Abstract: A rotor cooling circuit includes a cooling medium supply reservoir for supplying cooling medium to a plurality of axial passages through the turbine wheel and spacer disc rims. A passageway communicates cooling medium from the reservoir for flow in series and in a serpentine manner in and about the cavities between the wheels and discs. The flows are reunited at a plenum for flowing the cooling medium into first and second stage buckets. Return passages through the rims of the wheels and spacer discs return spent cooling medium from the buckets to a return location external of the rotor. Transition elements between the buckets and spacer plates in the rotor rim accommodate any thermal mismatch between the adjoining buckets and spacer plates to prevent cooling medium leakage at the joints.