It is 100 years of proven reserves. “Proving” a reserve is expensive, mining companies do that in order to secure favorable financing. But when there are 100 of proven reserves, there is little economic incentive to go and prove new reserves. However, if the industry demand increases, because of new build, then initially the proven reserves will suffice only for, say, 80 years, the incentive to prove new reserves appears, and mining companies spend the necessary funds to “prove” new reserves.

In reality, the amount of uranium available on this planet is virtually unlimited. There are billions of tons of it in seawater, and it is estimated it would take onle a 4x market price increase for seawater uranium extraction to be economically profitable.