ReviewResumption of nuclear glass alteration: State of the art
Introduction
Minor actinides and fission products arising from spent fuel reprocessing in France, Japan, Great Britain, Germany, etc. are confined in borosilicate glasses, which are considered particularly durable [1], [2], [3]. Deep geological disposal—currently the most consensual solution for these wasteforms—requires a study of their long-term behavior to assess their environmental impact. Work is now in progress to understand the physical and chemical mechanisms that determine glass behavior under repository conditions [4], [5]. Studies of long-term behavior [6], [7] combine experimentation (irradiation or doped materials, leaching in aqueous solution or in contact with environmental materials, characterization of the solid material, etc.) and modeling. They also call on studies of natural or archaeological analogs to provide evidence for validating the models [8].
Water is considered to be the main cause of corrosion of the glass packages responsible for the release and migration of radionuclides. Glass alteration involves several steps controlled by different limiting mechanisms [9], [10], [11], [12], [13]. The initial instants are dominated by a brief phase of interdiffusion between hydronium ions and cations bound to the glass network [14], [15], [16], [17]. Alteration is quickly controlled by dissolution of the glass network at the initial rate (Stage I in the American literature [18]) corresponding to hydrolysis of Si–O–X bonds (X = Si, Al, Zr, B, etc.) [15], [19]. When the silicon concentration in solution reaches a quasi-steady state, the glass dissolution rate diminishes to a “residual” rate (Stage II), generally several orders of magnitude lower than the initial rate. While dissolution at the initial rate is congruent for the major glass constituents (Si, B, Na, Al, etc.), the residual rate is characterized by incongruent dissolution due to the formation of a gel by silicon recondensation and the retention of some glass constituent elements [20], [21] and by slow precipitation of secondary phases (mainly clay minerals and rare earth phosphates). Incongruent dissolution is usually calculated with respect to boron, chosen as a tracer element of glass dissolution because it is not retained in secondary phases or gel once released from the glass. The reasons for the rate drop between initial and residual rates, and the way to model it, are subject to debate. For some authors, reorganizations in the gel lead to a closure of porosity giving to this layer passivating properties [9], [20], [22], [23], [24], [25]. These conclusions are in particular based on Monte Carlo simulations and specific surface area comparisons by gas adsorption and small angles X-ray scattering. An alternative explanation, based on the transition state theory, led some authors to propose a limitation of glass alteration kinetics by a reduction of the chemical affinity between hydrated glass and solution [26], [27], [28]. The fundamental mechanisms and the impact of the different modeling options of these mechanisms on the long-term behavior of glasses are still under study. In this review, the authors are more likely to support a layer-based conceptual model to interpret some of the reported data.
Even after the residual rate regime is established, a resumption of alteration—i.e. a sudden acceleration of the alteration rate—can occur (Stage III [29]). Associated with the precipitation of aluminosilicate minerals of the zeolite family at the gel/solution interface, resumptions of alteration are usually observed for specific glass compositions (e.g. high alkali metal concentrations) or specific experimental conditions (e.g. T > 90 °C and/or pH > 10.5, at high glass-surface-area-to-solution-volume (S/V) ratios). Experimentally the mechanisms and kinetics of glass dissolution are investigated through solution analysis and solid characterization to identify the nature and characterize the properties of the alteration products.
The resumption of alteration has been widely investigated. This article summarizes the results of studies demonstrating a resumption of alteration and/or attempting to understand its mechanisms. Cross-checking of the data allows an analysis of some key parameters affecting the occurrence of this phenomenon, potentially with synergistic effects: composition of the glass and the leaching solution, temperature and pH. The effect of the S/V ratio test parameter, affecting both solution composition and pH, is also considered because of its importance to design experiments and in the general approach to study long-term behavior of glasses. The difficulties associated with the mechanistic study and the predictability of this complex phenomenon are discussed. The objective is to review existing evidence to quantify the degree of glass alteration related to the zeolite precipitation mechanism and to highlight missing data. The scope is limited to nuclear glasses, although parallels with natural glasses or aluminosilicate minerals are sought, as in the approach proposed by long-term behavior science.
Section snippets
Demonstration of the resumption of alteration during vapor hydration tests
The acceleration of the glass dissolution rate after the residual rate phase was first demonstrated by vapor hydration tests (VHT). Originally developed to study glass behavior in an unsaturated repository at the Yucca Mountain site (volcanic tuff formation, Nevada, USA), these tests are equivalent to leaching at very high S/V ratios (107 to 109 m−1 [30], [31]) that generate high concentrations in solution.
Vapor hydration tests conducted at high temperatures considerably accelerate the
Resumption of alteration during leaching
The first instances of resumption of alteration during leaching experiments were observed in the mid-1980s with C31-3 glass leached in a saturated NaCl solution at 200 °C at a pressure of 15 bars [43], [44]. Analcime precipitation was observed after 3 days at S/V ratios ranging from 1.3 to 1000 m−1. It continued until consumption of all the aluminum in solution and played a major role in the consumption of dissolved silicon. The analcime crystals, with an Si/Al ratio ranging from 2 to 2.5 (compared
Thermodynamics and kinetics of the resumption of alteration
The resumption of glass alteration is accompanied by the nucleation and growth of phases that are more thermodynamically stable in the conditions of the alteration medium. At any time, the solution composition is controlled by the solubility of coexisting phases whose assemblage evolves as the overall system ages [30], [134].
Consider, for example, a solution containing a glass metastable with respect to an assemblage of secondary phases. Initially (at time t0 in Fig. 11), the solution in
First steps
A resumption of alteration was modeled for the first time in 1987 by Van Iseghem and Grambow [55] using the PHREEQE and GLASSOL codes for the Belgian SAN60 glass leached at 90 °C. The model took account of (i) a first-order dissolution law for the glass matrix in which only silicon was taken into account in the affinity term, (ii) hydrolysis and complexation of species in solution, (iii) the formation of secondary phases, and (iv) saturation of the solution with respect to silica [137], [138].
Summary and conclusions
The resumption of alteration was first observed in the early 1980s, and results in a sudden acceleration of the glass alteration rate following the establishment of the diminishing rate regime. Several hypotheses have been advanced to account for this phenomenon: phase separation during synthesis of the glass, cracking or exfoliation and secondary phase precipitation on the glass surface during aqueous alteration. The work performed to date ruled out the first two hypotheses and correlated the
Acknowledgements
This paper results from a partnership between CEA and AREVA. The authors wish to thank Dr. S. Mercado-Depierre for her advice, M. Pereau and CEA librarians for their help. The authors would also like to thank the anonymous reviewers for their careful review and constructive comments.
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