Spark plasma sintering-densified vanadinite apatite-based chlorine waste forms with high thermal stability and chlorine confinement

Abstract

Effective management of the long-lived radioactive 36Cl is necessary for the development of pyrochemical reprocessing of used fuels and also the molten salt reactors in which a large quantity of Cl-containing waste is encountered. Significant challenge exists in developing advanced waste form matrix with high Cl confinement capability and fabrication technology with minimized loss of Cl. In this work, Cl-bearing vanadinite apatite (Pb5(VO4)3Cl) powder samples are synthesized by solid state reaction at room temperature using high energy ball milling (HEBM), and dense pellets above 99% theoretical density can be consolidated by spark plasma sintering (SPS) at a temperature as low as 450 °C for 3 min. The effects of various sintering temperatures (300–800 °C) on physical density, microstructure, thermal stability and mechanical properties of the SPS densified pellets are investigated. Microstructure analysis shows that the average grain size of nanocrystalline ceramic is less than 190 nm when sintered at 450 °C, and the sintered microstructure is dominated by grain growth at higher sintering temperatures. No phase decomposition and significant chlorine loss can be identified during HEBM and low temperature SPS consolidation processes. Thermal gravimetric analysis (TGA) test indicates that the phase decomposition and chlorine loss occur only at an elevated temperature 830 °C for both HEBM-prepared powders and SPS densified pellets. Vanadinite pellets densified at 450 °C possess the maximum hardness of 4.6 GPa. The SPS-densified Cl-bearing vanadinite apatite displays high confinement of chlorine and a good thermal stability, and thus could be used as a potential nuclear waste form for the immobilization of the long-lived and highly volatile 36Cl.