Titre du document / Document title

The fate of the epsilon phase (Mo-Ru-Pd-Tc-Rh) in the UO₂of the Oklo natural fission reactors

Auteur(s) / Author(s)

UTSUNOMIYA S. ; EWING R. C. ;

Résumé / Abstract

In spent nuclear fuel (SNF), the micrometer-to nanometer-sized epsilon phase (Mo-Ru-Pd-Tc-Rh) is an important host of ⁹⁹Tc which has a long half life (2.13 x 10⁵ years) and can be an important contributor to dose in safety assessments of nuclear waste repositories. In order to examine the occurrence and the fate of the epsilon phase during the corrosion of SNF over long time periods, samples of uraninite from the Oklo natural reactors (∼ 2.0 Ga) have been investigated using transmission electron microscopy (TEM). Because essentially all of the ⁹⁹Tc has decayed to ⁹⁹Ru, this study focuses on 4d-elements of the epsilon phase. Samples were obtained from the research collection at University of Michigan representing reactor zone (RZ) 10 (836, 819, 687) and from RZ 13 (864, 910). Several phases with 4d-metals have been identified within UO₂ matrix at the scale of 50-700 nm; froodite, PdBi₂, with trace amounts of As, Fe, and Te, and palladodymite or rho-darsenide, (Pd, Rh)₂As. The most abundant 4d-metal phase is ruthenarsenite, (Ru, Ni)As, which has a representative composition: As, 59.9; Co, 2.5; Ni, 5.2; Ru, 18.6; Rh, 8.4; Pd, 3.1; Sb, 2.4 in atomic %. Ruthenarsenite nanoparticles are typically surrounded by Pb-rich domains, galena in most cases; whereas, some particles reveal a complexly zoned composition within the grain, such as a Pb-rich domain at the core and enrichment of Ni, Co, and As at the rim. Some ruthenarsenites and Rh-Bi-particles are embedded in surrounding alteration products, e.g., chlorite, adjacent to uraninite (no further than ∼ 5 μm). A few of those particles are still coated by a Pb-rich layer. Based on these results, the history that epsilon phases have experienced can be described as follows: (i) The original epsilon phase was changed to, in most cases, ruthenarsenite, by As-rich fluids with other trace metals. Dissolution and a simultaneous precipitation may be responsible for the phase change. (ii) All Mo and most of the Tc were released from the epsilon phase. Galena precipitated surrounding the 4d-metal phases. (iii) Once the uraninite matrix has dissolved, the epsilon nanoparticles were released and "captured" within alteration phases that are immediately adjacent to the uraninite.

Revue / Journal Title

Radiochimica Acta   ISSN 0033-8230   CODEN RAACAP 

Source / Source

2006, vol. 94, n^o9-11, pp. 749-753 [5 page(s) (article)]

Langue / Language

Anglais

Editeur / Publisher

Oldenbourg, München, ALLEMAGNE  (1962) (Revue)

Mots-clés d'auteur / Author Keywords

Epsilon phase ; o / Natural reactor / HRTEM ;

Localisation / Location

INIST-CNRS, Cote INIST : 10156, 35400015904437.0370