  Recent years have seen a renewed interest in the Oklo phenomenon,
particularly in relation to the study of time variation of the fine structure
constant. The neutron fluence is one of the crucial parameters for Oklo
reactors. Several approaches to its determination were elaborated in the past.
We consider whether it possible to use the present isotopic La138/139 data for
RZ10 as an additional indicator of neutron fluences in the active cores of the
reactors. We calculate the dependence of the Oklo La138 abundance on neutron
fluence and elemental lanthanum concentration. The neutron fluence in RZ10 can
be deduced from lanthanum isotopic data, but requires reliable data on the
primordial elemental abundance. Conversely, if the fluence is known, the
isotope ratio provides information on the primordial lanthanum abundance that
is not otherwise easily determined.
