The Minima of Viscosities.

The Trachenko-Brazhkin equation of the minimal possible viscosity is analysed, emphasising its validity by the account of multibody interactions between flowing species through some effective masses replacing their true (bare) masses. Pressure affects the effective masses, decreasing them and shifting the minimal viscosity and the temperature at which it is attained to higher values. The analysis shows that effective masses in the Trachenko-Brazhkin equation are typically lighter compared bare masses; e.g., for tin (Sn) the effective mass is m = 0.21mSn, whereas for supercritical argon (Ar), it changes from m = 0.165mAr to m = 0.129mAr at the pressures of 20 and 100 MPa, respectively.

On Crossover Temperatures of Viscous Flow Related to Structural Rearrangements in Liquids.

An additional crossover of viscous flow in liquids occurs at a temperature Tvm above the known non-Arrhenius to Arrhenius crossover temperature (TA). Tvm is the temperature when the minimum possible viscosity value ηmin is attained, and the flow becomes non-activated with a further increase in temperature. Explicit equations are proposed for the assessments of both Tvm and ηmin, which are shown to provide data that are close to those experimentally measured. Numerical estimations reveal that the new crossover temperature is very high and can barely be achieved in practical uses, although at temperatures close to it, the contribution of the non-activated regime of the flow can be accounted for.

NiTi2, a New Liquid Glass.

Many endothermic liquid-liquid transitions, occurring at a temperature Tn+ above the melting temperature Tm, are related to previous exothermic transitions, occurring at a temperature Tx after glass formation below Tg, with or without attached crystallization and predicted by the nonclassical homogenous nucleation equation. A new thermodynamic phase composed of broken bonds (configurons), driven by percolation thresholds, varying from ~0.145 to Δε, is formed at Tx, with a constant enthalpy up to Tn+. The liquid fraction Δε is a liquid glass up to Tn+. The solid phase contains glass and crystals. Molecular dynamics simulations are used to induce, in NiTi2, a reversible first-order transition by varying the temperature between 300 and 1000 K under a pressure of 1000 GPa. Cooling to 300 K, without applied pressure, shows the liquid glass presence with Δε = 0.22335 as memory effect and Tn+ = 2120 K for Tm = 1257 K.

The Flow of Glasses and Glass-Liquid Transition under Electron Irradiation.

Recent discovery and investigation of the flow of glasses under the electron beams of transmission electron microscopes raised the question of eventual occurrence of such type effects in the vitrified highly radioactive nuclear waste (HLW). In connection to this, we analyse here the flow of glasses and glass-liquid transition in conditions of continuous electron irradiation such as under the e-beam of transmission electron microscopes (TEM) utilising the configuron (broken chemical bond) concept and configuron percolation theory (CPT) methods. It is shown that in such conditions, the fluidity of glasses always increases with a substantial decrease in activation energy of flow at low temperatures and that the main parameter that controls this behaviour is the dose rate of absorbed radiation in the glass. It is revealed that at high dose rates, the temperature of glass-liquid transition sharply drops, and the glass is fully fluidised. Numerical estimations show that the dose rates of TEM e-beams where the silicate glasses were fluidised are many orders of magnitude higher compared to the dose rates characteristic for currently vitrified HLW.

Zirconolite Polytypes and Murataite Polysomes in Matrices for the REE-Actinide Fraction of HLW.

Electron backscatter diffraction (EBSD) has been used for more than 30 years for analyzing the structure of minerals and artificial substances. In recent times, EBSD has been widely applied for investigation of irradiated nuclear fuel and matrices for the immobilization of radioactive waste. The combination of EBSD and scanning electron microscopy (SEM/EDS) methods allows researchers to obtain simultaneously data on a specimen's local composition and structure. The article discusses the abilities of SEM/EDS and EBSD techniques to identify zirconolite polytype modifications and members of the polysomatic murataite-pyrochlore series in polyphase ceramic matrices, with simulations of Pu (Th) and the REE-actinide fraction (Nd) of high-level radioactive waste.

On Structural Rearrangements during the Vitrification of Molten Copper.

