For an example of how closely this is monitored see the Oklo fossil reactors[1]

The proportion of fissile isotopes being mined was off by a fraction of a percent, which caused the French government to launch an investigation. It turns out that millions of years ago the site had formed a natural fission reactor which depleted some of the fissile isotopes

[1]https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto...