a long nuclear-powered spacecraft with plasma rocket propulsion

Nuclear powered rockets have been proposed for decades. A spacecraft capable of supporting 200 personnel (including colonists) could realistically have an onboard nuclear fission powerplant rated at much greater than 100 megawatts.

[Personally, I think that pebble bed nuclear reactor protocols could be modified for both maritime and off-world applications. Adding thorium to the fuel mix might save mass & expense. The "pebbles" (or their substitutes) would facilitate safer nuclear waste disposal.]

Cooling the powerplant in outer space would be problematic. Hence, I'm assuming helium as the nuclear reactor's main coolant. Heat dissipation fins are visible in the picture.

Xenon is an ideal propellent for plasma & ion rockets. Since considerable radioactive material would already be onboard, could positrons (antielectrons) be harvested from select, radioactive isotopes?

(Using antimatter would enhance the effectiveness of the plasma rockets.)