Abstract: Operation of a liquid-metal-cooled fast-breeder reactor is enhanced by distributing coolant among the fuel assemblies at such a rate that the strain equivalent limiting temperature of each fuel assembly is closely approached but not exceeded. Further improvement is attained by proportionally allocating less coolant of a total prescribed coolant flow to the outer fuel assemblies of the reactor (yet satisfying the condition that the strain equivalent limiting temperature is not exceeded) whereby additional coolant flow is available for the inner fuel assemblies, thereby further reducing the radial thermal gradient in the coolant immediately above the fuel assemblies, a particular advantage being that the temperature of the upper internal structure of the reactor immediately above the inner zone of the reactor is reduced.