X-ray Absorption Fine Structure Studies of Speciation of Technetium in Borosilicate Glasses
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DD3.3.1
X-ray Absorption Fine Structure Studies of Speciation of Technetium in Borosilicate Glasses Wayne W. Lukens,1 David K. Shuh,1 Isabelle S. Muller,2 and David A. McKeown2 1 Chemical Sciences Division, Lawrence Berkeley National Laboratory Berkeley, CA 94720, USA. 2 Vitreous State Laboratory, The Catholic University of America Washington, D.C. 20064, USA. ABSTRACT A series of glass samples were prepared analogously to high level waste glass using either glass frit or glass precursors combined with a high level waste surrogate containing NaTcO4. Three different technetium species were observed in these glasses depending upon the synthesis conditions. If the glasses were prepared by reducing NaTcO4 to TcO2•2H2O with hydrazine or if a large amount of organic material was present, inclusions of TcO2 were observed. If no organic material was present, technetium was incorporated as TcO4-. If only a small amount of organic material was present, isolated Tc(IV) sites were observed in the glass. The relative technetium retention of these glasses was estimated from the Tc K-edge height, and had no correlation with the oxidation state of the technetium. Pertechnetate was well retained in these glasses. INTRODUCTION Borosilicate glass is the major waste form for the immobilization of the legacy high level nuclear waste created by years of plutonium production by the DOE. Glass was selected for several reasons including good durability and the fact that vitrification is a mature technology. Although most radionuclides present no challenges to the vitrification of high-level waste, certain radionuclides, especially 137Cs and 99Tc, are easily volatilized during vitrification.[1] The behavior of technetium and rhenium in borosilicate glasses has been reviewed,[2] and will be discussed only briefly. A number of Tc(VII) compounds have low boiling points. Both pertechnetyl chloride and fluoride (TcO3Cl and TcO3F) have boiling points under 100 °C. Tc(VII) oxide, Tc2O7, boils at 350 °C, and the alkali metal salts of pertechnetate, particularly CsTcO4, are volatile at high temperature. Little is known about the speciation of technetium in these glasses. Two studies about the speciation of technetium in borosilicate glasses have been reported.[1,3] Under reducing conditions, the glasses contained technetium metal, and under oxidizing conditions, the glasses contained TcO2(cr) inclusions. In none of these experiments was Tc(VII) observed. While these results may seem to imply that Tc(VII) is not stable in glass (presumably due to loss of volatile Tc(VII) species), the conditions used to prepare the glasses are not representative of those used to prepare actual waste form glasses, so the technetium speciation in these glasses may by different from those prepared under conditions more similar to those used for actual waste.
DD3.3.2
The main focus of this research is to characterize the speciation of technetium in borosilicate glasses as a function of experimental conditions, especially conditions similar to those used to prepare act
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