A rotor of a turbomachine includes a rotor drum located at a central axis and a plurality of buckets secured to the rotor drum. A rotor shell extends between axially adjacent buckets of the plurality of buckets and is secured to and supported by the plurality of buckets defining a cooling passage between the rotor drum and the rotor shell. A low pressure sink is located at an upstream end of the rotor receptive of a coolant flow through the cooling passage. A method of cooling a rotor of a steam turbine includes locating a rotor shell radially outboard of a rotor drum defining a cooling passage therebetween. A flow of steam is urged from a downstream portion of the steam turbine through the cooling passage toward a low pressure sink located at an upstream end of the steam turbine thereby cooling the rotor.