Charlie R. Scales

Density-structure relations in mixed-alkali borosilicate glasses by 29Si and 11B MAS-NMR

Abstract Densities and boron and silicate speciation in the mixed-alkali borosilicate glass system R (1/2Na 2 O+1/2Li 2 O)·B 2 O 3 · K SiO 2 have been measured over a wide range of R and K values. Previous models for the density of single-alkali borates and borosilicates and mixed-alkali borates have been used in an attempt to identify the roles of the two alkalies in this mixed-alkali borosilicate. Results are discussed in terms of the type and abundance of structural units present and the manner in which the alkali species apportion themselves between the borate and silicate units. Structural data obtained from 11 B and 29 Si magic angle spinning nuclear magnetic resonance (MAS–NMR) illustrate the similarities and differences between the mixed- and single-alkali systems. Below a critical value of R ( R max ), the current system exhibits a variation in N 4 , the fraction of tetrahedrally co-ordinated borons, similar to the Na-borosilicate system. Beyond this point, model N 4 predictions initially overestimate and then underestimate the experimental results seen in the current work. A semi-empirical formula, developed previously, has been employed to investigate the trends in density variation. Comparison, with previous reports, of the associated volumes of the silica Q 4 units shows a larger initial volume, V 6 , but a smaller value of d V 6 , the incremental change in volume of the [SiO 4 ] unit on introducing non-bridging oxygens (NBO). Some preliminary evidence is also presented for the existence of borosilicate units, below R max , not accounted for in most borosilicate density models.

Immobilisation of Simulated Plutonium-Contaminated Material in Phosphate Glass: An Initial Scoping Study

Vitrification is a potential route for the immobilisation of Plutonium Contaminated Material (PCM). This is an Intermediate Level Waste (ILW) arising from operations in which there is contact with Pu isotopes. PCM consists of low levels of Pu combined with metals, masonry, glass, ceramics, polymers and other carbonaceous materials. Simulated PCM containing CeO2 as a PuO2 surrogate was mixed with a phosphate precursor and vitrified. Pre-oxidation of PCM simulant prior to vitrification minimised the violence of batch reactions. No pre-oxidation produced inhomogeneous slag-like materials with high residual metals and particulates. Pre-oxidation at 600°C in air and at 1200°C in an O2-rich atmosphere produced more favourable results, with increasingly vitreous products resulting from more oxidised PCM simulant. The most oxidised PCM simulant produced phosphate glasses with low levels of particulate inclusions, as confirmed by x-ray diffraction and scanning electron microscopy. Particulates included iron-rich metallics and aluminous oxides. Increased melting times and temperatures may have reduced the number of inclusions slightly, but O2 bubbling during melting resulted in little additional benefit. Waste loading equivalent to ∼60 weight % of untreated waste may be possible. There was little evidence of Ce partitioning, indicating that it was immobilised within the glass matrix and had little preference for metallic or crystalline phases. These results demonstrate the potential feasibility for vitrification of PCM in phosphate glass, justifying further investigation into this potentially novel solution.

Survey of potential glass compositions for the immobilisation of the UK's separated plutonium stocks

The Nuclear Decommissioning Authority (NDA) has taken over ownership of the majority of the UKs separated civil plutonium stocks, which are expected to exceed 100 metric tons by 2010. Studies to technically underpin options development for the disposition of these stocks, for example by immobilization or re-use as fuel, are being carried out by Nexia Solutions on behalf of NDA. Three classes of immobilization matrices have been selected for investigation by means of previous studies and stakeholder dialogue: ceramic or crystalline wasteforms, storage MOx, and vitreous or glass-based wasteforms. This paper describes the preliminary inactive experimental program for the vitrification option, with results from a wide range of glass compositions along with conclusions on their potential use for plutonium immobilization. Following review, four glass systems were selected for preliminary investigation: borosilicate, lanthanide borosilicate, aluminosilicate and phosphate glasses. A broad survey of glass properties was completed in order to allow meaningful evaluation, e.g. glass formulation, waste loading, chemical durability, thermal properties, and viscosity. The program was divided into two parts, with silicate and phosphate glasses being investigated by Nexia Solutions and the Immobilisation Science Laboratory (ISL) at the University of Sheffield respectively.

Acoustic Emission Monitoring of Al Corrosion in Cemented-Based Wasteforms

An acoustic emission (AE) technique has been employed for nondestructive monitoring of the performance of a laboratory scale cementitious wasteform encapsulating Al. Recorded AE activity revealed the possibility for classification and differentiation of detected AE signals with a potential relationship between the corrosion rate of encapsulated Al and the accumulated structural damage within the cement matrix.

