Determination of transmutation effects in crystalline waste forms. 1998 annual progress report

摘要

In this report, the authors summarize the research that has taken place during the second year of the 3-year project. This project is being carried out at Pacific Northwest National Laboratory and Argonne National Laboratory. The original intent in this project was to use high-energy EXAFS to obtain information on the chemical state of the (sup 137)Ba in pollucite without opening the stainless steel capsules. Using this approach, the authors hoped to avoid changing the chemical state of the (sup 137)Ba.To evaluate the extent to which high-energy EXAFS data can be interpreted, the authors analyzed a number of Cs and Ba standards in addition to Ba-doped pollucite samples. The standards included a variety of structures from simple oxides and halides to titanates and silicates. While the EXAFS signal from the simple oxides and halides were low in amplitude the analysis of the EXAFS and the Fourier transforms resulted in reasonable data.The authors obtained extremely weak EXAFS signals from the more complex silicates and titanates with distant first neighbor atoms. In addition, the amplitudes of the Cs K-edge EXAFS are about half as intense as the corresponding Ba K-edge EXAFS (Figure 1). Several factors may have contributed to the apparent differences between the Cs and Ba K-edge data, such as core-hole lifetime broadening, weak photoelectron backscatters, and low charge density of the large low-valence cations. Also, long interatomic distances appear as low frequency oscillations in the XAS and these features could have been inadvertently removed when they tried to extract the EXAFS. Artifacts that have no possible structural origin often dominated the Fourier transforms from these compounds. A number of signal processing techniques were employed in an attempt to minimize these artifacts but they generally resulted in severe distortions of the weak EXAFS signal. Regardless of the specific origin, these factors reduce the extent to which the Cs K-edge EXAFS data can be interpreted. The authors conclude that Cs and Ba K-edge EXAFS can be used successfully for materials where the structure is relatively simple and the material is well ordered. However, in materials with low atomic number backscatterers and low symmetry such as pollucite, it is not possible to determine the effects of (sup 137)Cs beta decay and transmutation on the pollucite structure. However, one can reasonably hope to identify the presence of metallic Ba clusters or BaO, if either are the result of transmutation. To overcome the shortcomings with the high-energy EXAFS, they have explored the use of nuclear magnetic resonance (NMR) spectroscopy to examine the local structure of the cesium in some laboratory-prepared pollucite. Small quantities, typical of what they expect to recover from the sealed capsules, proved to be sufficient material to obtain spectra with both magic-angle spinning and static NMR techniques (Figure 2). The chemical shift is expected to be sensitive to local structure and to disorder. They expect to prepare a sample from the (sup 137)Cs pollucite when the authors open a capsule.'