Long-term corrosion of two nuclear waste reference glasses (MW and SON68): A kinetic and mineral alteration study |
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| Institution: | 1. State Key Laboratory of Environment-Friendly Energy Materials, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang, 621010, PR China;2. Institute of Quartz and Special Glasses, China Building Materials Academy, Beijing, 100024, PR China |
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| Abstract: | Powder samples of two inactive borosilicate glasses (MW and SON68), used as references for vitrified nuclear waste in Switzerland, were leached statically in pure water over more than 12a at 90 °C. Solution aliquots were taken at regular intervals in order to investigate the glass dissolution kinetics and the retention of elements representing radionuclides. At 5.7 and 12.2a, single tests were interrupted to investigate the corroded samples.Boron and Li concentration data indicate that the glass corrosion kinetics of the MW glass is about 10 times faster than for the SON68 glass, both in the transient and asymptotic phase of the leaching process. The alteration products were studied by X-ray diffraction (XRD), scanning (SEM) and transmission electron microscopy (TEM-STEM) coupled with energy dispersive spectrometry (EDS) analyses. Alteration of the MW glass produced abundant magnesian clay minerals, as well as nanoparticles of lanthanide and Zr phosphates, whereas only small amounts Ca–Zn–Fe–Ni clay minerals were formed as alteration product of SON68.Retention factors were above 99% for most trace elements, indicating almost quantitative fixation of many radionuclides in the secondary phases. Solution concentration data were used to calculate aqueous speciations and saturation indices of potential secondary solids. The solutions are close to saturation with respect to simple lanthanide phosphates (in agreement with the TEM data) and quartz. The presence of quartz in the altered SON68 samples is corroborated by XRD data.In conclusion, the corrosion and radionuclide retention properties of SON68 seem to be more favorable than those of the MW glass. A major finding of this investigation was that glass degradation may strongly depend on minor compositional changes in the glass composition. The presence of Mg in the MW glass triggers the nucleation of secondary clay minerals, thereby promoting glass corrosion via silica removal. In the Mg-free SON68 glass the formation of clays, and hence the glass degradation, were considerably slower. |
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