New Journal of Glass and Ceramics, 2011, 1, 105-111
doi:10.4236/njgc.2011.13015 Published Online October 2011 (
Copyright © 2011 SciRes. NJGC
Sol-Gel Processing of Silica Nuclear Waste Glasses
Andrzej Deptuła1, Magdalena Miłkowska1, Wiesława Łada1, Tadeusz Olczak1, Danuta Wawszczak1,
Tomasz Smolinski1, Fabio Zaza2, Marcin Brykala1, Andrzej G. Chmielewski1, Kenneth C. Goretta3
1Institute of Nuclear Chemistry and Technology (INCT), Warsaw, Poland; 2Italian National Agency for New Technologies, Energy and
Environment (ENEA), CR Casaccia, Rome, Italy; 3Asian Office of Aerospace Research and Development, Japan.
Received June 30th, 2011; revised August 9th, 2011; accepted August 21th, 2011.
A complex Sol-Gel process has been used for synthesis of silica glasses designed to contain high-level nuclear wastes.
Cs, Sr, Co, and Nd (generically denoted Me) were used, the last as surrogate for actinides. Gels in the form of powders
and sintered compacts were prepared by hydrolysis and polycondensation of tetraethoxide/Me nitrate solutions, which
contained ascorbic acid a s a catalyst. Thermal treatment stud ies were conducted on the resulting gels. Transformation
to final products was studied by thermog ravimetric analysis, infrared sp ectroscopy, and X-ray diffraction. Prelimina ry
testing of Me leaching was also completed in quiescent water. Only a single dense form was resistant to leaching.
Keywords: Sol-Gel, Silica Glass, Nuclear Waste, Thermal Treatment
1. Introduction
Vitrification of hazardous nuclear wastes has been shown
to be a viable technological alternative for effecttive ma-
nagement of spent fuel and radioactive waste. The advan-
tages of the method are that a large number of elements
that can be incorporated into the glass and a highly dura-
ble and small-volume waste is produced [1,2]. The prop-
erties of silica glasses, including good durability and
mechanical strength and ability to incorporated large
concentrations of metallic dopants, make them ideal can-
didates for matrices for nuclear waste storage [3,4]. The
most significant disadvantage of silica glasses for such
use is their high processing temperature of ~ 2000˚C. Sin-
tering of sol-gel-derived glasses can be accomplished at
much lower temperatures, and sol-gel techniques have
been successfully used for preparation of porous glass
hosts for nuclear wastes [3,4]. Appropriately prepared,
sintered ceramic bodies can have higher stabilities and be
more resistant to leaching than are many melt-processed
glasses [5,6].
In the present work, our proprietary complex sol-gel
process (CSGP) [7-9] has been adapted to synthesize
silica glasses capable of incorporating significant con-
centrations of high-level nuclear wastes. The heavy met-
als (denoted Me) Cs, Sr, Co, and, as a surrogate for acti-
nides, Nd, were incorporated into silica glasses and the
resulting waste forms were characterized and tested for
leaching response.
2. Experimental Details
A detailed flow-chart for the preparation Cs-, Sr-, Co-,
and Nd-doped (10 mole%) silica gels is shown in Figure
1. Gels in the form of powders or monoliths were pre-
pared by hydrolysis and subsequent polycondensation of
tetraethoxide/Me nitrate solutions containing ascorbic acid
(ASC) as a catalyst, instead of the HCl or NH4OH that
are routinely used for catalysis in glass synthesis. ASC
has not, to the best of our knowledge, been used previ-
ously in this type of processing of a nuclear waste glass.
Its use has been shown to decrease remarkably the time
needed to synthesize gels [7,9].
Thermogravimetric analysis (TG) and differential ther-
mal analysis (DTA) were conducted in air with a Hun-
garian MOM (Budapest, Hungary) [8,9]. The heating rate
was 10˚C /h.
All resulting products were analyzed by X-ray diffract-
tion (XRD) with a Rigaku Miniflex diffractometer (To-
kyo, Japan). Cu-Kα radiation was used. The tube output
voltage was 30 kV the tube output current was 15 mA,
the angular spread was 2θ = 3˚ - 90˚, with steps of 0.02˚
and a scanning rate of 2˚/min. Raw data were smoothed
by the Savitzky method, background was eliminated by
the Sonnevelt method, and Kα2 was eliminated. Infrared
measurements were conducted with a Perkin Elmer Mo-
del 983 Spectrometer (Waltham, MA). The potassium
bromide pellet technique was adopted.
