This review provides a comprehensive evaluation of the state-of-knowledge of radiation effects in crystalline ceramics that may be used for the immobilization of high-level nuclear waste and plutonium. The current understanding of radiation damage processes, defect generation, microstructure development, theoretical methods, and experimental methods are reviewed. Fundamental scientific and technological issues that offer opportunities for research are identified. The most important issue is the need for an understanding of the radiation-induced structural changes at the atomic, microscopic, and macroscopic levels, and the effect of these changes on the release rates of radionuclides during corrosion.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/09D70E1FB1A625EC09BB98883C3A5ED3/S0884291400044411a.pdf/div-class-title-radiation-effects-in-crystalline-ceramics-for-the-immobilization-of-high-level-nuclear-waste-and-plutonium-div.pdf

Radiation effects in crystalline ceramics for the immobilization of high-level nuclear waste and plutonium

Reverse Monte Carlo (RMC) modelling is a general method of structural modelling based on experimental data. RMC modelling can be applied to many different sorts of data, simultaneously if wished. Powder and single-crystal neutron diffraction (including isotopic substitution), x-ray diffraction (including anomalous scattering) and electron diffraction, extended x-ray absorption fine structure and nuclear magnetic resonance (magic angle spinning and second moment) have already been used to provide data. RMC modelling can also be applied to many different types of system - liquids, glasses, polymers, crystals and magnetic materials. This article outlines the RMC method and discusses some of the common misconceptions about it. It is stressed that RMC models are neither unique nor `correct'. However, they are often useful for aiding our understanding either of the structure itself, or of the relationships between local structure and other physical properties. Examples are given and the possibilities for further development of the RMC method are discussed.

http://slapper.apam.columbia.edu/bib/papers/mcgre_jpcm01.pdf

Reverse Monte Carlo modelling

The family of oxide glasses is very wide and it is continuously developing. The rapid development of advanced and innovative glasses is under progress. Oxide glasses have a variety of applications in articles for daily use as well as in advanced technological fields such as X-ray protection, fibre glasses, optical instruments and lab glassware. Oxide glasses basically consist of network formers, such as borate, silicate, phosphate, borosilicate, borophosphate, and network modifiers such as alkali, alkaline earth and transition metals. In the present review article, Raman spectroscopy results for the structures of borate, silicate, phosphate, borosilicate, borophosphate, aluminosilicate, phosphosilicate, alumino-borosilicate and tellurite glasses are summarized.

A review of the structures of oxide glasses by Raman spectroscopy

Very different materials are named “Glass,” with Young's modulus (E) and Poisson's ratio (ν) extending from 5 to 180 GPa and from 0.1 to 0.4, respectively, in the case of bulk inorganic glasses. Although glasses have in common the lack of long-range order in the atomic organization, they offer a wide range of structural features at the nanoscale and we show in this analysis that beside the essential role of elastic properties for materials selection in mechanical design, the elastic characteristics (E, ν) at the continuum scale allow to get insight into the short- and medium-range orders existing in glasses. In particular, ν, the atomic packing density (Cg) and the glass network dimensionality appear to be strongly correlated. Maximum values for ν and Cg are observed for metallic glasses (ν∼0.4 and Cg>0.7), which are based on cluster-like structural units. Atomic networks consisting primarily of chains and layers units (chalcogenides, low Si-content silicate, and phosphate glasses) correspond to ν>0.25 and Cg>0.56. On the contrary, ν<0.25 is associated with a highly cross-linked network, such as in a-SiO2, with a tri-dimensional organization resulting in a low packing density. Moreover, the temperature dependence of the elastic moduli brings a new light on the structural changes occurring above the glass transition temperature and on the depolymerization rate in the supercooled liquid. The softening rate depends on the level of cooperativity of atomic movements at the source of the deformation process, with an obvious correlation with the “fragility” of the liquid.

