If you're talking about a Pu238 natural decay type device, like RTGs use, then at the level of power output you'd need to be able to supply propulsion the fact

If you're talking about a Pu238 natural decay type device, like RTGs use, then at the level of power output you'd need to be able to supply propulsion the fact that you can't control the reaction rate would be a distinct negative, not a positive. Naval reactors are basically a miniaturised version of a power station reactor. Some of the latest HEU ones are designed for the fuel to last for the life of the boat.