Iodine-Conditioning this Long half-life Element in Alumino-borosilicate Nuclear glasses and glass-ceramics Under Pressure
The Iodine-CLEAN-UP project aims to propose a remediation to the environmental problem represented by the radioisotope I-129. This by-product of nuclear activity requires a specific matrix to be immobilized. Through an experimental approach using high-pressure, we will produce glass and glass-ceramic matrices that will accommodate a large quantity of iodine, and will be durable and stable over time for future storage in natural sites.
Propose a specific nuclear matrix formulation for the immobilization of radioactive iodine with high pressure conditioning
Iodine-129 is a by-product of nuclear activity and is considered as a long-lived intermediate-level waste. Although in small quantities, it represents an immediate and long-term environmental hazard due to its long half-life (15.7 Ma) and its high mobility in the environment. Currently, there is no completely satisfactory solution to immobilize this radioisotope.
In the nuclear industry, glass is a proven solution for the immobilization of nuclear wastes; however, due to the high volatility of iodine, this process cannot be applied to it. In the Iodine-CLEAN-UP project, we propose to use an original protocol under high-pressure conditions to solubilize iodine in large quantities in matrices allowing its immobilization in a stable and durable manner over time.
The objectives are to propose a specific glass or glass-ceramic matrix for which 1) the solubility of iodine is important, 2) the dissolution mechanisms and the form of dissolved iodine leaves the matrix stable and durable, and 3) the chemical durability is appropriate for deep geological storage over a long time.
The stakes are thus multiple: environmental to limit the dissemination of radiotoxic iodine in the environment, industrial to democratize the use of high-pressure techniques for the immobilization of nuclear waste, and, finally, fundamental on the behaviour of iodine in glass or glass-ceramic matrices, a subject that is currently little discussed in Materials Science.
High-pressure synthesis and characterization of iodine-doped glass and glass-ceramic matrices
We propose to use a high-pressure (0.1-2.0 GPa) iodine-doped glass matrix synthesis protocol. The range of compositions studied will be wide to consider the formulation of a specific matrix to immobilize iodine. We will test the impact of the nature of the network modifying cations on the solubility of iodine in glasses. We will study the effect of the degree of oxidation of iodine (from -1 to +7) on its solubility in glass matrices. In a more exploratory part, we will test the possibility of solubilizing iodine in pressurized glass-ceramics, which represent materials of interest for the storage of radioactive waste due to their high chemical durability.
The quantification of the elements will be done by inductively coupled plasma mass spectrometry (ICP MS) after dissolution (global analysis) and scanning microscopy equipped with an energy dispersive spectrometer (local analysis). This will allow determining the solubility of iodine in glasses and the homogeneity of its distribution. The determination of the redox state of iodine (i.e. speciation) will be performed by photoelectron spectroscopy, Raman spectroscopy and electron energy loss spectroscopy. The impact of iodine dissolution on the structure of glass matrices will be studied by Nuclear Magnetic Resonance and its impact on the physical properties of glass matrices by Differential Scanning Calorimetry.
The chemical durability of iodine-doped matrices will be studied through a series of experiments under different weathering regimes in aqueous conditions in climatic chambers and autoclaves (variable pH and temperature). The potential release of elements (i.e. iodine) in solutions will be monitored with different methods (e.g. ICP MS). The analysis of the thin weathering layers on the surface of the glass matrices will be done by ion probe and high resolution ICP MS. Mechanical tests will also be done to determine the evolution of the density and the degree of porosity.
The crucial aspect of the project is to propose a glassy matrix formulation 1) that dissolves a large amount of iodine in its structure; 2) for which the structure and physical properties are not affected negatively by the dissolution of the iodine; and 3) that retains a chemical durability that allows for deep geological storage.
A first level of results will be obtained with the iodine solubilities determined for our vitreous matrices. We will be able to establish a model of iodine solubility in glasses as a function of intensive conditions (i.e. pressure, temperature and redox) and matrix composition. The qualitative and quantitative analysis of the effect of network modifying cations nature of the glass matrix on the solubility of iodine will be a major aspect of our project.
We will focus on determining the impact of iodine on the degree of polymerization of the glass matrix: change in the distribution of the network forming species. Conjointly, the impact of iodine on the glass transition temperature will be determined. These two aspects account for the chemical stability of the matrix, which is an essential data for considering the storage of radioisotopes.
The results of the weathering experiments will aim to determine the iodine release rates in a medium of interest as a function of time and for iodine-doped glass matrices. Depending on the results, these will be implemented in a standard model of the evolution of the alteration of glass matrices (GRAAL) in order to evaluate the viability of storing iodine-doped glass matrices over geological time.
The Iodine-CLEAN-UP project is an original multidisciplinary project with a long-term environmental and industrial vocation. As a worldwide actor in nuclear energy, France produces nuclear waste that must be secured in storage matrices that are stable over time. Iodine-129 represents a special case of nuclear waste because of its high mobility in the environment and its long half-life. At its completion, the Iodine-CLEAN-UP project will be in a position to propose a specific vitreous matrix formulation (or glass-ceramic) allowing for the immobilization of 129I-based waste. The established protocol will be used to guide future developments on an industrial scale. As these wastes are produced in many industrialized countries, the prospects of the Iodine-CLEAN-UP project are international in scope.
Iodine in its 129I form is not the only radioelement with a high mobility and volatile nature. Thus, our protocol, which uses high-pressure synthesis, could be applied to other elements such as chlorine (36Cl), selenium (79Se) and technetium (99Tc), and a specific matrix formulation for the immobilization of these radioisotopes, by-products of the nuclear industry, could be envisaged.
The project coordinator is Yann Morizet, lecturer at Nantes University.