It's called a Pebble Bed Reactor
The Pebble Bed reactor takes advantage of a physics principle called Doppler Broadening:
When a reactor gets hotter, the accelerated motion of the atoms in the fuel increases the probability of neutron capture by U-238 atoms. When the uranium is heated, its nuclei move more rapidly in random directions, and therefore see and generate a wider range of relative neutron speeds. U-238, which forms the bulk of the uranium in the reactor, is much more likely to absorb fast neutrons.[1] This reduces the number of neutrons available to cause U-235 fission, reducing the power output by the reactor.In a nutshell, if the reactor temperature goes up, the amount of energy produced goes down, and the reactor temp stays stable. Instead of water, the PBR uses helium gas (which cannot be made radioactive by neutrons).In some reactor designs, such as the pebble bed reactor, this natural negative feedback places an inherent upper limit on the temperature at which the chain reaction can proceed. Such reactors are said to be "inherently safe" because a reactor failure cannot generate a criticality excursion. It is worth noting, however, that because of decay heat emitted from the decay of fission products, a meltdown is still theoretically possible if the ability to cool the reactor is lost, and thus the reactor design must be designed to prevent loss of coolant accident.
One cute aspect is the waste heat can be used in coal-to-gasoline conversion (which needs a high-temp steam input)