This article is protected by German copyright law. You may copy and distribute this article for your personal use only. Other use is only allowed with written permission by the copyright holder. Radiochim. Acta 94, 749–753 (2006) / DOI 10.1524/ract.2006.94.9.749  by Oldenbourg Wissenschaftsverlag, München The fate of the epsilon phase (Mo-Ru-Pd-Tc-Rh) in the UO 2 of the Oklo natural fission reactors By S. Utsunomiya 1 , ∗ and R. C. Ewing 1, 2, 3 1 Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, 1100N University Avenue, Ann-Arbor, Michigan, USA 2 Department of Nuclear Engineering & Radiological Sciences, University of Michigan, USA 3 Department of Materials Science and Engineering, University of Michigan, USA (Received August 15, 2005; accepted in revised form March 28, 2006) Epsilon phase / Oklo / Natural reactor / HRTEM Summary. In spent nuclear fuel (SNF), the micrometer- to nanometer-sized epsilon phase (Mo-Ru-Pd-Tc-Rh) is an important host of 99 Tc which has a long half life (2.13 × 10 5 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 dur- ing 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 99 Tc has decayed to 99 Ru, 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 UO 2 matrix at the scale of 50–700 nm; froodite, PdBi 2 , with trace amounts of As, Fe, and Te, and palladodymite or rho- darsenide, (Pd, Rh) 2 As. The most abundant 4d-metal phase is ruthenarsenite, (Ru, Ni)As, which has a representative compo- sition: 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 typ- ically 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 μ m). 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. Introduction Spent nuclear fuel (SNF) consists of more than 95 wt. % of U and contains ∼ 1 wt. % of Pu, ∼ 2–3 wt. % of the fission *Author for correspondence (E-mail: utu@umich.edu). products, and other transuranic elemements depending on the burn-up [1]. Speciation of the fission products are gener- ally of four types: (i) fission gases and volatiles, (ii) fission products as metals, including Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, and Te, (iii) fission products as oxides, includ- ing Rb, Cs, Ba, Zr, Nb, Mo, and Te, (iv) fission products dissolved as oxides in the fuel matrix, including Sr, Zr, Nb, and REE [1, 2]. Among these elements, 99 Tc is one of the elements of greatest concern, because it has a long half life (2.13 × 10 5 years) and can be an important contributor to the calculated dose after long times in safety assessments of nuclear waste repositories. In addition, Tc is predomin- antly present as soluble TcO 4 − under oxidizing conditions over wide range of pH (4–10), weakly adsorbed onto min- eral surfaces, and unlikely to be incorporated into secondary precipitated uranyl-minerals [3]. In SNF, the micron- to submicron-sized epsilon phase (Mo-Ru-Pd-Tc-Rh) is a major host of Tc. The epsilon- phase structure is hexagonal close packing [2]. The size and composition of epsilon particles vary depending on the fission yield [1, 2, 4]. The first TEM observation of the ep- silon phase in LWR (light-water reactor) SNF was done by Thomas and Guenther [5], which showed that the epsilon particles occur at ternary grain boundaries of the uraninite matrix associated with Xe-Kr phase and consist of 40 wt. % of Mo, 30 wt. % of Ru, 10 wt. % of Tc, 15 wt. % of Pd and 5 wt. % of Rh. Because the epsilon phase is a main host for Tc, the corrosion behavior has also been examined in a number of previous investigations. Finn et al. [6] reported that under the oxidizing conditions the UO 2 matrix dissolves rapidly, and the epsilon particles remain outside of the altered ma- trix, indicating relatively slower leach rate of the epsilon particles. The corroded epsilon particles also showed se- lective leaching of Mo among the five metals [4]. A cor- rosion test of epsilon particle using Re as a surrogate for Tc also showed a preferential leaching of Mo followed by Re and a precipitation of secondary products contain- ing Mo and Re [7]. Well-controlled dissolution experiments were conducted both under oxic and anoxic conditions by Cui et al. [8, 9]. There are two important aspects in the cor- rosion behavior: (i) The leach rate of the metals is in the order of: (fast) Mo > Tc > Ru ∼ Rh ∼ Pd (slow). The leach rate of Mo, under reducing conditions, is estimated to be 1.5 × 10 −7 g/cm 2 /day which is ∼ 40 times faster than that