PNNL

Abstract

In this document, we summarize our study of the effects of radiation induced damage to the titanate ceramics that were to be the immobilization form for surplus weapons-grade Pu. In this study, we made five ceramic materials: pure-phase pyrochlore, pure-phase zirconolite, pyrochlore-rich baseline, zirconolite-rich baseline, and impurity baseline. Two-hundred specimens were made of which 130 contained approximately 10 mass% 238Pu and 70 contained 10 mass% 239Pu. The specimens containing 239Pu served as materials against which the behavior of the 238Pu-bearing specimens could be compared. In our studies, we measured the “true” density (density exclusive of surface connected porosity), bulk density, crystalline-phase composition with X-ray diffraction (XRD), and dissolution rates as radiation induced damage accumulated in the 238Pu-bearing specimens. We routinely took photographs of the specimens during each characterization period. From our studies, we determined that these materials swell less than 10% and generally less than 5%. As the material swells, some open porosity can be converted to closed porosity, often causing the “true” density to decrease more rapidly than the bulk density. In general, 3?1018 a/g of damage accumulation were required for the materials to become amorphous as determined with the XRD method. The order in which the phases became amorphous was brannerite, pyrochlore, and zirconolite with brannerite being the most susceptible to radiation induced damage. However, we also show that Pu is not evenly distributed amongst the phases when multiple phases are present. We were unsuccessful in making a pure brannerite to study. Therefore, the brannerite was always present with other phases. For a material containing about 10 mass% 239Pu, 3?1018 a/g represent about 500 years in the geologic repository. At no time in our studies was there evidence for microcracking in these materials, even upon close examination in a scanning-electron microscope. Upon careful comparison between the dissolution behavior of non-radioactive, 238Pu-bearing, and 239Pu-bearing titanate ceramic specimens of the same composition, we see no difference in the dissolution rates of the three materials. Our results reported earlier suggested that the concentrations were affected by the radiolysis of the water in the dissolution tests with the 238Pu-bearing specimens, which have an intense local radiation field that does not exist for the 239Pu-bearing ceramics. This means that there is no effect of radiation induced damage on the forward dissolution rate of these ceramics. The results from this study show that the titanate ceramic is a viable immobilization form for the disposition of surplus weapons-grade Pu in a geologic repository. As the material becomes amorphous over approximately 500 years, no change to its dissolution rate will take place nor will the surface area of the ceramic increase from extensive microcracking. Therefore, the safety case that was used for the initial assessment of the performance of the titanate ceramic in the Yucca Mountain repository is valid.