Effects of complex irradiation scenarios on ISG nuclear glass long-term behavior
Résumé
Several countries have chosen the recycling of U and Pu in MOX fuels associated to the vitrification of the ultimate wastes composed of fission products (FP) and minor actinides (MA) in silicate glasses. In France, the borosilicate glass known as R7T7 is currently used to confine high-level radioactive waste remaining after reprocessing of spent nuclear fuel, in the perspective of the disposal of the glass canisters in a geological repository. The main challenge is therefore to predict their long-term behavior to ensure the safety of the disposal solution: the glass physical and chemical stabilities over geological timescale must thus be demonstrated, which require to study the glass response to the radiations emitted by the nuclear waste together with its behavior when submitted to water dissolution.This work focuses on the leaching behavior of a chemically simplified borosilicate glass, chosen by the international community, (called ISG as International Simple Glass). Pristine glass coupons were subjected to several external irradiations scenarios (electrons, Au ions, electrons followed by Au ions) to simulate the various irradiation sources of a real radioactive glass, inducing electronic and/or nuclear interactions. For both electron and Au ions irradiations, the doses values have been chosen to induce significant structural and properties evolutions in the glass and they also correspond to the dose level expected under disposal condition when the water will arrive in contact with the glass. Radiation effects on the glass structure and properties have been studied by respectively Raman and NMR spectroscopies, X-ray reflectivity, density, Vickers microindentation hardness and contact angle measurements.Pristine and irradiated glass coupons were then leached for several months in pure water, at 90 °C and at a high surface-area-to-volume ratio (SA/V = 200 cm-1), in order to quickly reach the residual alteration rate regime expected on the long-term under disposal conditions. The alteration layer thickness was monitored by ToF-SIMS to calculate the glass dissolution rate and cryo-TEM was perform to describe the alteration layer nanostructure.The results showed that the electron irradiation induces only small changes of both the structure and properties of the glass, including its alteration behavior. However, the irradiation by Au ions induces significant modifications of ISG glass structure and properties, together with an increase of the glass alteration rate. The results obtained for the sequentially (electrons followed by Au ions) irradiated glass are close to those obtained on the glass irradiated with Au ions only. It suggests that the long term glass leaching is sensitive to the resulting structural changes in the glass caused by the nuclear damage, induced here by Au ions.