We utilise displacement analysis of Cu-atoms between the chemical bond-centred Voronoi polyhedrons to reveal structural changes at the glass transition. We confirm that the disordered congruent bond lattice of Cu loses its rigidity above the glass transition temperature (Tg) in line with Kantor-Webman theorem due to percolation via configurons (broken Cu-Cu chemical bonds). We reveal that the amorphous Cu has the Tg = 794 ± 10 K at the cooling rate q = 1 × 1013 K/s and that the determination of Tg based on analysis of first sharp diffraction minimum (FDSM) is sharper compared with classical Wendt-Abraham empirical criterion.

Prediction of Second Melting Temperatures Already Observed in Pure Elements by Molecular Dynamics Simulations.

A second melting temperature occurs at a temperature Tn+ higher than Tm in glass-forming melts after heating them from their glassy state. The melting entropy is reduced or increased depending on the thermal history and on the presence of antibonds or bonds up to Tn+. Recent MD simulations show full melting at Tn+ = 1.119Tm for Zr, 1.126Tm for Ag, 1.219Tm for Fe and 1.354Tm for Cu. The non-classical homogeneous nucleation model applied to liquid elements is based on the increase of the Lindemann coefficient with the heating rate. The glass transition at Tg and the nucleation temperatures TnG of glacial phases are successfully predicted below and above Tm. The glass transition temperature Tg increases with the heating rate up to Tn+. Melting and crystallization of glacial phases occur with entropy and enthalpy reductions. A universal law relating Tn+ and TnG around Tm shows that TnG cannot be higher than 1.293Tm for Tn+= 1.47Tm. The enthalpies and entropies of glacial phases have singular values, corresponding to the increase of percolation thresholds with Tg and TnG above the Scher and Zallen invariant at various heating and cooling rates. The G-phases are metastable up to Tn+ because the antibonds are broken by homogeneous nucleation of bonds.

Correlation between chemical composition and 90Sr concentrations in groundwater of the Chornobyl NPP industrial site.

The volumetric activity of the divalent 90Sr ion in groundwater at the Chornobyl NPP industrial site ranges from 1 to 2 to 400-3800 Bq/l. The increase in groundwater radionuclides concentrations is associated with the reduced sorption properties of local sediments, which affect the migration capacity of radionuclides in the environment. The decrease of the 90Sr sorption properties of sediments is caused by changes in the chemical composition of groundwater. A new statistical method has been performed. Method based on the Monte Carlo method in order to evaluate the correlations between the 90Sr volumetric activity and the groundwater chemical composition components. Simulation results using this method suggest a correlation between the volumetric activity of 90Sr, the concentrations cations, the pH, and the oxidation index (organic contents). A direct correlation was established between the volumetric activity of 90Sr, Ca2+ concentrations and the pH of groundwater in the range from 7 to 12.4. It was revealed that the concentrations of Na+ and K+ do not affect the conditions of 90Sr migration with groundwater. There is an inverse correlation between the concentration of 90Sr and the oxidation index, which is an indirect indicator of the organic substances content in water. Thus, the presence of organic substances in the groundwater effectively promotes sorption of 90Sr. The proposed method of geochemical statistics enables a quantitative assessment of groundwater monitoring results.

On Structural Rearrangements Near the Glass Transition Temperature in Amorphous Silica.

The formation of clusters was analyzed in a topologically disordered network of bonds of amorphous silica (SiO2) based on the Angell model of broken bonds termed configurons. It was shown that a fractal-dimensional configuron phase was formed in the amorphous silica above the glass transition temperature Tg. The glass transition was described in terms of the concepts of configuron percolation theory (CPT) using the Kantor-Webman theorem, which states that the rigidity threshold of an elastic percolating network is identical to the percolation threshold. The account of configuron phase formation above Tg showed that (i) the glass transition was similar in nature to the second-order phase transformations within the Ehrenfest classification and that (ii) although being reversible, it occurred differently when heating through the glass-liquid transition to that when cooling down in the liquid phase via vitrification. In contrast to typical second-order transformations, such as the formation of ferromagnetic or superconducting phases when the more ordered phase is located below the transition threshold, the configuron phase was located above it.

Building and Breaking Bonds by Homogenous Nucleation in Glass-Forming Melts Leading to Transitions in Three Liquid States.

