Long-term aqueous alteration kinetics of A 99Tc-doped SON68 borosilicate glass

Fission products and minor actinides are currently immobilized by vitrification in a borosilicate glass known as 'R7T7'. One of the major functions of the glass matrix is to retain the radioactive elements in the event of water intrusion in the geological disposal, expected after several thousand years. The alteration of SON68 glass (nonradioactive surrogate of R7T7 glass) has been extensively studied during the last thirty years and the key mechanisms and kinetics of borosilicate glass alteration have been determined [1]: the first step corresponds to a congruent dissolution characterized by the initial dissolution rate. Then in-situ. recondensation of dissolved species results in the formation of a gel that decreases the alteration rate by several orders of magnitude to a residual alteration rate. The present work focuses on the study of the effects of electronic dose rate, which would induce electronic excitations and ionisations, on this residual alteration rate by studying a beta-doped glass containing 99Tc. Moreover, the behaviour of a long-life isotope, 99Tc (T1/2 = 2.1×105 years), which contributes to the long-term R7T7 glass activity, is considered. A 0.16%wt 99Tc-doped SON68 borosilicate glass is leached here under static conditions, in argon atmosphere at 90°C and at a high surface-area-to-volume ratio (initial S/V = 25 cm-1), in order to reach quickly the residual alteration rate. In addition, a reference experiment is performed by leaching a non-radioactive SON68 borosilicate glass in the same conditions (initial S/V = 26 cm-1, 90°C), but outside the shielded cell. The alteration rate is monitored by the releases of glass alteration tracer elements (B, Na and Li), which are measured by ICP-AES. Most of the leachate samples are analyzed directly. The last sampling is filtered (0.45 µm) and ultrafiltered (10 000 Dalton) to check for colloids. The releases in solution are expressed in terms of normalized mass losses (NL in g.m-2) [2]. Radiation effects on the glass leached and its gel network are characterised on raw grains by Scanning Electron Microscopy (SEM) analyses, and on ground altered glass powder by Transmission Electron Microscopy (TEM) analyses. Technetium releases are also measured by radiometry and its chemical oxidation state is assessed by measuring both pH and redox potential of the leachate. Results do not highlight any significant effect of beta irradiation on the residual alteration stage of this doped glass: similar results have been observed on the reference experiment. These observations are consistent with SEM and TEM characterizations, which show that a protective layer can be formed under beta irradiation. Concerning the behaviour of technetium, as described in figure 1, congruence between boron and technetium releases is observed, traducing the predominance of Tc(VII) in solution under these conditions (Eh = 380 mV/SHE, pH = 8 - 8,5)