Investigation of Natural and Artificial Zr-silicate Gels
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Investigation of Natural and Artificial Zr-silicate Gels Boris E. Burakov, Andrey Ph. Smetannikov, Evgeniy B. Anderson and Alexander Yu. Alexeev Laboratory of Applied Mineralogy and Radiogeochemistry, V.G. Khlopin Radium Institute, 28, 2-nd Murinskiy ave., St. Petersburg, 194021, Russia ABSTRACT Sample of natural Zr-silicate gel containing up to 13 wt.% U was characterized using scanning electron microscopy (SEM), X-ray powder diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and electron microprobe analysis (EPMA) method. It was found that gel matrix is amorphous in general; however, it contains non-identified nanocrystallites. No separated oxide phases of U, Zr or Si were observed in a gel matrix. After sintering in air at 1400°C for 1 hour gel transformed largely into crystalline zircon, (Zr,U)SiO4. Uranium was not found in any other phases besides zircon. It was assumed that high chemical durability of natural Zr-P-U-Ti-silicate gel is caused by two competing processes which exist under self-irradiation conditions: 1) crystallization of the gel and 2) metamictization of the crystallized zircon and other phases. Synthetic samples of Zr-silicate gel doped with Ce, U, Pu and Am were obtained and studied in comparison with natural samples. It was suggested to use artificial solid Zr-silicate gels for durable fixation of actinides for the goal of long-term or intermediate storage. INTRODUCTION Natural accessory minerals such as zircon, ZrSiO4, monazite, (Ce,La,Th)PO4; zirconolite, CaZrTi2O7; apatite, Ca5(PO4)3(Cl,F,OH); britholite, (Ce,Ca)5(SiO4,PO4)3(OH,F) etc. are well known concentrators of U , Th and their daughters isotopes. In general, all these minerals are crystalline and characterized by high chemical durability preventing radionuclide release from their matrices. Under self-irradiation most of them lose their crystalline structure and become metamict. This process might be accompanied with essential increase of radionuclide release, in particular, in the presence of underground water. However, some uranium ores in Kazakhstan contain chemically durable gel-analogues of accessory minerals [1,2]. These Zr-P-U-Ti-silicate gels contain (in wt.% “from-to”): Zr – 20-27; U – 4-13; Si – 8-10; Ca – 1-2; Fe – 1-14; Al – 1-2; Ti – 1-9; P – 0-2 plus water. In some cases gels contain mainly P instead Si. In general, chemical compositions of Zr-silicate gel is close to that of zircon, ZrSiO4, thorogummite or uranoan, (Th,U,Ce)(SiO4)1-x(OH)4x [3], hydrozircon, [(Zr,U)1-xFex][(SiO4)1-x(AsO4)x]x2H2O [4] and artificial hydroxylated zircon, Zr(SiO4)1-x(OH,F)4x [5]. Gels formed as a result of hydrothermal processes precipitating initially as the amorphous materials. Gels survived in an open geochemical system for millions of years demonstrating their high chemical durability. It was suggested to use artificial solid Zr-silicate gel for durable fixation of actinides for the goal of long-term or intermediate storage [6]. This paper discusses principal features of natural U-bearing Zr-silicate gel in comp
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