Processing Effects on the Behavior of Titanate Waste Forms in Aqueous Solutions
- PDF / 1,125,738 Bytes
- 8 Pages / 417.6 x 639 pts Page_size
- 66 Downloads / 177 Views
Inc.
PROCESSING EFFECTS ON THE BEHAVIOR OF TITANATE WASTE FORMS IN AQUEOUS SOLUTIONS
R. G. DOSCH Sandia National Laboratories,
Albuquerque,
New Mexico,
87185,
U.S.A.
ABSTRACT Titanate processing parameters including temperature, waste loading, redox conditions, and additives used to promote specific phases were evaluated with respect to their effect on leaching of Cs, Mo, Ca, Sr, Ba, U, Gd, and Ti in the range of 22'-150'C in leachates which included deionized water, acidic (pH 2), and basic (pH 12) solutions. Surface analyses and microstructural characterization were used to relate observed leaching behavior to processing parameters. Redox conditions were found to be very important with respect to Cs (and Mo) retention. Two Cs-bearing phases were found in titanate prepared under oxidizing conditions. One had a hollandite structure and the other contained Cs, Mo, Ca, and Fe as major constituents. The latter phase, which was more susceptible to leaching, was not observed in titanates prepared using adequate reducing conditions. Where applicable, a reference glass (PNL 76-68) was included in the leaching tests. Comparative leach rates for elements common to both waste forms were generally one to four orders of magnitude lower for the titanates within the range of conditions used. INTRODUCTION An essential property of any material designed as a vehicle for long term isolation of high level nuclear waste is resistance to aqueous attack under the range of conditions which may prevail in deep geologic repositories. Evaluation of this property constituted a significant fraction of the work done in our laboratories in the development of a titanate-based waste form directed at acidic, first cycle Purex waste streams [1,21. A matrix of approximately one hundred samples was prepared specifically to compare the effects of compositional and processing variations on the leaching properties of the product waste form. These included waste loading (10, 16, and 25% by weight), hot-pressing temperature (1100-12500C), oxidation state control, and additives (Ba and Al) used to promote the formation of the Cs-bearing hollandite phase. The general procedure used in comparative tests involved contacting samples, in monolithic form, with deionized water at 22°C, 60°C, 90'C, and 150°C and monitoring Cs dissolution after one day and subsequent three day periods. Selected samples were chosen for Ca, Ba, Sr, Mo, Ti, U, and Gd analyses. Based on the results of these tests, samples representing the best, average, and poorest titanate materials with respect to Cs retention were selected for further study which included leaching in solutions believed to represent the extremes of a probable repository pH range (2 to 12), leaching of powder samples, and leaching of pre-thinned transmission electron microscope (TEM) samples. Where applicable, samples of the current borosilicate reference waste form (PNL 76-68 glass, Bar #239, Lot #3) were tested under identical conditions to provide, by direct comparison, a point of reference for evaluating th
Data Loading...