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Retention of transuranic and actinide elements and bismuth at the Oklo natural reactor, Gabon: Application of Eh—pH diagrams

Elsevier B.V.
Publication Date
DOI: 10.1016/0009-2541(78)90085-2


Abstract Several tons of 239Pu and lesser amounts of 240Pu and 241Pu were produced during the 0.5 Ma lifetime of the Oklo natural reactor, Gabon, some 1.8 Ga ago. Production of Pu isotopes is evidenced by slight excesses of 235U in otherwise 235U-depleted U-rich zones as well as by even slighter excesses of 232Th and 209Bi in the same zones. The low-grade ore, unaffected by nuclear reactions, contains very little Bi and Th, and U possesses a normal isotopic composition. The 241Pu-produced Bi-excess demands that 241Am, 237Np and other members of the 241Pu decay chain must not have migrated, since loss of any member from the 241Pu decay chain would prevent the Bi excesses from accumulating in the high-grade ore zones. Eh—pH diagrams for all species of importance indicate the following: Po is retained as the native metal in the Eh—pH range of interest, whereas Pu and Np are retained as PuO 2 and NpO 2, respectively. Am is probably retained as Am(OH) 3 or as an (Eu,Am) 2(CO 3) 3 species. The field of Bi 2S 3 is wide enough to prevent migration even under slightly oxidizing conditions. Collectively, these data argue for the retention of Pu, Am, Np, U, Po and Bi at Oklo and suggest that geologic sites elsewhere should be suitable for waste isolation.

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