Nuclear reactor turbine vs piston

Just wondering, why do nuclear reactors heat up water and have the water turn a turbine? Wouldn't water pushing pistons be more efficient? Instead of the heat of the steam being wasted after the steam goes past the turbine, almost all of the energy could be used to move a piston.

the water* is heated to the point where it becomes steam. it is only this rapid expansion caused by evaporation that can drive the turbine(s). And since you use the turbine to drive a generator, it is FAR simpler to directly drive the shaft with the turbine instead of complicated piston engines.

* at least in modern nuclear power plants, there are actually 2 separated water cycles: the one to cool the reactor is closed (because of possible radioactive contamination - depending on reactor type). heat exchangers are used to heat the water in the second cycle that drives the turbine(s).


edit: sorry for the lecture about evaporation. you didn't mention steam in the first version of your post

but how do you propose to use more of the energy with a piston? after the piston has moved to the point where it gets pushed back from the other side, the steam probably is almost as hot as after the turbine?

thanks

btw: depending on the location of the power plant, cogeneration is a far better way to increase efficiency.

But that's only really useful in densely populated areas - not really the kind of area where nuclear power plants are normally built

There is absolutely no reason why you couldn't build a piston engine driven by nuclear-heated steam. And yes, it could be designed to capture more of the energy as done with multiple stage turbines and piston + turbos in automobiles. Steam engines used pistons for years. I'm sure the cost-versus-XXX curves deviate wildly when you push to large sizes and efficiencies.

A large heavy steam turbine for the same size/cost as a piston engine might also have fundamental advantages of reliability (fewer moving parts, all continuous motion), ease of maintenance and lower vibrations.

Doesn't the steam cool as it expands?