Oklo reactors and implications for nuclear science
  We summarize the nuclear physics interests in the Oklo natural nuclear
reactors, focusing particularly on developments over the past two decades.
Modeling of the reactors has become increasingly sophisticated, employing Monte
Carlo simulations with realistic geometries and materials that can generate
both the thermal and epithermal fractions. The water content and the
temperatures of the reactors have been uncertain parameters. We discuss recent
work pointing to lower temperatures than earlier assumed. Nuclear cross
sections are input to all Oklo modeling and we discuss a parameter, the
$^{175}$Lu ground state cross section for thermal neutron capture leading to
the isomer $^{176\mathrm{m}}$ Lu, that warrants further investigation. Studies
of the time dependence of dimensionless fundamental constants have been a
driver for much of the recent work on Oklo. We critically review neutron
resonance energy shifts and their dependence on the fine structure constant
$\alpha$ and the ratio $X_q=m_q/\Lambda$ (where $m_q$ is the average of the $u$
and $d$ current quark masses and $\Lambda$ is the mass scale of quantum
chromodynamics). We suggest a formula for the combined sensitivity to $\alpha$
and $X_q$ that exhibits the dependence on proton number $Z$ and mass number
$A$, potentially allowing quantum electrodynamic and quantum chromodynamic
effects to be disentangled if a broader range of isotopic abundance data
becomes available.
