The Long-Term Corrosion Behavior of Titanate Ceramics for Pu Disposition: Rate-Controlling Processes
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Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
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Days Figure 1. Normalized mass loss, based on Ca release for the zirconolite-rich ceramic in MCC-I tests. The error bars represent 35% relative uncertainty.
TEST MATERIALS The zirconolite-rich ceramic was fabricated at Lawrence Livermore National Laboratory. The precursor material was ground, dried, pulverized and calcined in air at 600'C for 1 hour. The calcined material was then broken up and mixed with the PuO 2. The resulting powder was cold pressed and sintered at 1325'C for about 4 hours. Further details of the fabrication method are described by Buck et al. [4]. The chemical composition of the zirconolite-rich ceramic was determined by dissolving it at 150°C in minersi acids, and analyzing the resulting solution with ICP-MS. The composition of the ceramic was measured to be: 8.0 mass % A12 0 3 , 7.3% CaO, 0.37% Cr,0 1 , 0.27% Fe 2 0 3 , 0.10% NiO, 0.26% ZnO, 0.2 1% CuO, 0.32% Ga,0 3, 3.6% BaO, 0.31% CeO_,, 9.5% Gd_,O, 37% TiO2, 16% ZrO2 , 0.35% HfO2 , and 14%PuO 2 . The phase composition of the ceramic is 60-70 vol. % zirconolite, about 30% rutile, less than 5% perovskite and brannerite, and less than 1% PuO,. This ceramic was developed several years ago at the beginning of the testing piogram. Long-term tests were initiated with this material. Newer formulations are composed primarily of pyrochlore. Because zirconolite and pyrochlore are chemically and crystallographically similar, we expect that the tests
described here will provide insight into the long-term corrosion behavior of the newer formulation. RESULTS AND DISCUSSION Previous corrosion studies with this zirconolite-rich ceramic, based on MCC-I test data, have shown an initial rapid release of material followed by a much slower release [5]. The NL(Ca) data in Figure I show rapid corrosion in the first three days and slower corrosion after three days. Similar changes were observed in the release rates of other elements found in this ceramic [5], and similar behavior has been observed with SYNROC ceramics [6]. One explanation for the decrease in the corrosion rate over time is that the leachate solutions approach saturation with respect to some solid phase. We would expect on this basis that the corrosion rate would decrease as the concentration of the element or elements contained in the phase increased. Possible controlling solid phases for the dissolution of this titanate ceramic include TiO2 and ZrO2 . Another explanation for the observed decrease in the corrosion rate over time is the formation of protective, Ti-rich alteration layers [6]. Such continuous TiO2 layers are often observed on corroded Ti metal surfaces [7]. Titanium-rich alteration phases have been observed on corroded perovskite and hollandite in SYNROC ceramics [8, 9]. In the same studies, however Ti-rich alteration layers were not observed on corroded zirconolite. Therefore, we do not expect that continuous, protective Ti-rich alteration layers are pr
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