As far as I understand, at least with some designs, the fuel elements have the property that it overall becomes less reactive above a certain temperature (that

As far as I understand, at least with some designs, the fuel elements have the property that it overall becomes less reactive above a certain temperature (that the elements can withstand without damage). So it's basically self- moderating, and the cooling system only has the job to extract the energy to generate electricity. At least that's the theory.