A nuclear reactor where fusioning atoms produce energy. Basically, you throw light atoms (usually a mix of deuterium and tritium as that is easiest to ignite) into a chamber where you heat it to incredible temperatures and compress it to incredible pressures. As soon as you reach the individual ignition condition of the reactor, you achieve nuclear fusion which keeps going all by itself and produces huge amounts of energy.

To cool the beast (it's grillions degrees centigrade hot!), you'd use something like lithium as your primary coolant and in turn cool that with water to generate steam. It might also work with Gründels, but that's really only water, too.

Pro: Clean and very powerful, as opposed to nuclear fission.

Contra: No one got it to work yet, but the Man is working on it.

The most promising type of fusion reactor currently being tested is the Tokamak type, where your reaction chamber is torus-shaped, wrapped in supraconducting coils to keep the incredibly hot plasma contained. Using a torus makes it possible to keep the extremely energised plasma particles (ions) going around and around all the time so you don't need to have them reflected by a magnetic mirror every so often. This is why end-to-end fusion test reactors with cylindrical chambers have gone largely out of fashion.