To save content items to your account,
please confirm that you agree to abide by our usage policies.
If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account.
Find out more about saving content to .
To save content items to your Kindle, first ensure firstname.lastname@example.org
is added to your Approved Personal Document E-mail List under your Personal Document Settings
on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part
of your Kindle email address below.
Find out more about saving to your Kindle.
Note you can select to save to either the @free.kindle.com or @kindle.com variations.
‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi.
‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
New glassy matrices, able to incorporate new highly concentrated radioactive liquid wastes (HLW), are being studied. Investigations were performed on rare earth-rich glasses, known as very durable matrices. The selected basic glass composition was (wt. %): 51.0 SiO2 – 8.5 B2O3–12.2 Na2O – 4.3 Al2O3 – 4.8 CaO – 3.2 ZrO2 – 16.0 Nd2O3. To determine both the environment around the rare earth in this glass and its evolution according to its concentration (1.3 – 30 wt. % Nd2O3), EXAFS (Extended X-Ray Absorption Fine Structure) spectroscopy at the LIII-edge of neodymium and optical absorption spectroscopy were used. By coupling these two characterisation methods, several hypotheses are proposed about the nature of the rare earth neighbouring in the glass.
Zirconolite (CaZrTi2O7) based glass-ceramics designed for the specific immobilization of plutonium wastes or minor actinides (Np, Am, Cm) from high level radioactive wastes were investigated. To reach an efficient double containment, actinides must be preferentially located in the crystalline phase, which is homogeneously dispersed in a calcium aluminosilicate residual glass. Several heat treatments (between 950° and 1350°C) of a parent glass belonging to the SiO2-Al2O3-CaO system and containing TiO2 and ZrO2 were performed to prepare glass-ceramics. Trivalent minor actinides were simulated introducing Nd2O3 in the glass composition. Electron microscopy, X-ray diffraction (XRD) and thermal analysis have shown that devitrification processes in the bulk and on glass surface are different. They lead to the crystallization of zirconolite in the bulk and to a mixture of titanite (CaTiSiO5) and anorthite (CaAl2Si2O8) near the surface. For heat treatment temperatures greater than or equal to 1250°C, baddeleyite (m-ZrO2) crystals form at the expense of zirconolite in the bulk of glass-ceramics. XRD indicates that the order in zirconolite Ca/Zr planes increases with heating temperature. At the same time, extended defects density decreases.
The investigations on enhanced reprocessing of nuclear spent fuel, and notably on separating the long-lived minor actinides, such as Am and Cm, from the other fission products have led to the development of highly durable specific matrices such as glass-ceramics for their immobilization. This study deals with the characterization of zirconolite (CaZrTi2O7) based glass-ceramics synthesized by devitrification of an aluminosilicate parent glass. Trivalent actinide ions were simulated by neodymium, which is a paramagnetic local probe. Glass-ceramics with Nd2O3 contents ranging from 0 to 10 weight % were prepared by heat treatment of a parent glass at two different growth temperatures: 1050° and 1200°C. X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX) and electron spin resonance (ESR) measurements clearly indicate that Nd3+ ions are partly incorporated in zirconolite crystals formed in the bulk of the glass-ceramic samples. The amount of neodymium in the crystalline phase was estimated using ESR results and was found to decrease with increasing either heat treatment temperature or total Nd2O3 content.
The heat treatment conditions are a key factor in fabricating zirconolite ceramics and glass-ceramics following high-temperature melting. An oxide mixture melted at 1450°C and subsequently heat-treated at 1200°C yielded a glass-ceramic containing crystallized zirconolite–2M. The silica-enriched residual glass represented about 60-70 vol% of the total; the actinide surrogates (Nd, Ce)were equally distributed between the residual glass and the zirconolite crystals. Zirconolite ceramics obtained after melting an oxide mixture at 1600–1700°C consisted of zirconolite, perovskite and rutile. Rapid cooling rates (> 100°#x00B0;··min-1) were obtained by pouring the melt into ingot molds; the resulting zirconolite ceramics were characterized by crystals of zirconolite-2M ranging from 1 to no more than 20 μm. Slow cooling (< 25°C#x00B0;··min-1 produced ceramics with crystals several hundred micrometers long. Despite the microstructural differences, the chemical durability of the zirconolite ceramics was identical. The initial alteration rates r0 were about two orders of magnitude lower than those measured for the residual aluminosilicate glass of the zirconolite glass-ceramics. Moreover, during long-term leach tests at high S/V ratios to obtain advanced degrees of reaction progress, the alteration rates of all the materials diminished by over 3 to 4 orders of magnitude below r0.
The fission products resulting from reprocessing of commercial spent fuel are currently vitrified industrially by COGEMA at La Hague. The properties of 21 non-radioactive borosilicate glass samples containing between 4 and 6% of the platinum-group metals (PGM: Pd and Ru) compared with 1.6% in the industrial glass were investigated for chemical composition variations covering the full specification range. After a brief morphological description of the undissolved PGM in the glass, the viscosity variations at temperatures ranging from 1100 to 1200°C are discussed with emphasis on the effects of the particle inclusions on the rheological properties of the glass. Variations in the chemical durability of quenched glass specimens are then discussed. The initial leach rate V0 at 100°C remained near the values obtained for the reference glass. The same tests were repeated on glass heat-treated to obtain maximum crystallization, and the results confirmed that the chemical durability of the glass is practically unaffected by the crystallization observed in this type of glass.
