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.
S. K. Fong et al., "Immobilization of Hafnium Surrogates in Fluorapatite", Advances in Science and Technology, Vol. 45, pp. 2018-2023, 2006