Lee A. Gerrard

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.

The Relative Merits of Oxides of Hafnium, Cerium and Thorium as Surrogates for Plutonium Oxide in Calcium Phosphate Ceramics

The choice of surrogate for plutonium oxide for use during the initial stages of research into the immobilization of intermediate level pyrochemical wastes containing plutonium andamericium oxides in a calcium phosphate host has been investigated by powder X-ray diffraction and X-ray absorption spectroscopy. Two non-radioactive surrogates, hafnium oxide and cerium oxide, together with radioactive thorium oxide were compared. Similarities in behaviour were observed for all three surrogates when calcined at the lowest temperature, 750°C but differences became more pronounced as the calcination temperature was increased to 950°C. Although some reaction occurred between all the surrogates and the host to form a substituted whitlockite phase, increasing the temperature led to a significant increase in the cerium reaction and the formation of an additional phase, monazite. Additionally it was observed that the cerium became increasingly trivalent at higher temperatures.

The Influence of Metal Diffusion and Interfacial Reactions on the Crystallization Behaviour of Glasses Employed in the Manufacture of Glass-Ceramic-to-Metal Seals

One of the major attributes of glass-ceramics is an ability to tailor their thermal expansion characteristics and this makes them ideal candidates for sealing to a wide variety of metals and alloys; however, during the sealing process, reaction of diffusing metal species with glass constituents may occur, and this can lead to the formation of undesirable phases within the interfacial region. In addition, diffusion of metal species into the bulk glass away from the interface may affect the overall crystallization kinetics and can result in the formation of unwanted crystalline phases which may be detrimental to the lifetime behaviour of a seal component. This contribution outlines and discusses the factors affecting the crystallization behaviour of glasses employed in seal manufacture and describe methods by which undesirable reactions can be alleviated or minimized through effective control of the process parameters and starting glass composition.