There's more Thorium in the crust, there's more Uranium in seawater. With either fuel (plus breeder reactors), the nuclear fuel supply of earth is unlimited on a several billion year timeframe at world-scale.
Here's an entire special issue (from 2016) of Industrial & Engineering Chemistry Research describing progress in extracting uranium from seawater: http://pubs.acs.org/toc/iecred/55/15