Vapour Phase Hydration of Blended Oxide - Magnox Waste Glasses

Vapour phase hydration studies of a blended Oxide / Magnox simulant high level waste glass were undertaken at 200°C, over a period of 5 – 25 days. The alteration of this simulant waste glass is characterised by a short incubation time of less than 5 days, leading to the formation of an alteration layer several microns thick. Following the incubation period, the alteration proceeds at a constant rate of 0.15(1)μmd −1 . The distribution of key glass matrix (Si, Na) and waste (Cs, Zr, Nd, Mo) elements was found to vary significantly across the alteration layer. Vapour phase hydration leads to formation of surface alteration products, identified as smectite, zirconium silicate and alkaline-earth molybdate phases.

Chemical Durability Studies of Waste-Simulant Doped Borosilicate Glasses

A mixed-alkali modified borosilicate base glass used in the vitrification of high-level nuclear waste (HLW) has been doped with a number of waste simulants to between 2 and 12 mol%. The simulants have been chosen to give two distinct series of glasses: one consisting of trivalent ions having the form M 2 O 3 (where M is La, Bi, Al or Fe) and the other consisting of divalent simulants of the form MO (where M is Pb, Zn or Ba). An international standard Soxhlet leach test procedure was performed on each glass to study the effect of prolonged, moderate-temperature, dynamic water corrosion. Results of these studies show that, except for BaO, as the amount of simulant is increased, the amount of Na and Li leached decreases showing them to become more chemically resistant. These corrosion tests have been correlated to ionic (D.C.) conductivity measurements, which show a decrease in the conductivity of the glass as the amount of waste simulant is increased, and 11 B magic-angle spinning nuclear magnetic resonance (MAS-NMR) studies, which have shown that, as more waste-simulant is loaded into the glasses the rate of conversion of [BO 4 ] to [BO 3 ] units increases. All of the data from these studies reflect the different network forming abilities of the divalent and trivalent cations.

The Environment of Fe3+/Fe2+ Cations in a Sodium Borosilicate Glass

The neutron diffraction isotopic substitution technique is employed to investigate the environment of Fe3+/Fe2+ cations in a sodium borosilicate glass matrix of composition 0.210Na2O0.18511B2O30.605SiO2. The neutron diffraction data were obtained using the D4c diffractometer at the Institut Laue-Langevin (ILL; Grenoble, France), and were recorded for three samples; the base glass, the base glass incorporating natural Fe2O3 (12 mol.%) and a similar glass containing Fe2O3 enriched in 57Fe. The data are Fourier transformed to yield the real-space total correlation function, T(r), and the first co-ordination shells of the Fe3+/Fe2+ cations are investigated via a peak fit to the isotopic difference correlation function TFe(r). It is concluded that the iron is mainly present as Fe3+ cations, both tetrahedrally and octahedrally co-ordinated by oxygen atoms, plus a small fraction (0.07 ± 0.01) of Fe2+ cations in octahedral co-ordination. The Fe3+ tetrahedral fraction is 0.45 ± 0.10, and appears to exist as FeO4 structural units incorporated into the network of silicate chemical groupings, with their negative charge being balanced by Na+ network-modifying cations. The remaining Fe3+ cations (fraction 0.48 ± 0.10) are thought to be predominantly octahedrally co-ordinated and associatedwith BO33 orthoborate anions in FeBO3 chemical groupings, which become non-stoichiometric due to the reduction of some of the Fe3+ cations to Fe2+.

Fission Product Immobilisation in Secondary Phases Formed During Magnox Waste Glass Dissolution at 60 °C: Experimental Results and Modelling.

Dissolution of a complex, simulated Magnox Waste (MW) glass in batch dissolution experiments at 60 °C over a period of 56 days is accompanied by extensive development of secondary gels. Gel development has been followed using a range of chemical, spectroscopic and physical means. Initially, a surface layer comprising (hydr)oxides of Fe, Zr and the lanthanides develops at the glass surface. Aluminosilicate gels containing Si, Al, Mg, Sr, Cs and Rb develop in systems where sufficient quantities of glass derived solutes accumulate in the leachate. These gels are hydrous and readily soluble in acidic oxalate solutions. Solution chemistry data is consistent with the development of Cs,Sr-bearing aluminosilicates, silica gel and (hydr)oxides of hydrolysis prone waste components. The experimental results are compared with the predictions of a model that considers kinetically constrained glass dissolution and the precipitation of secondary phases, including a Cs,Sr-bearing aluminosilicate gel.