Leaching tests were conducted on 2 g of material. Powder
Sol-Gel Processing of Silica Nuclear Waste Glasses
Figure 1. Flow-chart of CSGP for preparation Cs-, Sr-, Co-, and
Nd-doped silica gels.
agglomerates, in some cases with alumina entrained, were
placed in G4 crucibles, which were placed in 100 ml of
gently stirred deionized water at 23˚C. After each cruci-
ble was withdrawn, the water was filtered off. The Cs-
doped drops of glass, which attacked the platinum dish in
which they were heated, were handled similarly. After dry-
ing for 2 h at 110˚C, the specimens were weighed. These
procedures were repeated a total of 19 times. The maxi-
mum emersion time was 430 h. Solutions were analyzed
for Me and Si contents by ion chromatography [10] with
a Dionex 2000/sp ion chromatograph (Sunnyvale, CA,
USA) and with a Thermo Electron Corporation flame ato-
mic absorption spectrometer, Model SOLAAR MK2 (Ma-
rietta, OH, USA).
There is a wide range of possibilities for leaching tests.
Various specimen sizes and forms, solutions, fluid flows,
and temperatures can be used [11-15]. We selected leach-
ing of comparatively small samples of high surface area in
room-temperature deionized water as a reasonable method
to assess whether this type of silica-based, sol-gel-d-
erived glass may be a viable candidate as a nuclear waste
3. Results and Discussion
The thermal decomposition of gels (dried under vacuum
at 70˚C in Rotavapor) versus temperature, T, is illus
Figure 2. Thermal analysis of gel powders dried at 70oC:
CsSi (■■); SrSi (■■); CoSi (■■); NdSi (■■).
trated in Figure 2. Various minor endotherms and exo-
therms were observed, but no significant phase transfor-
mation appeared to take place. For all four gels, loss of
mass, m, became significant above approximately 70˚C.
The continuous mass losses are presumed to be con-
nected with the detachment of water and OH groups. The
mass losses became smaller above 350˚C, while the
thermal decomposition was virtually complete by ap-
proximately 800˚C. By 1000˚C, the Cs-, Co-, and
Nd-containing specimens lost 27-31% of their masses.
The Sr-containing specimen lost 48% of its mass. The
evidence from the thermal analyses conducted to 1000˚C
suggests formation of glasses for all of the heated gels.
Photographs of all materials calcined at various tem-
peratures are shown in Figure 3. Each of the well-formed
monoliths produced friable powders when heated to 70˚C.
Heating to 1200˚C produced little sintering and some color
changes. Heating to 1700˚C produced reactions with the
crucibles, but only limited extents of sintering, except, in
the case of Cs-containing glass, for which full melting
occurred. These results are consistent with literature data
[16], in which it is reported that amorphous silica pow-
ders sinter significantly by 1730˚C and the technologi-
cal melting temperature is given as 1800˚C - 2000˚C [4].
Alteration of thermal treatment coupled with a cold-
pressing step could perhaps produce denser monolithic
forms [17]. It is also possible that some sort of hot-pressing
step will be required to form dense monoliths from the
gels [3,4,18].
Infrared spectra and XRD patterns of gels and doped
silicate glasses are shown in Figure 4. The infrared spectra
Copyright © 2011 SciRes. NJGC
Sol-Gel Processing of Silica Nuclear Waste Glasses
Copyright © 2011 SciRes. NJGC
Figure 3. Photographs of materials obtained from sols according procedure described in Figure 1, heated for 2 h at tempera-
tures shown.
Sol-Gel Processing of Silica Nuclear Waste Glasses
Copyright © 2011 SciRes. NJGC
Sol-Gel Processing of Silica Nuclear Waste Glasses
Copyright © 2011 SciRes. NJGC
Figure 4. Infrared spectra and XRD patterns of gels and doped silicate glasses.
revealed that organic impurities did not exist in detect-
able concentrations at 70˚C. H2O and OH groups were
identified, but these groups were not observed at higher
temperatures (except for Co and Nd dopants at 450˚C).
Significant crystallization was observed in the Cs- and
Sr-doped specimens that were heated to 1200˚C.
only the substantially melted, dense form (the Cs-doped,
clear glass drops) exhibited a low leaching rate of the
dopant and the Nd-doped, sintered, but still-porous
specimen, was more resistant to leaching than were the
other sintered compacts. The porous Cs- and Sr-doped
specimens exhibited comparatively high leaching rates.
Analyses of the metal concentrations in the leachates
revealed similar trends, for both Me and Si concentra-
tions. The large differences in mass loss between many
of the individual weight-change tests may have been
caused by changes in the corrosion mechanism. In general,
glass corrosion in aqueous solutions is governed by dif-
fusion-controlled ion exchange and dissolution of the
glass network itself. Saturation of the solution can affect
the processes [25]. Precipitation of surface species can
also affect corrosion rates [1]. Detailed studies of the
corrosion processes and surface alteration [11,26] are
beyond the scope of this synthesis-based work.
