For resources to be counted as reserves, they have to be found. If no one bothers finding more resources, the resources that lie in wait are not counted as reserves. Here is some finding-cost data:

http://www.world-nuclear.org/info/inf75.html

the finding costs of crude oil have averaged around US$ 6/bbl over at least the past three decades. When finding costs of the two fuels are expressed in terms of their contained energy value, oil, at US$ 1050/MJ of energy, is about 300 times more expensive to find than uranium, at US$ 3.4/MJ. Similarly, the proportion of current market prices that finding costs comprise are lower for uranium. Its finding costs make up only 2% of the recent spot price of US$ 30/lb ($78/kgU), while the oil finding costs are 12% of a recent spot price of US$ 50/bbl.

By these measures, uranium is a very inexpensive energy source to replenish, as society has accepted far higher energy replacement costs to sustain oil resources.

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If those finding costs were considered too high, one would additionally have to hope that no one would be able to find the sea, or that if anyone were able to find the sea, that they would not be able to figure out how to extract uranium from it for, perhaps, only several hundred dollars per kg -- something which in fact has already been done, and which is being improved-upon by current research conducted by Japan and India.