/pdf/elastic-properties-and-short-to-medium-range-order-in-3swvedsyhi.pdf

Elastic Properties and Short-to Medium-Range Order in Glasses

Al environment in tectosilicate and peraluminous glasses: A 27Al MQ-MAS NMR, Raman, and XANES investigation

An empirical interaction potential has been developed primarily to simulate displacement cascades in a UO2 matrix. After a bibliographical survey revealed that the potentials discussed in the literature are not entirely suitable for describing point defects and their migration, we decided to adapt the most representative of the existing potentials. Our objective was specifically to reproduce transport phenomena, which are fundamental to a satisfactory description of atomic displacement cascades. The modified potential more correctly describes the experimental energies of formation and migration of point defects (interstitials, vacancies, etc.) as well as the oxygen diffusion coefficients in the superionic phase. Thermal expansion, heat capacity and enthalpy are also better modelled over a wide temperature range. The new potential is much more transferable than earlier potentials fitted mainly to the properties of perfect crystals. The initial simulated atomic displacement cascades showed that this potenti...

A new empirical potential for simulating the formation of defects and their mobility in uranium dioxide

Molecular dynamics and reverse Monte Carlo simulations are used to investigate the structure of calcium aluminosilicate glasses with low silica content. The polymerized network is close to that expected from a random distribution for all compositions with important amount of Al–O–Al linkages. Minor species such as triclusters and free oxygen atoms are present. The main structural variations are the changes in the ring size distribution or the Qn distribution. The ring statistics indicate the presence of 4-membered rings, with larger rings at high silica content. We observe that Al enters preferentially the tetrahedral polymerized units with increasing SiO2. A noteworthy structural property of the structure of these glasses is the presence of a significant amount of non-bridging oxygens, even in the fully charge-compensated glasses. The Ca environment correspond to a distorted polyhedra with seven oxygen neighbors. The variation of the glass transition temperature of these glasses are explained based on the preference of Al for more polymerized sites and the structural role of Ca.

Chemical dependence of network topology of calcium aluminosilicate glasses: a computer simulation study.

Structural role of molybdenum in nuclear glasses: an EXAFS study.

In inactive glasses used as models for French nuclear waste forms, Zr K- and L2,3-edge X-ray absorption spectroscopy spectra indicate that Zr is only 6-coordinated, with d(Zr-O) distances of 2.08 ± 0.01 A and with a small radial disorder. Beyond, the first coordination shell, next-nearest Si neighbors occur at about 3.4 A. These distances exclude the presence of [7]Zr and/or [8]Zr and suggest [6]Zr-O-Si angles close to 130°, indicating corner-shared ZrO6 and SiO4 polyhedra. This structural configuration is similar to those observed in some crystalline zirconosilicates and is in agreement with the stability of Zr in borosilicate melts and glasses.

Evidence for 6‐Coordinated Zirconium in Inactive Nuclear Waste Glasses

Wide-angle x-ray scattering (WAXS) has been used to investigate the structure of complex sodium (alumino) borosilicate glasses containing four, five, and six oxides. WAXS data were compared with previous molecular dynamics calculations on these glasses. Small differences were observed which are mainly due to a smaller Si-O distance in the models $(1.58 \AA{})$ compared to the experimental distance $(1.60 \AA{}).$ These discrepancies were removed by slightly moving the atomic positions using the reverse Monte Carlo code. Changes in the first cation-oxygen distances were sufficient to give an excellent fit of the WAXS data, with little modification of the medium range structure. This allows a determination of the structural features observed in the correlation functions and a validation of the numerical simulations for complex multicomponent glasses. The dependence of the glass structure on its chemical composition is discussed.

Dominique Ghaleb

Papers

A new empirical potential for simulating the formation of defects and their mobility in uranium dioxide

Chemical dependence of network topology of calcium aluminosilicate glasses: a computer simulation study.

Structural role of molybdenum in nuclear glasses: an EXAFS study.

Evidence for 6‐Coordinated Zirconium in Inactive Nuclear Waste Glasses

Competition for charge compensation in borosilicate glasses: Wide-angle x-ray scattering and molecular dynamics calculations