They are similar in nature to nuclear thermal rockets such as NERVA, but are considerably simpler and often have no moving parts.
In the rocket application the generator is removed, and the working fluid is instead used to produce thrust directly.
This means that the design, while highly efficient, can produce thrust levels of perhaps 1.3 to 1.5 N (0.29 to 0.34 lbf), making them useful only for thrusters.
In order to increase the power for medium-duration missions, engines would typically use fuels with a short half-life such as polonium-210, as opposed to the typical RTG which would use a long half-life fuel such as plutonium-238 in order to produce more constant power over longer periods of time.
Even without an exhaust, the photon pressure of the energy emitted by a thermal source can produce thrust, although an extremely tiny amount.