Record of Cycling Operation of the Natural Nuclear Reactor in the Oklo/Okelobondo Area in Gabon

Using selective laser extraction technique combined with sensitive ion-counting mass spectrometry, we have analyzed the isotopic structure of fission noble gases in U-free La-Ce-Sr-Ca aluminous hydroxy phosphate associated with the 2 billion yr old Oklo natural nuclear reactor. In addition to elevated abundances of fission-produced Zr, Ce, and Sr, we discovered high (up to $0.03{\mathrm{c}\mathrm{m}}^3\mathrm{S}\mathrm{T}\mathrm{P}/\mathrm{g}$) concentrations of fission Xe and Kr, the largest ever observed in any natural material. The specific isotopic structure of xenon in this mineral defines a cycling operation for the reactor with 30-min active pulses separated by 2.5 h dormant periods. Thus, nature not only created conditions for self-sustained nuclear chain reactions, but also provided clues on how to retain nuclear wastes, including fission Xe and Kr, and prevent uncontrolled runaway chain reaction.

Figure 1

Fission ${}^{136}\mathrm{X}\mathrm{e}$ and ${}^{86}\mathrm{K}\mathrm{r}$ in U oxides (open circles) and Al phosphates (solid squares) after minor correction to account for atmospheric contamination using ${}^{128}\mathrm{X}\mathrm{e}$ and ${}^{82}\mathrm{K}\mathrm{r}$. Xe concentrations were calculated from measured amounts of ${}^{136}\mathrm{X}\mathrm{e}$ and the amount of degassed material which was estimated from the specific density and geometry of the extraction crater. Fission ${}^{136}\mathrm{X}\mathrm{e}$ concentration and ${}^{86}\mathrm{K}\mathrm{r}/{}^{136}\mathrm{X}\mathrm{e}$ ratios in U-rich phases tend to be lower, suggesting a migration of fission products to Al phosphate.

Figure 2

Excess ${}^{130}\mathrm{X}\mathrm{e}$ (fission shielded) provides a mean for an estimation of neutron fluence from the slope of the dashed line. Data corrected to account for atmospheric contamination. Negative ${}^{130}\mathrm{X}\mathrm{e}/{}^{136}\mathrm{X}\mathrm{e}$ values are due to slight overcorrections.

Figure 3

Xe isotopic composition in U oxides and Al phosphates are related by a process that shifted points along the solid lines. Dotted lines illustrate a sole deficit of ${}^{136}\mathrm{X}\mathrm{e}$, which cannot explain the Xe anomalies in Al phosphate. Migration of all isotopes in each isobaric chain must be considered. Also shown are Xe components produced by the three potential progenitors (${}^{235}{\mathrm{U}}_{\mathrm{t}\mathrm{h}}$, ${}^{239}\mathrm{P}{\mathrm{u}}_{\mathrm{t}\mathrm{h}}$, ${}^{238}{\mathrm{U}}_{\mathrm{f}\mathrm{n}}$).

Figure 4

The calculated evolution of isotopic composition of intermediate fission products Sb, Te, and I, which hold up the production of stable Xe isotopes. Bold black lines correspond to the active period of the reactor, with the numbers indicating the duration of that period. Dashed lines illustrate free decay of Xe precursors. The gray horizontal lines show the compositions required to make Xe in Al phosphate from Xe in U oxides (as inferred by the slope of the solid lines observed in Fig. 3). The light gray line represents adjusted $⟨129⟩/⟨134⟩$ ratio assuming 37% losses of ${}^{129}\mathrm{I}$.