Today's nuclear reactors only extract about 2% of the energy in natural uranium, fast reactors could extract many times more by producing Pu239 and other fissile actinides from relatively abundant U238.

Since uranium is water soluble there are large amounts in the ocean and if fast reactors were made economical it would be practical to extract from seawater, seawater uranium is a renewable resource because uranium is always eroding from rocks and making its way to the ocean by rivers.

Thorium breeders do the same thing as plutonium breeders by converting Th232 to U233 except they can do it with a thermal spectrum. It's not so clear if terrestrial Th or U is really more abundant but Th is not in seawater because, like Pu it is not water soluble.