Abstract
In the prospect of deep geological disposal, the long-term behavior of high-level nuclear glasses has to be investigated regarding alpha radiation induced by long-life minor actinides. The present study focuses on the effects of alpha radiation on the long-term chemical reactivity of R7T7-type glasses, by separately considering the alpha dose rate and the alpha decay dose. Old SON68 glasses doped with 238/239PuO2 or 244CmO2 were studied to simulate high alpha dose rates corresponding to an early water ingress and a high level of alpha decay doses corresponding to long-term disposal conditions. A part of the 238/239Pu-doped glass block was annealed to fully recover the irradiation-induced damage accumulated since the glass was fabricated and to dissociate the effect of the alpha dose rate from that of the alpha decay dose. The glasses were then leached under static conditions at 90 °C for several years. The results showed that the residual alteration rate is not affected by the alpha dose rate over a wide range of dose rate values expected under disposal conditions: this glass remained relatively insensitive to the alpha radiolysis phenomena at the glass–water interface. However, over the duration of the experiments, the residual alteration rate of the damaged 238/239Pu-doped glass was enhanced compared to that of the annealed glass. This result is in agreement with those obtained on the 244Cm-doped glass and with reported values in the literature on simplified externally irradiated glasses, indicating that the ballistic effects of the recoil nuclei are responsible for this increase in the residual alteration rate.
Highlights
Several countries have chosen to recycle U and Pu in UOX and MOX fuels, and to vitrify the ultimate high-level radioactive waste (HLW) composed of fission products and minor actinides in silicate glasses
Experiments performed on heavily altered glasses[29] have shown that ballistic collisions from ion implantation could heal the porous gel part of the altered glasses to create a pore-free homogeneous microstructure. These results demonstrate that the nuclear decay dose affects the alteration layer, and the residual alteration rate, but the precise origin of these effects has not yet been clearly established
One study on the residual alteration rate regime, which focused on alpha dose rate (150 Gy.h−1), has been reported[20], and this study concluded that no radiation effect was present
Summary
Several countries have chosen to recycle U and Pu in UOX and MOX fuels, and to vitrify the ultimate high-level radioactive waste (HLW) composed of fission products and minor actinides in silicate glasses. Interdiffusion and ion exchange with water (H+/H3O+), followed by progressive in situ hydrolysis and subsequent recondensation[8,12,16,17,18] are the proposed mechanisms to explain the formation of the alteration layer In this last case, the alteration layer region, called gel, is constantly reorganized over time[8,19] and acts as a diffusive barrier. Experiments performed on heavily altered glasses[29] have shown that ballistic collisions from ion implantation could heal the porous gel part of the altered glasses to create a pore-free homogeneous microstructure These results demonstrate that the nuclear decay dose affects the alteration layer, and the residual alteration rate, but the precise origin of these effects has not yet been clearly established. One study on the residual alteration rate regime, which focused on alpha dose rate (150 Gy.h−1), has been reported[20], and this study concluded that no radiation effect was present
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