A rotor shaft adapted to rotate about a rotor axis thereof. The rotor shaft includes a rotor cavity configured concentrically to the rotor axis inside the rotor shaft. The rotor shaft further includes a plurality of cooling bores extending radially outward from the rotor cavity to feed cooling air into an internal cooling system in a blade. Each cooling bore includes a bore inlet portion and a distal bore outlet portion. The respective bore inlet portion ends in a plateau, projecting above the outer circumference contour of the rotor cavity. Thus, cooling bore inlets are shifted to a low stress area and the lifetime of the rotor is improved.