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Ming Zhang1

1, Institute of Materials, China Academy of Engineering Physics, Jiangyou, , China

The fundamental issues of particle-solid interactions and radiation-induced structural changes from a periodic-to-aperiodic state (or metamict state) are an active and important area of research. As different crystalline minerals have been proposed as actinide-bearing crystalline hosts for waste materials, it is important to understand radiation effect on these materials and to evaluate the durability and performance of these actinide-bearing phases. Studying metamicit state is also important for geochemistry, as U-Pd isotope system is used for age dating.
This work reviews early works/arguments on the relation of hydrogen and metamictization and reports new experiment data. Although it has been recognized that alpha-decay damaged minerals turns to have higher hydrogen contents, the relation between water and metamict state has been an unclear issue. Several important questions have been raised regarding the roles of hydrous species in metamictization and recrystallization, e.g., whether hydrous species stabilize the metamict state; why metamict minerals are generally "wet", even for some normally anhydrous minerals; whether hydrogen in radiation-damaged minerals is in the form of H2O or OH; what is the role of the presence of H2O in recrystallization? Since structurally incorporated OH or H2O can be seen as defects in the crystal structures of minerals, and since their sites and bonding are sensitive to the local structure, investigations into the behaviour of hydrogen-related species in radiation-damaged materials may provide insight into local defects cased by radiation damage. Behavior of hydrogen in radiation damaged single crystals of titanite CaTiSiO5 and zircon ZrSiO4 (both materials are proposed as nuclear waste forms) were studied by polarized spectroscopy. The results showed that in addition to structural damage, alpha-decay radiation causes an increase in hydrogen content in damaged regions and hydrogen commonly exists in the form of OH. The data show that heating radiation-damaged titanite and zircon to high temperatures leads to the diffusion of hydrogen from damaged amorphous region to crystalline region during recrystallinzation.

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