Steam power I can imagine will be with us for a long time yet.

Even many nuclear fusion reactor schemes entail stearm driven turbine powered electrical utility stations.

In current fission reactors such as those used for commercial nuclear power plants and also those used in some nuclear powered warships deployed by the various military powers around the globe, the heat generated within the fission reactor is partially converted into dynamo turbine rotation energy via steam pressure.

On can imagine that in the comming decades and centuries; nuclear fission, nuclear fusion, and perhaps matter-antimatter reaction heat produced within nuclear reactors and matter-antimatter reactors could be used to power ion, electron, or photon rockets, or electrodynamic-hydrodynamic-plasma drive mildly to highly relativistic velocites.
The heat generated would be used to drive highly efficient high power output steam turbine driven dynamos.

The use of well shielded accelerator technology to accelerate charged particles to very high gamma factors might make for considerably more effiecient fission and fusion powered space craft when compared to traditional fission and fusion rocket powered schemes. Note that fusion rockets are typically conjectured to have a maximum exhaust velocity of at most 0.1C while in theory, charged particles used as a reaction mass in an electrodynamic rocket can be accelerated to relativistic gamma factors many orders of magnitude greater than that which the Large Hadron Collidor at CERN will accelerate protons when it comes back on line next Summer.