Don't ignore the fact that uranium reserves would not last 100 years if we used it as our sole power source.
As demand goes up, so does supply, as additional reserves are prospected for and more costly uranium sources become economically viable.
There's not much reason to prove reserves if you won't be able to profitably mine them.
There just isn't much at current prices, but fuel costs are not significant so prices cam rise.
With that assumption, fission can last for a very long time if we get uranium from the oceans, and burn it in fast reactors. The oceans hold four billion tons of uranium, and with fast reactors we'd use it a hundred times more efficiently than we do today.