Study of natural minerals of U-pyrochlore-type structure as analogues of plutonium ceramic waste-form

摘要

At present, crystalline ceramic based on titanate pyrochlore, (Ca,Gd,Hf,Pu,U){sub}2Ti{sub}2O{sub}7, is considered as the US candidate waste form for the immobilization of weapons grade plutonium. Naturally occuring U-bearing minerals with pyrochlore-type structure: hatchettolite, betafite, and ellsworthite, were studied in orders to understand long-term radiation damage effects in Pu ceramic waste forms. Chemical shifts (δ) of U(Lα{sub}1)- and U(Lβ{sub}1) X-ray emission lines were measured by X-ray spectrometry. Calculations were performed on the basis of a two-dimensional δLα{sub}1 and δLβ{sub}1 correlation diagram. It was shown that 100% of uranium in hatchettolite and, probably, 95-100% of uranium in betafite are in the form of (UO{sub}2){sup}(2+). Formal calculation shows that in ellsworthite only 20% of uranium is in the form of U{sup}(4+) and 80% of the rest is in the forms of U{sup}(5+) and U{sup}(6+). The conversion of the initial U{sup}(4+) ion originally occurring in the pyrochlore structure of natural minerals to (UO{sub}2){sup}(2+) due to metamict decay causes a significant increase in uranium mobility.