and homogenization during fused bead preparation. For all samples, longer melting times gave rise to greater volatilization losses (including sulphates and halides) during fusion. This was demonstrated by a linear relationship between SO3 mass loss and time1/2, as predicted by a simple diffusion-based model. Iodine volatilization displays a more complex relationship, suggestive of diffusion plus additional mechanisms. This conclusion may have implications for vitrification of iodine-bearing radioactive wastes. Our research demonstrates that the nature of the sample material impacts on the most appropriate fusion times. For feldspars no less than ~25 min and no more than ~60 min of fusion at 1065 °C, using Li2B4O7 as the fusion medium and in the context of feldspar samples and the automatic fusion equipment used here, strikes an acceptable (albeit non-ideal) balance between the competing factors of fused bead quality, analytical consistency and mitigating volatilization losses. Conversely, for the silicate glass sample, shorter fusion times of less than ~30 minutes under the same conditions provided more accurate analyses whilst limiting volatile losses

X-ray Fluorescence Analysis of Feldspars and Silicate Glass: Effects of Melting Time on Fused Bead Consistency and Volatilisation

Reproducible preparation of lithium tetraborate fused beads for XRF analysis of glass and mineral samples is of paramount importance for analytical repeatability. However, as with all glass melting processes, losses due to volatilisation must be taken into account and their effects are not negligible. Here the effects of fused bead melting time have been studied for four Certified Reference Materials (CRM’s: three feldspars, one silicate glass), in terms of their effects on analytical variability and volatilisation losses arising from fused bead preparation. At melting temperatures of 1065 °C, and for feldspar samples, fused bead melting times shorter than approximately 25 min generally gave rise to a greater deviation of the XRF-analysed composition from the certified composition. This variation might be due to incomplete fusion and/or fused bead inhomogeneity but further research is needed. In contrast, the shortest fused bead melting time for the silicate glass CRM gave an XRF-analysed composition closer to the certified values than longer melting times. This may suggest a faster rate of glass-in-glass dissolution and homogenization during fused bead preparation. For all samples, longer melting times gave rise to greater volatilisation losses (including sulphates and halides) during fusion. This was demonstrated by a linear relationship between SO3 mass loss and time1/2, as predicted by a simple diffusion-based model. Iodine volatilisation displays a more complex relationship, suggestive of diffusion plus additional mechanisms. This conclusion may have implications for vitrification of iodine-bearing radioactive wastes. Our research demonstrates that the nature of the sample material impacts on the most appropriate fusion times. For feldspars no less than ~25 min and no more than ~60 min of fusion at 1065 °C, using Li2B4O7 as the fusion medium and in the context of feldspar samples and the automatic fusion equipment used here, strikes an acceptable (albeit non-ideal) balance between the competing factors of fused bead quality, analytical consistency and mitigating volatilisation losses. Conversely, for the silicate glass sample, shorter fusion times of less than ~30 min under the same conditions provided more accurate analyses whilst limiting volatile losses

Gamma irradiation-induced defects in borosilicate glasses for high-level radioactive waste immobilisation

Gamma irradiation-induced defects at doses of 0.5 and 5 MGy were studied in lithium sodium-borosilicate (LiNaBSi) and sodium barium-borosilicate (NaBaBSi) glasses, used for high-level radioactive waste immobilisation in the UK and India, respectively. X-band electron paramagnetic resonance (EPR), Raman and UV-Vis-nIR spectroscopies were used to characterise the glasses before and after irradiation. EPR and UV-Vis-nIR absorption spectroscopies revealed the formation of boron-oxygen hole centres (BOHC), electrons trapped at alkali cations or ET centres and peroxy-radicals (PORs) as defects common to both glasses. In addition, E− or polaron centres were observed in NaBaBSi glasses, possibly related to formation of elemental sodium colloids. Time-dependent thermal annealing at a range of temperatures, including those relevant to canister centreline cooling (CCC), which may be of relevance to geological disposal in future technical assessments, was carried out to study thermal stability of these radiation-induced defects. It was observed that PORs are the most thermally-stable defects in both glasses. The influence of glass composition on the segregation of sodium; possible formation of metal colloids upon irradiation has been discussed