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Oh, here is how...
from second link in post 13.
thanks RS
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Fission systems - propulsion

For spacecraft propulsion, once launched, some experience has been gained with nuclear thermal propulsion systems (NTR) which are said to be well developed and proven. Nuclear fission heats a hydrogen propellant which is stored as liquid in cooled tanks. The hot gas (about 2500°C) is expelled through a nozzle to give thrust (which may be augmented by injection of liquid oxygen into the supersonic hydrogen exhaust). This is more efficient than chemical reactions. Bimodal versions will run electrical systems on board a spacecraft, including powerful radars, as well as providing propulsion. Compared with nuclear electric plasma systems, these have much more thrust for shorter periods and can be used for launches and landings.

However, attention is now turning to nuclear electric systems, where nuclear reactors are a heat source for electric ion drives expelling plasma out of a nozzle to propel spacecraftspacecraft already in space. Superconducting magnetic cells ionise hydrogen or xenon, heat it to extremely high temperatures (millions °C), accelerate it and expel it at very high velocity (eg 30 km/sec) to provide thrust. Research for one version, the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) draws on that for magnetically-confined fusion power (tokamak) for electricity generation, but here the plasma is deliberately leaked to give thrust. The system works most efficiently at low thrust (which can be sustained), with small plasma flow, but high thrust operation is possible. It is very efficient, with 99% conversion of electric to kinetic energy. Heatpipe Power System