A mechanistic model for long-term nuclear waste glass dissolution integrating chemical affinity and interfacial diffusion barrier

Abstract

Abstract:Understanding the alteration of nuclear waste glass in geological repository conditions is critical element of the analysis of repository retention function. Experimental observations of glass alterations provide a general agreement on the following regimes: inter-diffusion, hydrolysis process, rate drop, residual rate and, under very particular conditions, resumption of alteration. Of these, the mechanisms controlling the rate drop and the residual rate remain a subject of dispute. This paper offers a critical review of the two most competitive models related to these regimes: affinity-limited dissolution and diffusion barrier. The limitations of these models are highlighted by comparison of their predictions with available experimental evidence. Based on the comprehensive discussion of the existing models, a new mechanistic model is proposed as a combination of the chemical affinity and diffusion barrier concepts. It is demonstrated how the model can explain experimental phenomena and data, for which the existing models are shown to be not fully adequate.Key words: nuclear waste glasses, long-term dissolution, mechanisms, modelling IntroductionRadioactivity wastes are generated at all stages of the nuclear fuel cycle, including the decommissioning of nuclear facilities, as well as from military applications. Of particular concern for the storage/disposal of radioactivity wastes are those containing long-lived radionuclides [1].The current plan ( countries such as Belgium, Finland , Sweden , France etc.)for long-term management of such wastes is to store them in deep, stable and lowpermeable geological formations [2].The storage design is based on the so-called multi-barrier concept, where several barriers prevent for a period of time, or slow down the release and migration of radionuclides through the geosphere [3,4].Within this concept, hazardous nuclides are immobilized into solidified bodies. The wasteform selection is difficult, since durability is not the sole criterion [5]. Currently, vitrification is regarded as the best solution for immobilizing radionuclides. This technology has been progressively developed over the last half-century, has matured and has become industrially robust.Data collected to date suggest that glass waste forms offer the advantages that they can accommodate a wide range of waste streams, are resistant to radiation damage, and are relatively inert to both chemical and thermal perturbations [6].The use of natural and archeological analogues supported further the durability argument of glass as waste forms [7][8][9][10][11]. Except for the alumino-phosphate glass used in Russia, the borosilicate glass has been universally selected by all other nations [12].In order to make scientifically-underpinned safety cases, the long-term behaviour of glassy wasteforms requires further understanding and assessment. Half-lives of some radionuclides extend to millions of years, requiring isolation for geological periods, while the period of investigation possible in the field and the la...

Towards a unifying mechanistic model for silicate glass corrosion

“…The new isotope tracer experiments revealed several observations that are not consistent with the classical SAL formation concept. 1,[20][21][22][23][24][25] In particular, the formation of several individual layers with largely different porosity inside a single SAL (exp. QBG-90/150) is difficult to explain by a leaching model that considers (i) the SAL as a residual and restructured glass and (ii) its porosity as a reflection of the free space created by the selective removal of certain network formers and modifiers from the glass, 26 and (iii) a transitional zone presented as hydrated glass 2 ahead of the SAL.…”

“…Moreover, although the count rates were low due to low B concentrations, the 10 B/ 11 B values in the SAL are typically on average higher than the natural B value in the glass (=0. 25), showing that some B from solution is incorporated in the SAL. Similar to B, Ca also depicts complex and unexpected profiles throughout the SAL.…”

“…[12][13][14][17][18][19] In the first model, the SAL is assumed to represent the product of repolymerisation reactions within a chemically modified, hydrated glass layer. 1,[20][21][22][23][24][25] This hydrated glass layer forms by solid-state, diffusion-controlled ion exchange of mobile glass network modifiers (e.g., Li, Na, or Ca) with protons from solution, forming silanol groups, while the cations are released into solution. This chemically leached layer eventually undergoes re-polymerization as the results of condensation of the (previously formed) silanol groups, releasing molecular water and producing porosity in the residual glass.…”

“…6,23,28 Whereas some researchers favor the idea that the SAL represents a diffusion barrier for reactive species, 6,23,24,28 others attribute this drop in the reaction kinetics to solution saturation conditions (chemical affinity) and related inhibiting effects, 29 or to both. 13,25 In the second model, the SAL is assumed to form by the precipitation of amorphous silica directly from solution. 8,12,14,18 The formation of such a silica layer is thereby spatially and temporally coupled to the congruent (stoichiometric) dissolution of the glass, i.e., both the glass dissolution and the silica precipitation reactions proceed at an inwardly moving, atomically sharp reaction interface.…”

Dissolution Kinetics of R-Glass Fibres: Influence of Water Acidity, Temperature, and Stress Corrosion

“…They reported that a strong acid attack was dependent on the composition of the glass [13]. Ma et al presented a model for the glass dissolution, based on the combination of the chemical affinity and diffusion [35]. Bashir et al studied the kinetics of the dissolution of E-glass fibres in hot alkaline solutions using zero-order and shrinking cylinder models [23,36].…”