Rapid microwave synthesis of Pb 5 (VO 4 ) 3 X (X = F, Cl, Br and I) vanadinite apatites for the immobilisation of halide
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Rapid microwave synthesis of Pb5(VO4)3X (X = F, Cl, Br and I) vanadinite apatites for the immobilisation of halide radioisotopes. Martin C. Stennett, Ian J. Pinnock and Neil C. Hyatt Department of Materials Science and Engineering, Sir Robert Hadfield Building, Mappin Street, Sheffield, S1 3JD, UK. ABSTRACT Microwave dielectric heating was utilized to synthesize potential host phases for halide radioisotopes. Synthesis of Pb5(VO4)3I was successfully achieved, without the use of a sealed container, using a modified domestic microwave oven (DMO) operating at 2.45 GHz. Rapid synthesis of Pb5(VO4)3X (X = F, Cl, Br) was also achieved using a commercially available microwave muffle furnace, also operating at 2.45 GHz. The combination of rapid heating rate and inverse temperature profile characteristic of microwave heating was found to promote formation of the target phases whilst retarding the volatilization of the halide species. Pb5(VO4)3I ceramic bodies produced in the DMO exhibited a heterogeneous zoned microstructure, whereas Pb5(VO4)3X phases with X = F, Cl, and Br fabricated in the microwave muffle furnace were dimensionally uniform suggesting this could be a promising route to fabricating single phase, dense halide containing ceramics. INTRODUCTION The current renaissance in nuclear energy is driven by increased global energy requirements combined with the need to reduce dependence on, and the environmental impact of, conventional fossil fuels. A significant expansion in nuclear energy will require improvements in the processing and disposal of spent nuclear fuel containing problematic nuclear waste isotopes, some of which have no currently accepted disposal route. Radioactive halides, most notably 129I, constitute an important fraction of the fission product inventory in spent 235U fuel. 129 ,LVDȕDQGȖHPLWWHUDQGDVDUHVXOWRILWVORQJKDOIOLIH [6 y), high mobility in the environment, and bioactivity poses a potential long term dose risk [1]. Historically 129I has been discharged into the marine environment but future regulatory changes will probably require it, and other radioactive halides, to be immobilised in a safe, robust wasteform. Audubert et al. proposed a lead iodovanadate phase, Pb5(VO4)3I, as a potential matrix for the immobilisation of 129 I [2]. Pb5(VO4)3I adopts the apatite structure incorporating the large iodine anions in one dimensional tunnels formed by corner sharing VO4 polyhedra. Due to the volatility of iodine, solid state synthesis of Pb5(VO4)3I is only possible in a closed system e.g. by isostatically pressing a mixture of Pb3(VO4)2 (formed from PbO and NH4VO3) and PbI2 in a sealed metal can. Typical reaction conditions are 500 – 700 °C and 9 – 200 MPa pressure [2-4]. In an attempt to develop a rapid and direct synthesis route for Pb5(VO4)3X (X = F, Cl, Br, I), under ambient pressure conditions, mixtures of PbO, V2O5 and PbX2 were reacted by microwave dielectric heating in a modified domestic microwave oven (DMO) and a CEM Phoenix microwave muffle furnace. V2O5 is known to
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