I've heard very little about pebble bed reactors in recent times. They are inherently more stable, and don't use control rods to limit the reaction. They're meant to be FAR safer.
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The core generates less power as its temperature rises, and therefore cannot have a criticality excursion when the machinery fails, it is power-limited or inherently self controlling due to Doppler broadening. At such low power densities, the reactor can be designed to lose more heat through its walls than it would generate. In order to generate much power it has to be cooled, and then the energy is extracted from the coolant.
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Wiki: Pebble Bed Reactors -
http://en.wikipedia.org/wiki/Pebble_bed_reactor