Brian L. Metcalfe

The immobilization of high level radioactive wastes using ceramics and glasses

An overview is given of the immobilization of high level radioactive waste (HLW) and surplus materials from a variety of commercial and defence sources employing glass and ceramic hosts. A number of specific host materials are reviewed, including borosilicate and phosphate glasses, glass-ceramics and crystalline ceramics. Topics covered include wasteform processing and manufacture, in addition to wasteform stability, durability and mechanical behaviour. Although, at the present time, borosilicate glass is the generally accepted first generation wasteform for the immobilization of HLW, the emergence of new sources of radioactive materials requiring immobilization has renewed interest in many of the alternative candidates. These include, in particular, titanate, zirconate and phosphate based ceramics, together with iron phosphate based glasses and basaltic glass-ceramics. The relative merits and limitations of each host material are compared and discussed, with particular reference to processing considerations and to current and likely future requirements.

The preparation, properties and applications of some glass-coated metal filaments prepared by the Taylor-wire process

The Taylor-wire method offers a versatile and intrinsically inexpensive route for the manufacture of glass-coated metal filaments a few micrometres in diameter in a single operation directly from the melt. The preparation by this process of a number of microcrystalline and amorphous microwires is reported. Materials investigated have included copper, four different Ni-Si-B alloys and a Co-Mo alloy. The resultant properties of the microwire products prepared from these materials are summarized. One potential application for microwire is in the area of composite materials and data are presented outlining the preparation directly from microwire of metal filament-reinforced glass-matrix composites. In conclusion, a number of alternative potential applications for microwire are briefly discussed.

Development and properties of a SiC fibre-reinforced magnesium aluminosilicate glass-ceramic matrix composite

The preparation of a magnesium aluminosilicate glass-ceramic matrix composite having an approximately matched thermal expansion coefficient to the silicon carbide fibre reinforcement is described. Data are presented on the process conditions necessary to produce a composite with matched thermal expansion coefficients, the strength and work of fracture of the composite at ambient temperature, and also the effect of temperature on the mechanical properties.

The preparation and properties of some lithium borate based glasses

The preparation and properties of lithium borate based glasses are described. The properties which were monitored included the glass-forming ability, the thermal stability, the chemical durability and the mechanical behaviour. Methods which were used in an attempt to improve the mechanical properties of these glasses are also discussed, including controlled crystallization, chemical ion-exchange and fibre reinforcement. The strongest glass, which had a mean strength of ~ 280 MPa in biaxial flexure, was produced by ion-exchange in molten sodium nitrate.

Influence of transition metal oxide additions on the crystallization kinetics, microstructures and thermal expansion characteristics of a lithium zinc silicate glass

Lithium zinc silicate glasses can be used to prepare moderately high thermal expansion glass-ceramics, and these materials are ideally suited for the manufacture of hermetic seals to both nickel-based superalloys and stainless steel. On the basis of earlier work by the present authors, one particular composition from the lithium zinc silicate system was chosen for detailed investigation. This composition contains Na2O and B2O3 fluxing agents, together with P2O5 as the primary nucleating agent. The crystallization kinetics and resultant microstructures of this composition have been studied as a function of the heat-treatment parameters using differential thermal analysis, dynamic mechanical thermal analysis, scanning and transmission electron microscopy, ambient and high-temperature X-ray diffraction, and small-angle neutron scattering. Indirect evidence from the dynamic mechanical thermal analysis and small-angle neutron scattering suggests that the nucleated glass is phase separated on a very fine scale, of the order of 16 nm. A number of crystalline phases have been positively identified in the heat-treated glasses, including cristobalite, quartz, tridymite, β1-Li2ZnSiO4 and γo-Li2ZnSiO4, the precise phases that are formed depending strongly on the heat-treatment parameters. The influence of a number of transition metal oxide additions on the resultant properties of the lithium zinc silicate composition has also been investigated, and it has been shown that the crystallization kinetics, microstructures and thermal expansion characteristics are all strongly affected by these additions. In particular, the activation energy for crystallization (which is related to the nucleating efficiency) is dependent on the ionic field strength of the transition metal ion species employed, with crystallization being favoured by solutes of high field strength.

The preparation and properties of some lithium zinc silicate glass-ceramics

Lithium zinc silicate glasses are of interest for the preparation of moderately high thermal expansion glass-ceramics which are suitable for sealing to a number of nickel-based superalloys. The effect of composition, in particular the variation of nucleating species, on the crystallization behaviour of a number of these glasses has been examined using differential thermal analysis, X-ray diffraction, and electron microscopy. Various crystal phases have been identified, including cristobalite, quartz, tridymite andγ0 Li2ZnSiO4. In addition, most of the glass-ceramics also contain an unidentified phase which may be related to theβ-series of lithium zinc silicates. Heat-treatment schedules have been derived on the basis of these results in order to produce a number of glass-ceramic materials. The resultant thermal expansion characteristics of the glass-ceramics have been monitored using dilatometry, and expansions in the range ≈12.3 to 17.1×10−6° C−1 (20 to 460° C), have been obtained, depending on the precise glass composition and heat-treatment schedule employed. In addition, the mechanical properties of a number of selected samples have been monitored, employing a biaxial flexure technique.

