An approximate description of the Oklo uranium deposit is 90% uraninite (UO₂) by mass, saturated with water that acted as the moderator. Assume that the Oklo deposit was homogeneously saturated with water with a ratio of 3.5 uranium nuclei per water molecule and compute the minimum enrichment (²³⁵U/²³⁸U) needed to sustain a fission reaction. You can ignore the moderating effect of the extra oxygen in the uraninite (it is negligible). You can also ignore the neutron-absorption cross section of the oxygen in the uraninite and of any other material in the deposit except for the water.