Ceram. Trans., Vol. 93, pp. 357-363, (1999) SOLID SOLUTIONS OF Ce, U, AND Th IN ZIRCON S.V. Ushakov 1 , W.Gong 2 , M.M. Yagovkina 3 , K.B. Helean 2 , W. Lutze 2 and R.C. Ewing 4 1 V.G. Khlopin Radium Institute, 28, 2nd Murinskiy Ave, 194021 St. Petersburg, Russia 2 Center for Radioactive Waste Management, Univ. of New Mexico, Albuquerque, NM87131, USA 3 JSC “Mekhanobr-Analyt”, 8a, 21 Linija, 199026, St. Petersburg, Russia 4 Dept. of Nuclear Engineering&Radiological Sciences, University of Michigan, MI48109, USA ABSTRACT Zircon (ZrSiO 4 ) has been proposed as a candidate host phase for storage and disposal of excess weapons plutonium. Zr can be replaced by a variety of elements including Hf, Am, Pu, Np, U, Pa, and Th. Little to nothing is known about miscibility in most of these systems. We have synthesized a variety of solid solutions of (Zr,A)SiO 4 where A = Ce, U, and Th using the sol-gel technique. Though more experiments need to be conducted to refine our results, it is obvious that the solubility of these elements are limited to less than 10 at%. The solubility limits determined by analytical electron microscopy and X-ray diffraction data are in at.%: 7±1 (Ce), 4±1 (U), and 2±0.5 (Th). Ce (as Ce(IV)) was used to simulate Pu. The measured solubility limit is an approximate value. The presence of some Ce(III) in the solid solution could not be excluded. The solubility of Pu in ZrSiO 4 is unknown. The highest Pu concentration reported in the literature is 9.2 at.%. INTRODUCTION Zircon (ZrSiO 4 ) has been proposed as a candidate host phase for storage and disposal of excess weapons plutonium 1,2 . The waste form could be a single phase zircon ceramic or a dual phase zircon/zirconia ceramic 3 . Zircon is a ubiquitous mineral and contains small amounts of uranium and thorium naturally. Long-term chemical durability of zircon has been evidenced by geochronologists who found that zircon alteration is small even under extreme conditions in nature 4 . Over 4,000 references were found (georef since 1960) on zircon dating. Zircon is one of the most durable minerals and the oldest dated mineral on earth and in the universe (about 3.4 billion yr.). Incorporation of plutonium into the zircon structure requires information on solubility limits not only for Pu but also for its daughter product uranium. A variety of compositions of ASiO 4 in which A 4+ = Hf, Zr, Am, Pu, U, Pa, and Th have been synthesized. HfSiO 4 (hafnon) ZrSiO 4 (zircon), ThSiO 4 (thorite), and USiO 4 (coffinite) occur naturally. Complete miscibility has been reported between hafnon and zircon 5,6 . Limited miscibility has been reported between zircon and coffinite and zircon and thorite 7 . We have synthesized various solid solutions of (Zr,A)SiO 4 where A 4+ = Ce, U, Th by the sol-gel method. These compositions and the observed phase assemblages were used to calculate solubility limits of the respective A cations in zircon. Uranium and thorium were studied to compare our experimental approach and the results with reported data by Mumpton and Roy 7 . Ce as Ce 4+ is a good substitute for Pu. The ionic radii of Pu 4+ and Ce 4+ are close: 0.96 and 0.97 Å,