A 1966 Aerojet concept for a space probe with a nuclear reactor and ion engines. Note the largish thermal radiator “wings;” such things are usually left off spacecraft in science fiction, but they are a vital part of any nuclear spacecraft. Nukes, after all, are simply heat sources; in order to get useful electrical power out of them, the heat must be used to boil a working fluid which runs a turbogenerator; and the hot gas then needs to be condensed back to a liquid by radiating the heat way to space. And thermal radiation is a terribly slow and weak process, necessitating large radiators. Electricity can also be created with thermionic systems, which generate electricity across a thermal differential… hot on one side, cold on the other. But unless the cold side it attached to some radiators, the cold side will soon be just as hot as the hot side, and then… no thermal differential, no power generation.
Note also that even with a substantial powerplant and the sizable bank of ion engines, acceleration is going to be creakingly slow. Thus you can get away with spindly structures. The reactor itself is the tiny little tin can-looking thing, top and centerline; the U-shaped structure around it is a radiation shield protecting the electronics, structure and radiators from the radiation spat out by the reactor.