Influence of composition of nuclear waste glasses on vapor phase hydration

Abstract

12For the first time, the influence of glass composition on the vapor hydration kinetics of the French AVM 13 nuclear waste glass simulants was investigated. Three complex borosilicate glasses (> 20 oxides) along 14 with three simplified Na/(Ca, Na)/(Mg, Na)-alumino-borosilicate glasses with four or five oxides were 15 altered at 50°C in water vapor (95% RH) for up to 557 days. The solid characterization of the altered 16 samples (by SEM, TEM, XRD, ToF-SIMS, SAXS) revealed that the rate-controlling vapor hydration 17 mechanism is composition dependent. The vapor hydration rate of the more durable glasses, whose 18 molar ratio of Al 2 O 3 /(CaO or MgO) is ≥1, seems to be limited by network-hydrolysis, whereas the overall 19 glass alteration rate of the less durable glasses, whose Al 2 O 3 /MgO ratio is <1, seems to be driven by the 20 precipitation of Mg-rich secondary phases. All the vapor hydrated glasses show the presence of a 21 homogeneous gel layer of a few tens of nm thickness. The more durable glasses have a smaller quantity 22 of secondary phases precipitated on their surface. The less durable glasses have a relatively significant 23 quantity of secondary phases precipitated above the gel layer and irregularly shaped, scattered and 24 highly porous altered zones formed beneath the gel layer. The overall alteration rate of the less durable 25 glasses was 10-20 times faster than the more durable glasses. For all glasses, FTIR spectroscopy indicated 26 an inflexion in the vapor hydration rate after 120-200 days of alteration, likely due to a passivating effect 27 of the altered layer formed under unsaturated conditions. The average pore size of the altered layer in 28 50°C [34]. The vapor hydration of SON68 (French inactive reference nuclear waste glass) at high 58 temperatures (90-200°C, activation energy (E a )=43-47 KJ/mol) [28] and lower temperatures (35-90°C, 59 by studying the behavior of three AVM (Atelier de Vitrification de Marcoule) glasses in contact with water 87 vapor. The AVM glasses are complex borosilicate glasses containing more than 20 oxides including fission 88 products issued from the UNGG (Uranium Naturel Graphite-Gaz) reactor at Marcoule. The range in the 89 AVM glasses composition is extensive due to the variation in the composition of fission products 90The synthesis of the AVM glasses has already been described by Thien [46]. The glass samples for this 113 study were retrieved from the same batch. Q, QCa and QMg were prepared using the oxide precursors 114

Glass alteration in atmospheric conditions: crossing perspectives from cultural heritage, glass industry, and nuclear waste management

“…Carbonates are also observed on vapor hydrated nuclear waste glasses, especially on calcium containing glasses. The morphology of the alteration layer may either be a homogeneous hydrated layer or heterogeneously altered zones adjacent to visibly unaltered zones or sometimes both 70,[73][74][75] .…”

“…Various protocols have been used to control RH and temperature in glass alteration experiments: (i) desiccator with or without saline solutions 18,19,78 , (ii) climatic chamber 64,75,[79][80][81][82] , (iii) hermetic box enclosing a saline solution and placed in an oven 83 , (iv) teflonlined autoclave in an aluminum container (to homogenize T) 70,84 , with samples placed on a teflon holder above a saline solution, and placed in an oven. Humidity conditions can be unsaturated (less than 100% RH) or saturated (100% RH), although we focus on the unsaturated conditions in this paper.…”

“…(ii) In some cases, glasses that classify as the most durable in aqueous phase become the least durable in vapor phase: such an order reversal occurs in a series of Mg-rich 75 , and another series of Fe-rich 73 , sodium aluminoborosilicate glasses.…”

“…In this respect, it is important to emphasize that these alkali salts do not generally hold all the alkalies of the alteration layer. Indeed, many studies report that alkalies are significantly retained in the hydrated layer 5,64,73,75,79,80,84 . It is generally supposed that this retention is due to the saturation of the water film that cancels the chemical gradient, and that Na + is released but present in the pore water 75,84 .…”

“…Indeed, many studies report that alkalies are significantly retained in the hydrated layer 5,64,73,75,79,80,84 . It is generally supposed that this retention is due to the saturation of the water film that cancels the chemical gradient, and that Na + is released but present in the pore water 75,84 . Dissolution experiments of glass samples pre-altered in vapor conditions show a spike in alkali release that supports this idea 84 , although the alkalies released in the solution account for less than 30% of the alkalies initially present in the hydrated layer in the case of the unstable mixed alkali lime glass of CH 79 .…”