Spark plasma sintering of iodine-bearing apatite

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Abstract

The high chemical durability of iodine-bearing apatite makes it strongly prospective for conditioning of radioactive iodine. The synthesis and consolidation of iodine-bearing compounds require low temperatures to avoid iodine volatilization. Spark plasma sintering therefore appears to be a suitable process because of its shorter treatment time and lower sintering temperature compared with other processes such as HUP or HIP. Two alternatives were examined: SPS sintering of iodine-bearing apatite powder and SPS reacting of a stoichiometric lead iodide and lead phosphovanadate powder mixture. The degree of densification and the microstructure of bulk materials in both cases are described and compared. Reactive sintering appears to involve a three-stage mechanism: (i) PbI2 coalescence, (ii) solid-state iodoapatite synthesis and consolidation and, (iii) iodoapatite consolidation in the presence of a liquid phase. The SPS reacted products reveal the finest and most homogeneous microstructure, and a density exceeding 96%.

Introduction

One of the management strategies examined for long-lived nuclear wastes involves a deep geological disposal. Repository safety is based on conditioning the waste in a chemically inert matrix to ensure radionuclide retention in the event of contact with a dissemination vector. In the case of iodine-129, with a half-life of 15.7 million years, apatite-derivative minerals with the generic formula Pb10(VO4)6(1−x)(PO4)6xI2 are promising conditioning materials [1], [2]. They are capable of high iodine loading (>8 wt.%) with strong containment properties based on normalized leaching tests [3]. Ceramics with no open porosity are required to limit the potential exchange surface area; this implies the use of materials with a densification exceeding 92% of the theoretical density, a threshold generally acknowledged as marking the elimination of open porosity. However, the densification of iodine-bearing materials requires low sintering temperatures to prevent volatilization; iodoapatite, Pb10(VO4)4.8(PO4)1.2I2, decomposes in air above 500 °C. The densification of this type of iodine-bearing apatite was therefore initially envisaged only in a confined environment [3], [4]. This can be obtained with an excess of lead phosphovanadate, Pb3(VO4)2(1−x)(PO4)2x, which is densified at relatively low temperatures [5], [6] and can be consumed as a reagent in the event of PbI2 diffusion during heating. The balance equation for the formation of iodine-bearing apatite can be written as follows:3Pb3(VO4)2(1-x)(PO4)2x+PbI2Pb10(VO4)6(1-x)(PO4)6xI2

This work concerns spark plasma sintering (SPS) of iodine-bearing apatite: Pb10(VO4)4.8(PO4)1.2I2. Earlier work in a confined environment showed that spark plasma sintering of this phase was possible at temperatures below 500 °C and therefore theoretically without the need for a gangue material [7], [8]. Spark plasma sintering of iodoapatite without a gangue was investigated first, then the synthesis and consolidation of this phase by reactive sintering was examined.

Section snippets

Experimental procedure

Iodoapatite Pb10(VO4)4.8(PO4)1.2I2 was synthesized from lead phosphovanadate Pb3(VO4)1.6(PO4)0.4 and lead iodide PbI2. Iodoapatite is formed with a volume shrinkage by 1.804 cm3 per mole. This calculation is based on the theoretical reaction written above, taking into account the molar mass for each phase and the following densities: 7.302, 6.099 and 7.117 g cm−3 for Pb3(VO4)1.6(PO4)0.4, PbI2 and Pb10(VO4)4.8(PO4)1.2I2 respectively.

Lead phosphovanadate was obtained by calcining a powder mixture of

Reactive versus non-reactive sintering

No pressure rise was observed in the chamber during sintering, regardless of the conditions, indicating that no iodine volatilization occurred during these tests. XRD analysis showed that the Pb10(VO4)4.8(PO4)1.2I2 phase was conserved during SPS sintering and, that the reactants were completely converted to iodoapatite in the case of SPS reacting. The iodoapatite crystallites were systematically larger in the case of non-reactive sintering, except for sample IAp-001 (51 nm) compared with the

Conclusion

Non-reactive sintering of iodine-bearing apatite Pb10(VO4)4.8(PO4)1.2I2 by spark plasma sintering results in about 91% densification in the temperature range between 400 °C and 450 °C. Densification is limited mainly by abnormal particle growth. At lower temperatures and pressures a density gradient appears between the pellet periphery and core. This gradient reflects the temperature gradient in the material during sintering. In the case of SPS reacting, samples with homogeneous composition and

Acknowledgement

The authors would like to thank the GNR 3051 Matinex for funding this research.

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