http://www.ted.com/talks/kirk_sorensen_tho...clear_fuel.html

Good video. but thinking the downsides? and we haven't done this already.

The obstacles are:
1) lack of experience in materials that can handle molten salts at high temperatures for long periods of time
2) safety issues with circulating liquid fuel and fission product barriers to prevent release
3) safety issues with circulating liquid fuel and reactivity control
3) requirement to use high enriched uranium/plutonium
4) requirement to have on-site reprocessing (proliferation risk)
5) NRC inability to consider alternative nuclear technology that falls outside of their PWR/BWR pigeonhole

All except 3 and 4 are valid. These are addressed in Sorensen's talks. It was impressed to watch.

Hope the technology will make a comeback in the future.