Here is a link on this.
The argument for pebble bed is very convincing.
http://en.wikipedia.org/wiki/Pebble_bed_reactor
It combines much lower power density, much higher temperatures and well established pyrolytic materials technology.
A major plus, from thermodynamics budgets, is the single stage cooling system. It does not require a secondary cooling system and the primary coolant, CO2 or He or N2 drives the turbines.
This could very well be the future. It should have been for about 25 years.
Ed