[Yo-Yo]

I assume Rolls-Royce is using the nuclear reactor to generate electricity for an Ion Drive engine.

The hardest part about using a nuclear reactor for space travel is making it light enough to launch, yet robust enough to survive an unplanned 'launch event.'

[Lonesome in Massachussets]

The New Horizons vehicle that went to Pluto uses (unironically) a plutonium power source for electricity but not for propulsion.