The thermal history of melts leads to three liquid states above the melting temperatures Tm containing clusters-bound colloids with two opposite values of enthalpy +Δεlg × ΔHm and -Δεlg × ΔHm and zero. All colloid bonds disconnect at Tn+ > Tm and give rise in congruent materials, through a first-order transition at TLL = Tn+, forming a homogeneous liquid, containing tiny superatoms, built by short-range order. In non-congruent materials, (Tn+) and (TLL) are separated, Tn+ being the temperature of a second order and TLL the temperature of a first-order phase transition. (Tn+) and (TLL) are predicted from the knowledge of solidus and liquidus temperatures using non-classical homogenous nucleation. The first-order transition at TLL gives rise by cooling to a new liquid state containing colloids. Each colloid is a superatom, melted by homogeneous disintegration of nuclei instead of surface melting, and with a Gibbs free energy equal to that of a liquid droplet containing the same magic atom number. Internal and external bond number of colloids increases at Tn+ or from Tn+ to Tg. These liquid enthalpies reveal the natural presence of colloid-colloid bonding and antibonding in glass-forming melts. The Mpemba effect and its inverse exist in all melts and is due to the presence of these three liquid states.

On Viscous Flow in Glass-Forming Organic Liquids.

The two-exponential Sheffield equation of viscosity η(T) = A1·T·[1 + A2·exp(Hm/RT)]·[1 + C·exp(Hd/RT)], where A1, A2, Hm, C, and Hm are material-specific constants, is used to analyze the viscous flows of two glass-forming organic materials-salol and α-phenyl-o-cresol. It is demonstrated that the viscosity equation can be simplified to a four-parameter version: η(T) = A·T·exp(Hm/RT)]·[1 + C·exp(Hd/RT)]. The Sheffield model gives a correct description of viscosity, with two exact Arrhenius-type asymptotes below and above the glass transition temperature, whereas near the Tg it gives practically the same results as well-known and widely used viscosity equations. It is revealed that the constants of the Sheffield equation are not universal for all temperature ranges and may need to be updated for very high temperatures, where changes occur in melt properties leading to modifications of A and Hm for both salol and α-phenyl-o-cresol.

Revealing Structural Changes at Glass Transition via Radial Distribution Functions.

Transformation of glasses into liquids is discussed in terms of configuron (broken chemical bond or transformation of an atom from one to another atomic shell) percolation theory with structural changes caused. The first sharp diffraction minimum (FSDM) in the pair distribution function (PDF) is shown to contain information on structural changes in amorphous materials at the glass transition temperature (Tg). A method to determine the glass transition temperature is proposed based on allocating Tg to the temperature when a sharp kink in FSDM occurs. The method proposed is more sensitive compared with empirical criterion of Wendt-Abraham; e.g., for amorphous Ni the kink that determines Tg is almost twice sharper. Connection between the kink in fictive temperature behavior of PDF and Wendt-Abraham criterion is discussed.

Special Issue: Materials for Nuclear Waste Immobilization.

Nuclear energy is clean, reliable, and competitive with many useful applications, among which power generation is the most important as it can gradually replace fossil fuels and avoid massive pollution of environment. A by-product resulting from utilization of nuclear energy in both power generation and other applications, such as in medicine, industry, agriculture, and research, is nuclear waste. Safe and effective management of nuclear waste is crucial to ensure sustainable utilization of nuclear energy. Nuclear waste must be processed to make it safe for storage, transportation, and final disposal, which includes its conditioning, so it is immobilized and packaged before storage and disposal. Immobilization of waste radionuclides in durable wasteform materials provides the most important barrier to contribute to the overall performance of any storage and/or disposal system. Materials for nuclear waste immobilization are thus at the core of multibarrier systems of isolation of radioactive waste from environment aimed to ensure long term safety of storage and disposal. This Special Issue analyzes the materials currently used as well as novel materials for nuclear waste immobilization, including technological approaches utilized in nuclear waste conditioning pursuing to ensure efficiency and long-term safety of storage and disposal systems. It focuses on advanced cementitious materials, geopolymers, glasses, glass composite materials, and ceramics developed and used in nuclear waste immobilization, with the performance of such materials of utmost importance. The book outlines recent advances in nuclear wasteform materials including glasses, ceramics, cements, and spent nuclear fuel. It focuses on durability aspects and contains data on performance of nuclear wasteforms as well as expected behavior in a disposal environment.

Ceramic Mineral Waste-Forms for Nuclear Waste Immobilization.

