Phase Evolution in Dissolution Reactions between Power Reactor High Level Liquid Waste Calcines and Basic Glass

The dissolution reactions between high level liquid waste (HLLW) calcines and molten basic glass are key to the HLLW vitrification by two-step cold crucible induction melter (CCIM) technology. The phase evolution occurring in the melts of glass and calcines at different melting temperatures and periods was investigated. A mixture of basic glass powders and calcine powders in Pt-Au disks was isothermally heated for 15-90 min at 650-1 100 ℃. The HLLW calcines are mainly composed of Na₂MoO₄, Ln₂Zr₂O₇, ZrO₂, CsNO₃ and (Sr, Ba)MoO₄. Based on X-ray diffraction, Raman spectroscopy and scanning electron microscopy results, dissolution reactions began to take place at temperatures above 650 ℃, and the resultant phases reached stability within 30 min. Specifically, the platelike Na₂MoO₄ particles from HLLW calcines were initially dissolved in glass melts to form regional melts that were oversaturated with \mathrmMoO_4^2- ions, which subsequently gave rise to crystallization of powellite CaMoO₄ in the samples. The formed CaMoO₄ particles were partially dissolved in glass melts at temperatures below 1 000 ℃ and completely dissolved at 1 050 ℃ or above. It is also found that a small amount of Sr²⁺ joins the crystallization process, leading to slight shifts in X-ray diffraction angles and Raman shift of formed CaMoO₄ crystals. Results of Raman spectra also suggest that the structure of \mathrmMoO_4^2- in glass network changes with dissolution reactions. Amorphous \mathrmMoO_4^2- ν₁ band moves to higher frequencies for samples heated at higher temperatures and longer time, which is probably attributable to the incorporation of high strength cation Ln³⁺ into glass network interstices. Ln₂Zr₂O₇ phases from calcines are reacted with basic glass to form oxyapatite (Ca₂Ln₈(SiO₄)₆O₂) and ZrO₂ crystals. This reaction starts from 700 ℃ and almost completes at 850 ℃. The formed oxyapatite are stable at 700-800 ℃, partially dissolves again into glass at 850 and 900 ℃, and completely dissolves at 950 ℃ in the melts. The residual ZrO₂ particles remaines undissolved in glass until 850 ℃, and gradually dissolves at higher temperatures. In summary, during the dissolution reaction of HLLW calcines and basic glass, the molybdates, lanthanide zirconia and cesium nitrates are initially reacted with glass surface. The abundant Ca²⁺ ions participate in the reactions by forming intermediate products of CaMoO₄ and (Ca₂Ln₈(SiO₄)₆O₂), which are again melted into glass at higher temperatures. Such process helps to the dissolution of HLLW calcines. The dissolution of intermediate products is not completed until 1050 ℃for 60 min, therefore it is suggested to the CCIM operation that the calcines and basic glass should be melted at temperatures higher than 1 050 ℃ for not shorter than 60 min to form homogeneous glass melts.