Flow distribution system for coolant in a nuclear reactor and method

A nuclear reactor including an inner core containing both fuel and blanket assemblies and means for cooling the assemblies by passing liquid coolant, specifically sodium, across and in contact with the latter is disclosed herein. The nuclear reactor also includes a particular technique for distributing the liquid coolant into first and second separate plenums, each of which includes an inlet adapted to receive the coolant. These first and second plenums are maintained in fluid communication with the fuel and blanket assemblies, respectively, whereby to pass received coolant across and in contact with all of the assemblies. In addition, the distribution of the coolant is automatically changed, at least to a limited extent, between these first and second plenum in response to predetermined thermal changes in said assemblies