Hollandite-rich Ceramic Melts for the Immobilisation of Cs
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+ROODQGLWHULFK&HUDPLF0HOWVIRUWKH,PPRELOLVDWLRQRI&V M.L. Carter1,2, E.R. Vance1 and H. Li1 Australian Nuclear Science and Technology Organisation, New Illawarra Rd, Lucas Heights, NSW 2234, Australia 2 School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia
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$%675$&7 Ceramic wasteforms designed to be processed by melting in air has been developed to immobilise Cs-rich wastes. Detailed characterisation electron microscopy is presented on versions of these melted materials which are rich in Cr-, Ni-, Zn or Co- substituted titanate hollandites and which have PCT-B normalised Cs leachate concentrations of < 0.2 g/L. To assist in understanding the general crystal chemistry of titanate hollandites, this study also investigates the solubility limits of Cs in single-phase hollandite BaxCsy(M3+)2x+yTi8-2x-yO16 where M = Cr and BaxCsy(M2+)x+y/2Ti8-x-y/2O16 formulations where M = Zn, Co or Ni. ,1752'8&7,21 In recent years there has been renewed interest in the development of ceramic wasteforms for separated radionuclei, notably in Europe. A French law passed in December, 1991, specified three major areas of French research on long-lived radioactive waste management: 1. Develop processes capable of enhanced separation and transmutation of long-lived radionuclides. 2. Investigate reversible or irreversible waste disposal in deep geological formations. 3. Investigate conditioning processes and long-term interim surface or sub surface storage options. Within the context of the third research topic, of particular interest are the cesium isotopes, 137 Cs and more particularly 135Cs. The immobilisation of Cs in borosilicate glass is well known and in recent years development of new vitreous matrices has taken place to improve leach resistance. This has resulted in a 10 -fold improvement in leach rates [1]. To further improve leach resistance work has been carried out on crystalline matrices. The titanate synroc phase hollandite (BaxCsyM3+2x+yTi8-2x-yO16 where trivalent M = Al in oxidising conditions and Al and Ti in reducing conditions) is well known for its ability to incorporate Cs when produced by hot pressing [2,3] with excellent leach resistance. But attempts to melt (hollandite-bearing) synroc-C under air atmospheres resulted in all the synroc titanate phases, including hollandite, being formed [4,5], but leachable Cs molybdate phases due to the presence of Mo6+ were also observed. Day HWDO[6] found that the Cs did not enter the hollandite phase in melted synroc-C and in order to incorporate the Cs in the hollandite the synroc-C melts had to be processed under reducing atmospheres. We have previously developed a hollandite-rich material, which can be fabricated by ceramic technology [7] to immobilize Cs. But it would also be attractive to be able to use melting technology for this purpose, using an air atmosphere. In recent work by Carter HWDO[8] on the melting in air of titanate hollandite-rich samples containing Cs and with no Mo present, it was found that the Cs would enter the Al contai
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