Systems and methods for cooling a rotor assembly disposed within a cavity of an expander fluidly coupled with a cooling source are provided. The system may include an annular body disposed on a rotor disc of the rotor assembly. The rotor disc may also include a plurality of rotor blades mounted thereto via respective roots. The annular body may define at least one fluid passageway fluidly coupling the roots and the cooling source. The annular ring may be configured to substantially prevent mixing of the flue gas with a coolant provided by the cooling source and flowing through the at least one fluid passageway and contacting at least one root. The system may also include a plurality of seal members, each disposed between respective platforms of adjacent rotor blades and configured to substantially prevent the flue gas flowing though the expander from mixing with the coolant.