Crystalline ceramics are intensively investigated as effective materials in various nuclear energy applications, such as inert matrix and accident tolerant fuels and nuclear waste immobilization. This paper presents an analysis of the current status of work in this field of material sciences. We have considered inorganic materials characterized by different structures, including simple oxides with fluorite structure, complex oxides (pyrochlore, murataite, zirconolite, perovskite, hollandite, garnet, crichtonite, freudenbergite, and P-pollucite), simple silicates (zircon/thorite/coffinite, titanite (sphen), britholite), framework silicates (zeolite, pollucite, nepheline /leucite, sodalite, cancrinite, micas structures), phosphates (monazite, xenotime, apatite, kosnarite (NZP), langbeinite, thorium phosphate diphosphate, struvite, meta-ankoleite), and aluminates with a magnetoplumbite structure. These materials can contain in their composition various cations in different combinations and ratios: Li-Cs, Tl, Ag, Be-Ba, Pb, Mn, Co, Ni, Cu, Cd, B, Al, Fe, Ga, Sc, Cr, V, Sb, Nb, Ta, La, Ce, rare-earth elements (REEs), Si, Ti, Zr, Hf, Sn, Bi, Nb, Th, U, Np, Pu, Am and Cm. They can be prepared in the form of powders, including nano-powders, as well as in form of monolith (bulk) ceramics. To produce ceramics, cold pressing and sintering (frittage), hot pressing, hot isostatic pressing and spark plasma sintering (SPS) can be used. The SPS method is now considered as one of most promising in applications with actual radioactive substances, enabling a densification of up to 98-99.9% to be achieved in a few minutes. Characteristics of the structures obtained (e.g., syngony, unit cell parameters, drawings) are described based upon an analysis of 462 publications.

Updating irradiated graphite disposal: Project 'GRAPA' and the international decommissioning network.

Demonstrating competence in planning and executing the disposal of radioactive wastes is a key factor in the public perception of the nuclear power industry and must be demonstrated when making the case for new nuclear build. This work addresses the particular waste stream of irradiated graphite, mostly derived from reactor moderators and amounting to more than 250,000 tonnes world-wide. Use may be made of its unique chemical and physical properties to consider possible processing and disposal options outside the normal simple classifications and repository options for mixed low or intermediate-level wastes. The IAEA has an obvious involvement in radioactive waste disposal and has established a new project 'GRAPA' - Irradiated Graphite Processing Approaches - to encourage an international debate and collaborative work aimed at optimising and facilitating the treatment of irradiated graphite.

Long-term field and laboratory leaching tests of cemented radioactive wastes.

Experiments with real and simulated radioactive cementitious wasteforms were set up to compare the leaching behaviour of cementitious wasteforms containing nuclear power plant operational waste in field and laboratory test conditions. Experiments revealed that the average annual (137)Cs leach rate in deionised water was about thirty-five times greater compared with the measured average value for the 1st year of the field test. Cumulative leached fraction of (137)Cs for 1st year (3.74%) was close to values reported in literature for similar laboratory experiments in deionised water, however more than two orders of magnitude higher than the 1st year leached fraction of (137)Cs in the repository test (0.01%). Therefore, to compare field and laboratory test results, a scaling factor is required in order to account for surface to volume factor difference, multiplied by a temperature factor and a leach rate decrease coefficient related to the ground water composition.

Thermodynamic parameters of bonds in glassy materials from viscosity-temperature relationships.

Doremus's model of viscosity assumes that viscous flow in amorphous materials is mediated by broken bonds (configurons). The resulting equation contains four coefficients, which are directly related to the entropies and enthalpies of formation and motion of the configurons. Thus by fitting this viscosity equation to experimental viscosity data these enthalpy and entropy terms can be obtained. The non-linear nature of the equation obtained means that the fitting process is non-trivial. A genetic algorithm based approach has been developed to fit the equation to experimental viscosity data for a number of glassy materials, including SiO2, GeO2, B2O3, anorthite, diopside, xNa2O-(1-x)SiO2, xPbO-(1-x)SiO2, soda-lime-silica glasses, salol, and α-phenyl-o-cresol. Excellent fits of the equation to the viscosity data were obtained over the entire temperature range. The fitting parameters were used to quantitatively determine the enthalpies and entropies of formation and motion of configurons in the analysed systems and the activation energies for flow at high and low temperatures as well as fragility ratios using the Doremus criterion for fragility. A direct anti-correlation between fragility ratio and configuron percolation threshold, which determines the glass transition temperature in the analysed materials, was found.