[wtallis]

Going based on numbers from Wikipedia, U-235 is good for 83.14 TJ/kg, but that isotope only has a natural abundance of 0.72%. So 3 micrograms of uranium in the natural proportions would get you around 1800 J ignoring any contribution from other isotopes (ie. you're not using a breeder reactor). That means your uranium extraction method needs to be very energy efficient indeed.

[acidburnNSA]

Oh yeah, you pretty much have to use breeder reactors to use seawater uranium. That has been the plan all along, e.g. Cohen 83: http://large.stanford.edu/publications/coal/references/docs/...