Fun fact: the ratio of U-235 to U-238 in natural uranium varies over time, as U-235 has a half-life of about 700 M years, while U-238 has a half-life of 4.5 G years. In Earth's distant past, natural U-235 was plentiful enough that criticality could be reached without enrichment, and actually occurred at what are now known as the Oklo fossil reactors.

Check out my other nuclear mods:

Nuclear Power Rebalance: Rebalances solar and nuclear energy values to make nuclear energy realistically overpowered.

Plutonium Breeding: Adds Pu-239 to the nuclear fuel cycle and replaces Kovarex enrichment with plutonium breeding.