2Àunits are not bond to Si or B. 4,6 The same kind of isolated tetrahedral molybdate entities are present in the Na 2 MoO 4 and CaMoO 4 phases that can form in some nuclear borosilicate glasses. [4][5][6]11 The similarity between the local environment of MoO 4 2À entities in glass and in crystalline molybdate phases may explain the low solubility of molybdenum in glass and its tendency to crystallize during slow cooling (%1°C/min) of the melts after beingpoured into canisters or during prolonged glass heat treatment above the transformation temperature, Tg. 12 Moreover, recent works 13-16 on soda-lime borosilicate glass or more simplified silicate glasses have shown that increasing MoO 3 amount induced an increase of the silicate network reticulation. The presence of less than 1 mol% MoO 3 is sufficient to cause a 3% drop in the number of nonbridging oxygen atoms and clearly shows the influence of MoO 3 on glass structure based on the increase of the polymerization. 15 In a previous study, 17 the solubility limit of MoO 3 in the simplified glass composition chosen in the SiO 2 -B 2 O 3 -Na 2 O-CaO system derived from the HLW glass formulation was about 1.0 mol% (2.3 wt%) for a cooling rate of 1°C/min. Beyond this limit, the glass was partially crystallized into Ca-MoO 4 (water-durable phase) and Na 2 MoO 4 (water-soluble phase). When the cooling rate was increased to 10 4°C /min, the amount of MoO 3 incorporated increased up to 2.5 mol% C. Jantzen-contributing editor Manuscript No. 29912.

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The long-term behavior of high-level nuclear glass subjected to alpha/beta radiation by long-life radionuclides must be investigated with respect to geological disposal. This study focuses on the effects of alpha and beta radiations on the chemical reactivity of R7T7 glass with pure water, mainly on the residual alteration rate regime. Glass specimens doped with 0.85 wt % 239 PuO 2 (a emitter) and 0.24 wt% 99 TcO 2 (b emitter) that simulate alpha and beta dose rate corresponding to long-term disposal conditions are leached under static conditions in argon atmosphere at 90°C, in initially pure water and at a high surface-area-to-volume ratio (S/V = 20/cm). The alteration rate is monitored by the release of glass alteration tracer elements (B, Na, and Li). Radiation effects on the leached glass and its gel network are characterized by SEM and TEM analyses. Plutonium and technetium releases are also measured by radiometry, and their chemical oxidation state is assessed by measuring the pH and reduction-oxidation potential of the leachates. The results do not highlight any significant effect of alpha/beta radiation on the residual alteration of these doped glasses. These observations are consistent with solid characterizations, which show that a protective layer can be formed under such irradiation fields. Under our experimental conditions (Eh~380 mV/ *magaly.tribet@cea.fr