The uranium ore mined today contains only 0.72% of fissionable 235U, too little to make reactor fuel for thermal-neutron fission. For this reason, the mined ore must be enriched with 235U. Both 235U (T1/2 = 7.0 ( 108 y) and 238U (T1/2 = 4.5 ( 109 y) are radioactive. How far back in time would natural uranium ore have been a practical reactor fuel, with a 235U/238U ratio of 3.0%?