Ree Elements Distribution

Distribution of REE in Uranium Minerals in Oklo-Yucca Mountain’s Future

Abstract

Rare earth elements concentration and mobility in Oklo which was a natural reactor zone in 2 billion years ago was investigated via LA-ICP-MS. Changes in concentration were observed as a function of alteration. The spent nuclear fuel which was uraninite mineral with impurities showed high concentration of REE while the altered part was depleted in REE because of the high mobility of REE. Some REE such as 140Ce, 146Nd and 147Sm showed higher mobility compared to other REE. Because of the nature of Oklo which can be an analogue of spent nuclear fuel repository the results could predict the future of Yucca Mountain which is a nuclear repository below the groundwater level. It could be concluded that the when the Yucca mountain is exposed to an oxidizing condition, it will be altered and the fissiogenic REE elements can leave the spent nuclear fuel spread out in the environment.

Introduction

The ‘‘Oklo phenomenon’’, is one of the most amazing and surprising discovery of the 20th century that was made in the domains of geosciences and nuclear physics (Bodu et al., 1972; Naudet, 1991; Neuilly et al., 1972). The ratio between two most abundant U isotopes, 235U (0.72%) and 238U (99.28%), is remarkably constant and only small deviation from the present-day value of 0.0072 have been observed in nature. There is, however, one place on Earth that we know, where the 235U/238U deviates dramatically and can be as low as 0.0038. This is Oklo in which sustained fission chain reactions occurred in these U deposits. From 1972 to 1988, fifteen natural nuclear reaction zones (RZ) in the 2 billion years old uranium deposits in Gabon, namely Oklo and Bangombe have been discovered. Isotopic analyses of U in the ore and the discovery of fission products provided firm evidence that spontaneous nuclear chain reactions had indeed occurred at Oklo (Neuilly et al. 1972). The location therefore represents a natural analogue of a nuclear waste disposal site and has been intensely studied with respect to the possible migration of radionuclides. The abundant mineral of U is Uraninite (UO2+X) which is stable under reducing condition and when it is exposed to oxidizing condition  U4+ within uraninite oxidizes and make U6+. After time, addistional oxygen atoms as well as rare earth elements (REE) may be incorporated to maintain charge balance. When this mineral become altered as a primary alteration or secondary alteration, REE and other impurities of uraninite can move and redistribute while carrying U with themselves. It is of major concern to investigate the behavior of REE in geosphere, since REE are highly produced by fission and their distribution behavior is closely related to geochemical events. In this study the mobility of REE in Oklo has been investigated in order to define and predict the future of Yucca Mountain which is a spent nuclear fuel repository.

Previous Studies

The possibly that large-scale nuclear recations may have occurred in the Earth’s crust was first proposed by Noetzelin in 1939 and Odagiri in 1940 (cf. Kuroda 1975). In order to understand under which geological and physical conditions natural fission reactions could spontaneously start in a natural environment, to determine the effect of such reactions on the surrounding rocks (Pourcelot and Gauthier-Lafaye, 1999) and finally to get information on the behavior of actinides (Bros et al., 1993; Hidaka and Holliger, 1998; Kikuchi et al., 2007) and fission products that were stored for a long period of time in a geological environment, advanced geological, geochemical and physical studies were conducted (AIEA, 1975, 1977; Blanc, 1996; Gauthier-Lafaye et al., 2000; Naudet, 1991; Stille et al., 2003).

Geological Setting

The Oklo, Okélobondo, and Bangombé uranium ores occur in Palaeoproterozoic deposits in the Franceville basin, southeast Gabon.

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Fig.1: Cross-section through the Oklo and Oke´lobondo deposits and location of the different reaction zones (after Gauthier-Lafaye et al., 1996).

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Fig.2: Cross-section through the Oklo reaction zone 2 (after Gauthier-Lafaye et al., 1996).

Sample Preparation and Analysis

A purchased thin section of Oklo-Reactor zone 10 was prepared for analyses. The sample had 2 main parts: uraninite with dark color and altered uraninite with yellow color. At first, the sample was analyzed with minimal sample preparation using XRF (X-Ray Fluorescence) spectrometer. The elemental concentration and the XRF map of the thin section showing the probable existing area of each element were gained from XRF. The XRF analysis could give us better view on the part of sample interesting to us.

Secondly, the sample was analyzed through SEM (Scanned Electron Microscope). 100 images were taken via SEM from the different phases of different regions in order to choose the best part of each region of the sample- uraninite in black or altered uraninite in yellow color. The SEM analyzed 10 desired areas and gave us the elemental concentration of each area. The elemental concentration for Al which was more accurate and precise compared to XRF analyses was used as internal standard for LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry).

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Fig.3: SEM analyses of the altered part of the sample-yellow

Finally, 8 selected region of the different part of the sample, uraninite and altered uraninite, were ablated using a New Wave Research UP213 solid—state laser ablation system. A 30 μm spot size was used with 5 Hz repetition rate and 50% intensity. Analysis of each location included one minute each of background and sample time.

The finnigan ELEMENT2 High Resolution ICP-MS was used for determining the concentration of rare earth elements. Standard sample bracketing was done via NIST SRM 610 which was chosen based on the approximate concentration of rare earth elements in the sample. The GLITTER (GEMOC Laser ICPMS Total Trace Elements Reduction Software package) software processed the data obtained from ELEMENT2. The concentration of Al which was obtained from SEM analyses was the input internal standard element for GLITTER.