Shirley K. Fong

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.

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.

Development Of A Glass-Encapsulated Ceramic Wasteform For The Immobilization Of Chloride-Containing ILW

Pyrochemical reprocessing of plutonium generates several ILW waste-streams and a simple process has been developed at AWE to immobilize the chloride and actinide ions present in one of these waste-streams (Type 1) by calcining the waste with calcium phosphate. We have now investigated the possibility of using this process as the basis for treating a more complex waste-stream (Type 2) and have determined that with some minor modification the original process can effectively immobilize the greater range of cations present in Type 2 waste to again produce a free-flowing non-hygroscopic powder. Weight for weight replacement of the Type 1 waste by Type 2 produced a powder which was approximately 20% deficient in PO 4 3- when compared with stoichiometric powder. Addition of phosphorus pentoxide to the simulant waste/calcium phosphate powder mixture prior to calcination to produce stoichiometric powder significantly improved the chloride ion uptake. In order to comply with safety requirements it is necessary to convert the free-flowing powder into a monolithic wasteform suitable for long term storage. Conversion of the powder to a monolithic wasteform by sintering using a sodium aluminophosphate glass as a binder has been investigated and has proved successful, but to achieve high densities it has been necessary to cold press the samples prior to sintering.