Henry Spencer wrote:

Likely recoverable reserves of U-235 (not counting seawater and granite,
both of which are very difficult to mine economically) correspond to about
300 TW-yr of energy.

I don't understand why you say this about seawater uranium. Estimates
for the cost of extraction using the Japanese amidoxime technology
are as low as 10,000 yen/pound (with some development):

http://npc.sarov.ru/english/digest/1...appendix8.html

(a web page containing a copy of the results of a Japanese
study.)

IMO, the advances needed to reach this performance level for
this technology are much less than those needed to bring SPS
to maturity. At $100/lb -- less than a factor of three above
the last spot market price of U I saw -- the uranium should
be affordable in a once-through cycle. Reprocessing doesn't
make economic sense (from avoided uranium costs) until natural
uranium reaches around $700/lb, IIRC.

Paul