It is interesting to note that bidentate 3
O ions were
observed only with the Sr- and Nd-doped silicate gels
dried at 70˚C, whereas with the Cs- and Sr-doped gels,
only free, non-coordinated nitrate ions were observed in-
stead (ν3 NO3 at 1384 cm–1, v. strong and ν1 + ν3 3
at 2360 cm–1, v. weak), which is in accord with the ob-
servations of Bogard et al. [19]. Free nitrate ions were
observed with all samples heated at 450˚C and their tra-
ces appeared even at 800˚C with Sr or Nd dopants present.
It is difficult to explain the occurrence of a strong bond-
ing mode of nitrates in the examined systems, because
nitrates typically decompose at lower temperatures: e.g.,
Sr(NO3)2 at 640˚C; CsNO3 at 400˚C; Co(NO3)2 at
290˚C [16]. A possible explanation may come from
the fact that all of the oxygen atoms of the NO3 groups
were engaged in coordination sites. In all of the spectra,
typical SiO2 vibrations bands (4000 cm–1 and 2510 cm–1)
were observed: Si-O-Si symmetric bending at 455 - 470
cm–1, Si-O-Si symmetric stretching at 783 - 815 cm–1,
and asymmetric stretching at 1074 - 1080 cm–1.
The leaching data are difficult to compare directly
with those of other studies, owing to variations of speci-
men form and test conditions, but data from similar tests
[20,27] indicate generally higher rates for the waste
forms produced in this study. The Cs-doped glass drops
and the Nd-doped (actinides surrogate) lightly sintered
forms were reasonably resistant to leaching, within the
analytical errors. The other forms, all porous, were clear-
ly susceptible to leaching. In forms for which the dopants
were more strongly bonded, the structure of silica matrix
was also more resistant leaching (Table 1).
Leaching tests consisted of weight-change measure-
ments and analysis of leachates. Use of stirred room-
temperature, deionized water will produce more rapid
leaching than will, for example static, ground water tests
[20], but will not allow for extrapolation to actual re-
pository conditions and long duration [21-24]. It should
allow for assessment of the applicability of the sol-gel
glasses produced in this study as nuclear waste forms.
Data from the studies are shown in Figure 5 and Table 1.
The most obvious trends in the data in Figure 5 are that
These studies yielded mixed results. Uniform products
containing high concentrations of dopants were produced,
but the products remained porous after prolonged sinter-
ing at elevated temperatures. Such porous products are
not suitable for use as nuclear waste forms. Future efforts
will seek to fabricate denser forms. It is possible that col-
d-pressing [1-5] or a tailored reaction-sintering step [6]
Sol-Gel Processing of Silica Nuclear Waste Glasses
Figure 5. Leaching experiments of specimens sintered at 1700˚C, except for Nd-doped specimen, which was sintered at 1200˚C:
Cs (includes agglomerates from alumina support (Figure 3), Cs (transparent glass drops (Figure 3), Sr, Co
and Nd.
Table 1. Analysis of leached components in at.%; Me de-
termined by ion chromatography and Si by flame atomic
absorption spectrometry.
Number of
leachings Me oxide at.% Me at.% of SiO2
1 Cs agregates 0.10 0.18
4 “ 0.06 0.22
9 “ 0.02 0.00
1 Cs drops of glass 0.001 0.09
4 “ 0.00003 0.005
9 “ 0.00003 0.02
1 Sr 0.021 0.21
4 “ 0.026 0.20
9 “ 0.018 0.35
1 Co 0.001 0.054
4 “ 0.006 0.03
9 “ 0.0002 0.085
1 Nd 0.00002 0.025
4 “ 0.00004 0.075
9 “ 0.00002 0.186
could result in denser final products. It is perhaps more
likely that some type of hot-pressing step [3,4,18] will be
4. Conclusions
Silicate glass typically containing high-level nuclear waste
elements Cs, Sr, Co, and Nd (as actinides surrogate) were
synthesized directly by a complex sol-gel process, by
including the dopants in starting in the starting sols. Ap-
plication of ascorbic acid as a catalyst, instead of the
more-common HCl or NH4OH that are generally used,
and elevated temperature allowed for processing within
several hours. XRD patterns indicated that all studied
elements were, for all temperatures of heat treatment,
integrally bonded in the silicate glass structures. Infrared
spectra indicated that in general organic impurities did
not exist after drying in vacuum at 70˚C. H2O and OH
groups existed at this temperature, but they were not ob-
served at 450˚C, except when Co and Nd dopants were
present. Leaching test revealed that stabilities of powders
and agglomerates sintered at 1700˚C were not sufficient.
