Systems for increasing the efficiency of a turbine and an overall power plant system are disclosed. In one embodiment, a turbine includes: an outer shell including a set of grooves configured to complement components of a wheel and diaphragm steam path section; a drum rotor disposed within the outer shell; a set of shell converts connected to the outer shell via the set of grooves, the set of shell converts configured to complement components of the drum rotor; and a working fluid passage substantially defined by the drum rotor and the set of shell converts.