And China is building some right now. The HTR-PM pebble bed reactor is actually two reactors connected to a single steam turbine, for a total of 210 MWe. They're building one of these two-reactor modules now, and they're planning another 18 reactors on that same site once they've got the kinks worked out and the factories set up to start really cranking them out. More on the current status and technical and economic details here:

http://nextbigfuture.com/2010/02/current-status-and-technica...