Distribution and solubility of radionuclides and neutron absorbers in waste forms for disposition of plutonium ash and scraps, excess plutonium, and miscellaneous spent nuclear fuels. 1998 annual progress report

'The objective of this research is to gain a fundamental understanding of the distributions and the solubility limits for actinides Pu and U and rare earth neutron absorbers such as Gd and Hf in waste forms. This will be accomplished by systematically studying the local structural environments of these constituents in representative waste forms such as glass, ceramics, and vitreous ceramics. Basic knowledge of these issues will provide a technical and scientific basis that can be used by the US Department of Energy (DOE), Environment Management (EM) Program in developing, evaluating, and selecting waste forms for the safe disposal of Pu, spent nuclear fuel, and other transuranic wastes. The work presented here is a summary of the research activity from November 1997 to May 1998. The elucidation of the correlations between the local structural environments of actinides and rare earth neutron absorbers in waste forms as functions of waste form compositions, and waste form processing conditions will also advance basic material science. The work presented here is a summary of the research activity from November 1997 to May 1998. Currently being studied is the effect of the Pu oxidation state on its solubility in borosilicate-based glasses. When glasses are melted in ambient atmosphere, Pu(IV) has been shown to be the dominant oxidation state as determined by ultraviolet-visible-near infrared spectroscopy (UV-VIS-NIR) and x-ray absorption fine structure (XAFS) techniques. However, no literature data are available for glasses containing Pu predominantly as Pu(III) nor the solubility for Pu(III) in the glass. The results of the study demonstrate that in borosilicate glass, Pu(III) is significantly more soluble than Pu(IV). Using x-ray diffraction analysis the solubility of Pu(III) as oxide was determined to be at least 25 mass% in the reduced glass, while it was no greater than 10 mass% in the same glass under oxidizing conditions (glass melting temperature was 1,400 C). The oxidation states of Pu in the glasses were determined by XAFS analysis using the Pu L III edge as shown in Figure 1. Using standard reference XAFS spectra of Pu(IV) and Pu(III), the authors estimated that about 90% of the Pu was present as Pu(IV) in the oxidized glass and about 95% of the Pu was Pu(III) in the reduced glass. In contrast to the redox effect on the solubility limit of Pu, the solubility of U in soda-lime (SL) silicate glass is favored under oxidizing conditions or glass with high Na 2 O concentration. In ambient atmosphere at 1,500 C, the solubility, in terms of UO{sub 3}, is found to be 30 mass% in the baseline glass (SRM 1,830 from NIST) and at least 40 mass% in the baseline glass with additional Na{sub 2}O.