Uranium Redistribution Under Oxidizing Conditions in Oklo Natural Reactor Zone 2, Gabon
- PDF / 2,640,681 Bytes
- 8 Pages / 414.72 x 648 pts Page_size
- 20 Downloads / 194 Views
URANIUM REDISTRIBUTION UNDER OXIDIZING CONDITIONS IN OKLO NATURAL REACTOR ZONE 2, GABON H. ISOBE*, T. OHNUKI*, T. MURAKAMI**, AND F. GAUTHIER-LAFAYE*** *Japan Atomic Energy Research Institute, Tokai, Ibaraki, 319-11, Japan **Ehime University, Matsuyama, 790, Japan ***CNRS-Centre de G6ochimie de la Surface, 1 rue Blessig, F-67084 Strasbourg Cedex, France
ABSTRACT This mineralogical study was completed to elucidate the relationships between uranium distribution and alteration products of the host rock of natural reactor zone 2 at Oklo. The samples used were from the reactor core and reactor zone clays just below the reactor core. Uraninite is preserved without any alteration in the reactor core. Uranium minerals found to be present in the fractures in the reactor zone clays associated with iron-mineral veins, galena and Ti-bearing minerals. Uranium, for which the phases could not be identified, occurs in iron-mineral veins and the iron-mineral rim of pyrite grains in the reactor zone clays. Uranium is not associated with granular iron minerals occurring in the illite matrix of the reactor zone clays. The degree of crystallinity and uranium content of the three iron-bearing alteration products suggest that they formed under different conditions; the granular iron minerals, under alteration conditions where uranium was not mobilized while the iron-mineral veins and the ironmineral rim of pyrite, under conditions in which uranium is mobilized after the formation of the granular iron minerals. INTRODUCTION The natural nuclear chain reactions which occurred at an Oklo uranium deposit, Gabon 2 billion years ago provides us with information on the migration behavior of transuranium elements (TRU) and fission products and their decay products, as well as uranium series nuclides. Oklo is, thus, a unique natural analogue site for testing radioactive waste disposal concepts, as compared to normal uranium ore deposits, e.g., Koongarra, Australia. It is very important, because of this uniqueness, to examine "the natural reactors" and their near-field area geochemically, mineralogically, and radiochemically in order to develop a better understanding of the migration behavior of uranium and the other elements contained in the high level waste forms [e.g., 1]. The Oklo deposit is in a sandstone layer of 4 to 10 m in thickness [2,3]. The deposit is overlain with pelitic facies FB and underlain with sandstone FA [2,3]. The mineralized zone has a dimension of 900 m north-south by 600'm east-west. The layer is inclined to the east approximately at 100 to 500 from the horizon. Consequently, the depth of the ore body from the ground surface varies from 50 m to 500 m. More than 10 reactor zones having high grade uranium ores, are scattered across the whole ore body [3]. The first discovered reactor zones (No. 1 to 6) are located in a shallower region, and they were removed out by open pit mining. Mat. Res. Soc. Symp. Proc. Vol. 353 01995 Materials Research Society
1212
The reactor zones have experienced several geological events from t
Data Loading...
