The Use of Cerium Valence State for Evaluation of Accessory Minerals Durability to Radiation Damage
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The Use of Cerium Valence State for Evaluation of Accessory Minerals Durability to Radiation Damage Roman V.Bogdanov1, Yuri M. Zaytsev1, Andrey S. Sergeev2 Department of Chemistry, Saint-Petersburg State University, 199034, St. Petersburg, Russia; e-mail: [email protected] 2 Department of Geology, Saint-Petersburg State University, 199034, St. Petersburg, Russia 1
ABSTRACT Cerium-actinide bearing natural minerals which demonstrate their long-time physicochemical durability under the environment effect would be considered as analogues of actinide ceramic waste forms. Radiation damage of crystalline materials causes oxidation of cerium from initial Ce(III) to Ce(IV). Therefore, cerium valence state in actinide-cerium bearing natural minerals in some cases reflects the resistance of such minerals to radiation damage. Cerium valence state was determined in the following natural minerals of similar age and similar U-Th-contents: monazite (four samples), britholite (two samples), and aeschynite (one sample). The method of chemical shifts of X-ray emission (CeKα1 line) was used. The following contents of Ce(IV) were observed: more then 30 % in britholite, 11 % in aeschynite, 0 % in monazite. The results obtained suggest that durability of these actinide host phases with respect to radiation damage decreases in the monazite-aeschynite-britholite series. INTRODUCTION When we consider the properties of minerals which are natural analogues of matrices for the conservation of actinides and, in particular, weapon-grade Pu, the main attention is drawn to structural changes in the host-phase during the geological history of the sample. The chemical aspect of this problem is investigated to a lesser extent, namely is the change in the redox-state of atoms in the cation sublattice of the mineral into which actinide atoms can be inserted by the mechanism of isomorphic substitution. Minerals stability to oxidation processes under the effect of radiation determines to a considerable extent the applicability of the corresponding mineral matrices for the conservation and long-term storage of actinides in underground repositories. In the study of REE-bearing minerals, cerium transformation during the geological history of minerals may serve as a good test for the radiation stability of the corresponding structures as matrices for actinides conservation: Ce4+ + e- ⇔ Ce3+ (1). A Ce-bearing pyrochlore from a rare earth element ore deposit of Inner Mongolia Northern China displayed two zones of this mineral with different degrees of metamictization [1]. According to EELS data, in the metamict part of pyrochlore 84 % of cerium is in the form of Ce4+, whereas in the slightly metamict zone almost all cerium (95%) is trivalent. The authors reach a natural conclusion that the initial form of cerium insertion into natural pyrochlore was as Се3+. This corresponds to low oxygen fugacity in mineral formation of this pyrochlore: the oxygen fugacity (logfO2) ranges from (-30) to (-25) [1]. In contrast, the formation of tetravalent cerium was ca
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