The melted Cs-doped glass was substantially more re-
sistant to leaching. Results indicate need to focus on den-
sification of the final forms, either by adding pressing
steps or significantly altering the sintering protocols.
5. Acknowledgements
This work is an initial part of studies that will be contin-
ued within Polish Governmental Project “Technology
Supporting Development of Safe Nuclear Power,” Part
“Development of Techniques and Technologies Suppor-
ting Management of Spent Nuclear Fuel and Radioactive
Waste.” The authors thankful to INCT team: Dr. L. Fuks
for performing IR spectra, Dr. W. Skwara for flame spec-
trometry analysis and Dr. K. Kulisa for ionic chroma-
tography analysis.
[1] M. I. Ojovan and W. E. Lee, “An Introduction to Nuclear
Waste Immobilization,” Elsevier, London, 2005.
[2] M. I. Ojovan, J. M. Juoi and W. E. Lee, “Application of
Glass Composite Materials for Nuclear Waste Immobili-
sation,” Journal of The Pakistan Materials Societ y, Vol. 2,
No. 2, 2008, pp. 72-76.
[3] T. Woignier, J. Reynes, J. Phalippou and J. L. Dussossoy,
“Nuclear Waste Storage in Gel-Derived Materials,” Jour-
nal of Sol-Gel Science and Technology, Vol. 19, 2000, pp.
Copyright © 2011 SciRes. NJGC
Sol-Gel Processing of Silica Nuclear Waste Glasses111
[4] T. Woignier, J. Reynes, J. Phalippou and J. L. Dussossoy,
“Sintered Silica Aerogel: A Host Matrix for Long Life
Nuclear Wastes,” Journal of Non-Crystalline Solids, Vol.
225, 1998, pp. 353-357.
[5] D. R. Clarke, “Ceramic Materials for the Immobilization
of Nuclear Waste,” Annual Review of Materials Science,
Vol. 13, 1983, pp. 191-218.
[6] W. L. Gong, W. Lutze and R. C. Ewing, “Reaction sin-
tered glass: A Durable Matrix for Spinel-Forming Nu-
clear Waste Compositions,” Journal of Nuclear Materials,
Vol. 278, No. 1, 2000, pp. 73-84.
[7] A. Deptula, W. Lada, T. Olczak, M. T. Lanagan, S. E.
Dorris, K. C. Goretta and R. B. Poeppel, “Method for
Preparing High-Temperature Superconductors,” Polish Pa-
tent 172618, 1997.
[8] A. Deptula, J. Chwastowska, W. Lada, T. Olczak, D.
Wawszczak, E. Sterlinska, B. Sartowska and K. C. Gor-
etta, “Sol-Gel-Derived Hydroxyapatite and Its Applica-
tion to Sorption of Heavy Metals,” Advances in Science
and Technology, Vol. 45, 2006, pp. 2198-2203.
[9] A. Deptula, W. Lada, T. Olczak, D. Wawszczak, M. Bry-
kala, F. Zaza and K. C. Goretta, “Novel Sol-Gel Synthe-
sis of LiMn2O4 and LiNixCo1-xO2 Powders,” Advances in
Science and Technology, Vol. 63, 2010, pp. 14-23.
[10] P. R. Haddad, “Comparison of Ion Chromatography and
Capillary Electrophoresis for the Determination of Inor-
ganic Ions,” Journal of Chromatography A, Vol. 770, No.
1-2, 1997, pp. 281-290.
[11] J. A. Stone, “An Overview of Factors Affecting the Lea-
chability of Nuclear Waste Forms,” Nuclear and Chemi-
cal Waste Management, Vol. 2, No. 2, 1981, pp. 113-118.
[12] A. Barkatt, E. E. Saad, R. Adiga, W. Sousanpour, Al.
Barkatt, M. A. Adel-Hadadi, J. A. O'Keefe and S. Al-
terescu, “Leaching of Natural and Nuclear Waste Glasses
in Sea Water,” Applied Geochemistry, Vol. 4, No. 6, 1989,
pp. 593-603. doi:10.1016/0883-2927(89)90069-3
[13] W. L. Ebert, “Effects of the Glass Surface Area/Solution
Volume Ratio on Glass Corrosion: A Critical Review,”
ANL-94/34, Argonne National Laboratory, 1994.
