What is surprising is that the idea has gotten so little traction before now and even now the interest is far less than it ought to be.
Our military navies have decades of safe experience with small reactors for transportation - submarines and aircraft carriers.
Yet, given the heavy weight of those power plants, we first might need to be planning on two stage long distance manned space flight.
The first stage would be on some sort of reusable vehicles that spend most of their fuel on a trip up to very high earth orbit where waits a space docking station for longer range space vehicles.
Those longer range vehicles will stay there for maintenance as well and avoid the heavy lifting costs of getting back to earth orbit.
The short range vehicles will need far less fuel to safely ferry their passengers and payloads back to earth than they needed to get to the orbiting docking station.
It is at that docking station the second stage would begin, using nuclear powered vehicles that will not need additional fuel to make many long range round trip space voyages. Keeping the reactors from overheating will use the cold void of space instead of seawater. Those longer range vehicles will stay there at the docking station for maintenance and awaiting subsequent voyages, avoiding the heavy lifting costs of getting back to earth orbit again.
Many science fiction writers already figured out what it has been taking NASA so long to admit.
Using what as reaction mass?