Calcium Phosphate: A potential host for halide contaminated plutonium wastes.

The presence of significant quantities of fluoride and chloride in four types of legacy wastes from plutonium pyrochemical reprocessing required the development of a new wasteform which could adequately immobilize the halides in addition to the Pu and Am. Using a simulant chloride-based waste (Type I waste) and Sm as the surrogate for the Pu3+ and Am3+ present in the waste, AWE developed a process which utilised Ca3(PO4)2 as the host material. The waste was successfully incorporated into two crystalline phases, chlorapatite, [Ca5(PO4)3Cl], and spodiosite, [Ca2(PO4)Cl]. Radioactive studies performed at PNNL with 239Pu and 241Am confirmed the process. A slightly modified version of the process in which CaHPO4 was used as the host was successful in immobilizing a more complex multi-cation oxide–based waste (Type II) which contained significant concentrations of Cl and F in addition to 239Pu and 241Am. This waste resulted in the formation of cation-doped whitlockite, Ca3-xMgx(PO4)2, β-calcium phosphate, β-Ca2P2O7 and chlor-fluorapatite rather than the chlorapatite and spodiosite formed with Type I waste.

Diffusion of Metallic Species and their Influence on Interfacial Reactions in Glass-Ceramic-to-Metal Seals

This is a preliminary investigation aimed at assessing the influence of individual metallic elements on the sealing characteristics of glass-ceramic-to-metal seals in order to aid in the analysis of interfacial reactions in more complex practical alloy systems. In the present study, a lithium zinc silicate (LZS) glass nucleated with P2O5 has been sealed to high purity Fe, Ni and Cr metals and the resultant diffusion into the glass of each metal monitored as a function of sealing temperature and time. The initial data obtained are compared with similar data noted for multi-component alloys.

Immobilization of Hafnium Surrogates in Fluorapatite

To immobilize the halide and actinide ions present in four Intermediate Level Waste (ILW) waste-streams the following process has been developed at AWE. The waste streams are initially calcined with CaHPO4 to yield apatite which is then sintered with a sodium aluminophosphate glass to produce a monolithic wasteform. As each waste stream composition is expected to vary widely it is necessary to determine the safe limits of waste loading at which the actinides will be adequately immobilized via this solid state synthesis route. In these initial non-active studies hafnium was used as a surrogate for plutonium. Samples having nominal composition (Ca10-2xHfx)F2(PO4)6 (x = 0, 0.125, 0.25, 0.5, 0.75, 1.0 and 1.25) were prepared at 850 and 1050 °C. These were studied by XRD to determine the phase assemblage and solid solution limits in the apatite phase. Phase pure fluorapatite (Ca10F2(PO4)6) was obtained at 1050 °C (x = 0). At x = 0.125, on XRD patterns, additional reflections assigned to HfO2, Ca0.5Hf2(PO4)3 and Ca3(PO4)2 were observed. Proportions of these phases increased with x. Synthesis at 850°C (x = 0), yielded a two phase mixture of Ca10F2(PO4)6 and β-Ca2P2O7. At x ≥ 0.250 HfO2 was detectable by XRD, thereafter proportions of HfO2 and β-Ca2P2O7 increased with x.

Preparation, Characterization and Applications of Glass-Ceramic-to-Metal Seals

It is recognized that many factors need to be taken into consideration in the successful design and manufacture of high quality glass-ceramic-to-metal seals, particularly if an adequate component lifetime is to be achieved. During their preparation, undesirable reactions may occur between diffusing metal species and glass constituents, and these can lead to the formation of highly localized internal stresses, the presence of which can initiate failure of a seal either during manufacture or, more seriously, whilst in service due to the influence of static fatigue. In the case of high temperature systems, reactions under hostile operating conditions also need to be taken into consideration. A thorough understanding of the relevant glass-ceramic/metal interactions is therefore required in order that steps can be taken to avoid or at least minimize reactions within the interfacial region that may lead to localized modifications of the glass-ceramic microstructure. In this contribution, factors influencing the lifetime behaviour of glass-ceramic/metal systems are reviewed and discussed, with particular reference given to SOFC sealants and also to advanced electrical components developed at AWE including seals to stainless steels and Ni-based superalloys. Fundamental studies on bonding to pure Fe, Ni and Cr are also included.