Influence of gamma irradiation effects on the residual alteration rate of the French SON68 nuclear glass
Résumé
The residual rate regime of SON68 nuclear glass has been investigated here by static experiments under external irradiation. The experiments were carried out in a gamma irradiator with Co-60 sources. Four dose rates were studied: two high dose rates (5 and 10 kGy h(-1)) to exacerbate electronic radiation effects, and two lower dose rates (0.8 and 0.05 kGy h(-1)). The objective of this work is not to assess potential LET effects but to assess more widely electronic effects on the residual rate regime which controls the long term stability of the glass under geological disposal. Fresh glasses were leached in argon atmosphere at 90 degrees C and at a high surface-area-to-volume ratio (SA/V = 10 000 m(-1)). Under these conditions the system quickly reaches the residual alteration rate regime. The alteration rate was monitored by solution analyses: the release of glass alteration tracer elements-in particular boron, sodium and lithium-was measured by ICP-AES. The hydrogen peroxide produced by water radiolysis was quantified by chemiluminescence. Radiation effects on the glass and its gel network were characterized by scanning electron microscopy (SEM) and Transmission Electron Microscopy (TEM). Higher H2O2 concentrations were measured on irradiated samples in the presence of glass, suggesting an irradiation effect on the glass and its alteration layer. However, the data obtained on glass alteration show that potential defects or changes induced by gamma irradiation do not modify the kinetics of alteration up to a gamma dose rate of 10 kGy h(-1): the residual rates measured for the different dose rates studied are similar and comparable to experiments performed without irradiation. Moreover, these observations are consistent with SEM and TEM characterizations which show that the alteration layer formed under gamma irradiation is similar to the alteration layer formed without irradiation. This study demonstrates that species produced by pure water radiolysis and defects induced by gamma irradiation in the glass and/or its alteration layer may remain limited and localized enough not to affect glass alteration under these conditions.