[14] J. R. Martinez, E. Espericueta and G. Ortega-Zarzosa,
“Effect of Aging of Chlorophyll Species Embeded in Sil-
ica Xerogels Matrix,” New Journal of Glass and Ceram-
ics, Vol. 1, No. 1, 2011, pp. 7-12.
[15] L. Werme, I. K. Björner, G. Bart, H. U. Zwicky, B. Gram-
bow, W. Lutze, R. C. Ewing and C. Magrabi, “Chemical
corrosion of highly radioactive borosilicate nuclear waste
glass under simulated repository conditions,” Journal of
Materials Research, Vol. 5, No. 5, 1990, pp. 1130-1146.
[16] E. M. Rabinovich, “Preparation of Glass by Sintering,”
Journal of Materials Science, Vol. 20, No. 12, 1985, pp.
4259-4297. doi:10.1007/BF00559317
[17] J. Phalippou, M. Prassas and J. Zarzycki, “Crystallization
of Gels and Glasses Made from Hot-Pressed Gels,”
Journal of Non-Crystalline Solids, Vol. 48, No. 1, 1982,
pp. 17-30. doi:10.1016/0022-3093(82)90243-5
[18] M. Nishioka, S. Hirai, K. Yanagisawa and N. Yamasaki,
“Solidification of Glass Powder with Simulated High-
Level Radioactive Waste During Hydrothermal Hot-Pres-
sing”, Journal of the American Ceramic Society, Vol. 73,
No. 2, 1990, pp. 317-322.
[19] J. S. Bogard, S. A. Johnson, R. Kumar and P. T. Cun-
ningham, “Quantitative Analysis of Nitrate Ion in Ambi-
ent Aerosols by Fourier-Transform Infrared Spectroscopy,”
Environmental Science and Technology, Vol. 16, No. 3,
1982, pp. 136-140. doi:10.1021/es00097a004
[20] A. N. Sharaf El-Deen, M. M. El-Dessouky, M. A. Helmy,
M. Wagdy, A. Raouf and M. I. El-Dessouky, “Charac-
terization and Leach Investigation of Sodium Silicate
Matrices Used for Immobilization of Radioactive Waste,”
in M. F. Barakat, Ed., Proceedings of Second Arab Con-
ference on the Peaceful Uses of Atomic Energy, Cairo,
5-9 November 1994, pp. 375-391.
[21] G. Malow, W. Lutze and R. C. Ewing, “Alteration Effects
and Leach Rates of Basaltic Glasses: Implications for the
Long-Term Stability of Nuclear Waste Form Borosilicate
Glasses,” Journal of Non-Crystalline Solids, Vol. 67,
1984, pp. 305-321. doi:10.1016/0022-3093(84)90156-X
[22] L. Werme, I. K. Björner, G. Bart, H. U. Zwicky, B. Gram-
bow, W. Lutze, R. C. Ewing and C. Magrabi, “Chemical
Corrosion of Highly Radioactive Borosilicate Nuclear
Waste Glass Under Simulated Repository Conditions,”
Journal of Materials Research, Vol. 5, No. 5, 1990, pp.
1130-1146. doi:10.1557/JMR.1990.1130
[23] G. Leturcq, G. Berger, T. Advocat and E. Vernaz, “Initial
and Long-Term Dissolution Rates of Aluminosilicate
Glasses Enriched with Ti, Zr and Nd,” Chemical Geology,
Vol. 160, 1999, pp. 39-62.
[24] J. Sterpenich and G. Libourel, “Using Stained Glass Win-
dows to Understand the Durability of Toxic Waste Ma-
trices,” Chemical Geology, Vol. 174, 2001, pp. 181-193.
[25] M. I. Ojovan, R. J. Hand, N. V. Ojovan and W. E. Lee,
“Corrosion of Alkali—Borosilicate Waste Glass K-26 in
Non-Saturated Conditions,” Journal of Nuclear Materials,
Vol. 340, 2005, pp. 12-24.
[26] W. L. Gong, L. M. Wang, R. C. Ewing, E. Vernaz, J. K.
Bates and W. L. Ebert, “Analytical Electron Microscopy
Study of Surface Layers Formed on the French SON68
Nuclear Waste Glass during Vapor Hydration at 200˚C,”
Journal of Nuclear Materials, Vol. 254, 1998, pp.
249-265. doi:10.1016/S0022-3115(97)00349-8
[27] P. R. Aravind, L. Sithara, P. Mukundan, P. Krishna Pillai
and K. G. K. Warrier, “Silica Alcogels for Possible Nu-
clear Waste Confinement—A Simulated Study,” Materi-
als Letters, Vol. 61, No. 11-12, 2007, pp. 2398-2401.
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