The development of glass materials for long-term storage of high-level waste implies determining the glass thermal stability and notably assessing the risk of devitrification. Previous studies of the French nuclear waste glass have identified the crystalline phases and the domains in which they form, and have shown that devitrification is minimal. Modeling the long-term crystallization behavior requires an investigation of the thermodynamic and kinetic mechanisms liable to induce crystallization during cooling. The first step in this approach is to determine the nucleation and growth curves for each of the component phases. Crystals including CaMoO4, CeO2 and ZnCr2O4 were identified in glasses heat-treated at temperatures between 630°C and 1110°C, and the time and thermal dependence of the CaMoO4 morphology were evaluated. The nucleation and growth kinetics of these phases were determined by optical microscopy and SEM, and the impact of impurities was addressed by studying two glasses, with and without platinoid elements. The results indicated enhanced nucleation kinetics in glass containing platinoid elements. No induction time was observed before permanent nucleation in either of the glasses, and rapid saturation of nucleation kinetics—synonymous with the depletion of active centers of nucleation— was detected after a few hours. Furthermore, similar growth kinetics were observed in both glasses. The nucleation and growth curves coincided for all the phases. Peak values were much higher for nucleation than for growth kinetics, confirming the need for a thorough investigation of the mechanisms occurring in and below the glass transition range, i.e. in the non-equilibrium state.
Structural and bonding characteristics of simplified (Pd, Te) precipitates have been determined in a simulated nuclear French glass using extended x-ray absorption fine structure (EXAFS) and x-ray diffraction. In this sample, these precipitates have a homogeneous composition, with about 10 wt.% Te. They retain a face-centered cubic structure as in pure Pd with a cell parameter which obeys Vegard's law. Pd K-edge EXAFS shows the presence of Te in the Pd coordination shell, with (Pd–Te) distances of 2.80 Å. These distances, higher by 0.05 Å than the (Pd–Pd) distances, may result in a lower packing efficiency of the CFC lattice. The comparison with the average distances derived from x-ray diffraction shows the nonmetallic character of the Pd–Te bond in these precipitates. These bonding modifications may cause the limited solubility of Te in metallic Pd.
The chemical durability of aluminosilicate glasses was investigated experimentally between 90 and 200°C. In order to evaluate their potential for containment of minor actinides, these glasses were doped with Nd (to simulate the presence of trivalent actinides) or U. The proportions of the glass network formers, Si and Al, were near those found in basaltic glasses (tholeitic end members). The differences between the chemical compositions allow the clarification of the influence of glass network modifier elements on the chemical durability of silica glasses. More precisely, we tested the ability of Ti, Zr, Nd and N to potentially improve the chemical durability.
Two types of leach tests were conducted:
Dynamic leach tests to determine the initial dissolution rates at 90, 150 and 200°C and the activation energy (Ea) of the glass dissolution reaction;
Static leach tests at high SA/V (200 cm−1) and 90°C to determine the long-term alteration rates and the apparent silica solubility.
For all the glass compositions tested, the initial dissolution rates at nearly neutral or weakly basic pH are similar at the same temperature. Consequently all aluminosilicate glasses have the same activation energy, 60 kJ/mol. This suggests that the initial hydrolysis mechanism is controlled by the breakdown of Si-O and/or Al—O bonds, whatever the nature of glass network modifier elements.
On the other hand, modifier elements have major effects on the formation of protective films at the glass surface. The nature of this protective film depends on the chemical composition of the glass. A phenomenon similar to metal passivation, has been observed on a glass highly enriched with Nd (53.8 oxide wt%) which may lead to alteration rates in undersaturated media up to two orders of magnitude lower than those observed for basaltic glasses. This “passivation” effect disappears when a sodium sulfate solution (a Nd complexing agent) is used as a leachant. Nitrogen does not improve the chemical durability of aluminosilicate glasses. Finally, all these glasses have the same low dissolution rate (10−4 g/m2/day) under saturated conditions.
Neodymium-doped zirconolite materials may be synthesized by two melting processes. One involves devitrification of an aluminosilicate parent glass containing titanium, zirconium and neodymium oxides, yielding a glass ceramic comprised of submicron zirconolite needles embedded in a silica-rich glass matrix. The second method consists of melting an oxide mixture with the stoichiometry of a highly Nd-enriched zirconolite, then quickly cooling the melt to produce a ceramic rich in zirconolite crystals several hundred microns long containing a large fraction of the initial neodymium.
Models have been developed to calculate the density, molten-state viscosity and initial corrosion rate according to the chemical composition of glass formulations used to vitrify high-level fission product solutions from reprocessed light water reactor fuel. Developed from other published work, these models have been adapted to allow for the effects of platinoid (Ru, Pd, Rh) inclusions on the molten glass rheology.
Nine samples of the “R7T7” glass composition selected to vitrify fission product solutions in France were prepared with added platinoid elements (ruthenium, rhoditun and palladium) in soluble form and as insoluble metal particles in solution, and their major properties were measured. Regardless of the initial form when added to the glass the platinoids always formed the same heterogeneous inclusions in the final glass: RuO2 precipitates which were often found as aggregates, and polymetallic (Pd, Rh & Te) inclusions.
The particles tended to settle in the molten glass. The viscosity increased by about 20% at 1100°C. The mechanical properties and short-term leach rates were not significantly affected. Crystallization increased by a factor of 2 or 3 in heat-treated glass specimens but did not exceed a few volume percent. However, as the short-term leach rate did not significantly increase, the glass properties were very satisfactory.
R7T7 glass samples were tested to determine their sensitivity to variations in the chemical composition and in industrial scale operating parameters. Variations investigated included the composition of the feed solution and the glass frit, the frit/glass ratio and the glass melting temperature. The iniportant properties of the resulting glasses were measured. Permissible variation ranges defined on the basis of the results obtained ensure that the glass properties remain acceptable compared with the reference glass.
Email your librarian or administrator to recommend adding this to your organisation's collection.