Modeling heat and fluid transfers during natural nuclear reaction in the Oklo Uranium deposit (Gabon)

in: 4th Joint Oklo Working Group CEC-CEA and final meeting of the "Oklo as a natural analogue" programme 1991-1995, Saclay

Abstract

The temperatures and water circulation around a fission reactive zone in the Oklo uranium deposit has been quantified using a numerical modeling approach. Mass balance, maximum fluid velocities and the extension of the dispersion zone in the surrounding rocks have been evaluated assuming several hypothesis on the duration of the fission reaction, the mean energy released during the functioning of the reactor and the permeability of the geological formations. During the fission reaction, the temperature at the center of the·reactor increases by 50 to 250°C above the regional value, depending on the assumed permeability and the heat production caused by the reactor. However; the thermal perturbation around the center of the reactor extends to less than 50 m. Because the temperature increased rapidly around the reaction zone, the pressure of the fluids trapped in .the porous medium induced over-pressuring in the surrounding rocks, causing hydraulic fracturing in the vicinity of the reaction zone. The extent of fracturing can be estimated to be 15 to 20m. The modeling of the transitory episode during which the reaction started shows that the center of the reactor reached its maximum temperature (over 400°C) in a very short period of time, less than 1000 yr., compared with the assumed duration of the fission reaction (more than 100 000 yr., depending on the reactor to be considered). This implies that fracturing of the surrounding rocks would have occurred during the early stages of the process, leading to an increase in the permeability of the surrounding rocks. Depending on the assumed hypothesis concerning the permeability in the vicinity of the reactor, the filtration velocities are low (about 10-8 to 10-12 m.s·' , the most realistic case being a few mm.yr1), compatible with a conductive thermal regime. Several models have been tested to estimate the maximal temperature range in the vicinity of the reactor. To reach a maximum temperature over 400°C, a value recorded by the fluid inclusion data (V. Savary, M. Pagel and F. Weber, (submitted», the functioning of the reactor must have continued for only a short period of time (less than 100000 yr.), and the mean heat production must have been greater than 500W.m-3 . Moreover, despite the low values of the fluid velocity, the duration of the reaction in the zone 10 (SF 84), implies that the quantity of fluid which circulated through the fission zone may have had an important effect on the dissolution of primary minerals or precipitati?n of secondary minerdls in the surrounding rocks.

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    BibTeX Reference

    @INPROCEEDINGS{,
        author = { Royer, Jean-Jacques and Gerard, B. and Le Carlier de Veslud, Christian },
         title = { Modeling heat and fluid transfers during natural nuclear reaction in the Oklo Uranium deposit (Gabon) },
         month = { "jun" },
     booktitle = { 4th Joint Oklo Working Group CEC-CEA and final meeting of the "Oklo as a natural analogue" programme 1991-1995 },
          year = { 1995 },
      location = { Saclay },
      abstract = { The temperatures and water circulation around a fission reactive zone in the
    Oklo uranium deposit has been quantified using a numerical modeling approach. Mass
    balance, maximum fluid velocities and the extension of the dispersion zone in the
    surrounding rocks have been evaluated assuming several hypothesis on the duration of
    the fission reaction, the mean energy released during the functioning of the reactor and
    the permeability of the geological formations.
    During the fission reaction, the temperature at the center of the·reactor increases
    by 50 to 250°C above the regional value, depending on the assumed permeability and
    the heat production caused by the reactor. However; the thermal perturbation around
    the center of the reactor extends to less than 50 m. Because the temperature increased
    rapidly around the reaction zone, the pressure of the fluids trapped in .the porous
    medium induced over-pressuring in the surrounding rocks, causing hydraulic fracturing
    in the vicinity of the reaction zone. The extent of fracturing can be estimated to be 15
    to 20m. The modeling of the transitory episode during which the reaction started shows
    that the center of the reactor reached its maximum temperature (over 400°C) in a very
    short period of time, less than 1000 yr., compared with the assumed duration of the
    fission reaction (more than 100 000 yr., depending on the reactor to be considered).
    This implies that fracturing of the surrounding rocks would have occurred during the
    early stages of the process, leading to an increase in the permeability of the surrounding
    rocks. Depending on the assumed hypothesis concerning the permeability in the
    vicinity of the reactor, the filtration velocities are low (about 10-8 to 10-12 m.s·' , the
    most realistic case being a few mm.yr1), compatible with a conductive thermal regime.
    Several models have been tested to estimate the maximal temperature range in
    the vicinity of the reactor. To reach a maximum temperature over 400°C, a value
    recorded by the fluid inclusion data (V. Savary, M. Pagel and F. Weber, (submitted»,
    the functioning of the reactor must have continued for only a short period of time (less
    than 100000 yr.), and the mean heat production must have been greater than 500W.m-3
    . Moreover, despite the low values of the fluid velocity, the duration of the reaction in
    the zone 10 (SF 84), implies that the quantity of fluid which circulated through the
    fission zone may have had an important effect on the dissolution of primary minerals or
    precipitati?n of secondary minerdls in the surrounding rocks. }
    }