Effect of compositional variations on charge compensation of AlO4 and BO4 entities and on crystallization tendency of a rare-earth-rich aluminoborosilicate glass

doi:10.1016/j.materresbull.2009.05.009

Materials Research Bulletin

Volume 44, Issue 9, September 2009, Pages 1895-1898

https://doi.org/10.1016/j.materresbull.2009.05.009 Get rights and content

Abstract

This paper presents the structural and crystallization study of a rare-earth-rich aluminoborosilicate glass that is a simplified version of a new nuclear glass proven to be a potential candidate for the immobilization of highly concentrated radioactive wastes that will be produced in the future. In this work, we studied the impact of changing the nature of alkali (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) or alkaline-earth (Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) cations present in glass composition on glass structure (by ²⁷Al and ¹¹B nuclear magnetic resonance spectroscopy) and on its crystallization tendency during melt cooling at 1 K/min (average cooling rate during industrial process). From these composition changes, it was established that alkali cations were preferentially involved in charge compensation of (AlO₄)⁻ and (BO₄)⁻ entities in the glassy network comparatively to alkaline-earth cations. Whatever the nature of alkali cations, glass compositions containing calcium gave way to the crystallization of an apatite silicate phase bearing calcium and rare-earth (RE) cations (Ca₂RE₈(SiO₄)₆O₂, RE = Nd or La) but melt crystallization tendency during cooling strongly varied with the nature of alkaline-earth cations.

Introduction

The rising demand for energy, the risks of losing oil and gas sources of procurement and the need to reduce green-house gas emissions are among the reasons of current and future nuclear energy development. However, the reprocessing of spent nuclear fuel generates highly radioactive liquid wastes (HLW) that must be isolated from the biosphere in very durable solid matrices. Today, HLW are immobilized by dissolution in glasses (mainly borosilicate) that takes advantage of both the tolerance of their disordered structure to waste chemical complexity (nuclear waste solutions may contain more than 40 elements) and of their relatively easy process by melting in comparison with ceramic waste forms for instance [1], [2], [3], [4], [5].

New confinement glasses, aimed at immobilizing more concentrated nuclear waste solutions than today, stemming for instance from the reprocessing of high discharge burn up spent nuclear fuel, are currently investigated in France [6], [7], [8], [9], [10], [11]. A complex rare-earth-rich aluminoborosilicate glass containing 16 wt% rare-earth oxides has already proved to be a good candidate for the immobilization of these wastes (good chemical durability, good waste capacity and low crystallization tendency) [6], [9]. In this glass composition, high amounts of alkali and alkaline-earth cations are present coming either from the waste or from the glass frit that will be mixed with the waste in the glass melter. For instance Rb₂O, Cs₂O, SrO and BaO oxides will represent about 17 wt% of all the fission products in waste solutions. A high quantity of sodium is also present in solutions and stems from spent fuel reprocessing. Both sodium and lithium are also added in the glass frit composition to facilitate glass fabrication. To increase the chemical durability of nuclear glasses, calcium is also added to their composition [12], [13].

In previous works [7], [10], [14], we mainly studied the structure and the crystallization tendency of a simplified seven-oxide rare-earth-rich aluminoborosilicate glass belonging to the SiO₂–B₂O₃–Al₂O₃–CaO–Na₂O–ZrO₂–RE₂O₃ system with RE = La or Nd, derived from the complex nuclear glass composition by using the most abundant Ca²⁺ and Na⁺ cations to simulate respectively all the alkaline-earth and alkali cations occurring in wastes. In this paper, we present the impact of totally changing the nature of alkali or alkaline-earth cations in the composition both on glass structure (impact on the charge compensation of (AlO₄)⁻ and (BO₄)⁻ entities) and on melt crystallization tendency during cooling at a rate close to the average cooling rate in the bulk of glass canisters during industrial process. Even if K⁺ and Mg²⁺ cations are not present in the complex nuclear glass composition envisaged in this work, their effect on the simplified glass structure was also investigated for sake of completeness of the present study.

Section snippets

Experimental

The simplified seven-oxides glass composition studied in this work is the following (in mol%): 61.79 SiO₂–3.05 Al₂O₃–8.94 B₂O₃–14.41 R₂O–6.32 R′O–1.89 ZrO₂–3.60 RE₂O₃ (glass A) with RE = La or Nd. Two glass series were prepared from this composition for both RE = La and Nd. For the first series (R series), the nature of R⁺ cation was varied from Li⁺ to Cs⁺ (keeping R′ = Ca). For the second series (R′ series), the nature of R′²⁺ cation was varied from Mg²⁺ to Ba²⁺ (keeping R = Na). All glasses were

Glass structure

Concerning the silicate network, Raman results on the R and R′ series [11] showed that increasing R⁺ or R′²⁺ cations field strength induced a displacement of the 2Q₃ ↔ Q₂ + Q₄ equilibrium to the right in the melt in agreement with the increasing glass-in-glass phase separation tendency with cations field strength in binary silicate glasses [18] (Q_n represents SiO₄ units with 4-n non-bridging oxygen atoms). Using the field strength parameter defined by Dietzel [19] as Fs = Z/d² where Z is the cation

Conclusion

The structural and crystallization results obtained on the new complex aluminoborosilicate glass composition envisaged in this paper to immobilize more concentrated radioactive waste solutions demonstrate that:

•
The distribution of the main alkali and alkaline-earth cations (i.e. Na⁺ and Ca²⁺) within the glassy network (as charge compensators near (AlO₄)⁻ or (BO₄)⁻ units or near non-bridging oxygen atoms) will be not significantly modified by the presence of Rb⁺, Cs⁺, Sr²⁺ and Ba²⁺ cations

Acknowledgements

The CEA (Commissariat à l’Energie Atomique, France) is gratefully acknowledged for its financial support to this study.

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Effect of compositional variations on charge compensation of AlO4 and BO4 entities and on crystallization tendency of a rare-earth-rich aluminoborosilicate glass

Abstract

Introduction

Section snippets

Experimental

Glass structure

Conclusion

Acknowledgements

Chem. Phys. Lett.

J. Non-Cryst. Solids

J. Non-Cryst. Solids

Solid State Nucl. Mag. Reson.

J. Mater. Sci.

Adv. Appl. Ceram.

MRS Bull.

Nucl. Sci. Eng.

Phys. Chem. Glasses

Mat. Res. Soc. Symp. Proc.

Effect of compositional variations on charge compensation of AlO₄ and BO₄ entities and on crystallization tendency of a rare-earth-rich aluminoborosilicate glass