How is NASA expecting this nuclear propulsion technology to become feasible? This NASA announcement provides some hope of nuclear-powered rockets for long trips.
I understand that, unlike chemical rockets that would be high thrust for shorter periods of time, a nuclear-powered rocket would be lower thrust (perhaps a lot less than 1 G) but applied all of the time for nearly as long as the first half of the trip.
Now you need propellant and you want to minimize the mass of the entire spaceship being accelerated.  It seems to me that to do that you need to propel the propellant out the back end as fast as possible so that you get as much thrust as possible from as little propellant as possible.
Do they use a linear accelerator in this?  It's hard for me to imagine that you would get a feasible rocket from just heating up the liquid into gas with the nuclear reactor.  It seems to me that you want to toss those xenon or krypton atoms out the back end at nearly the speed of light.
How are they planning to do this?
 A: 
Nuclear electric propulsion systems use propellants much more efficiently than chemical rockets but provide a low amount of thrust. They use a reactor to generate electricity that positively charges gas propellants like xenon or krypton, pushing the ions out through a thruster, which drives the spacecraft forward. Using low thrust efficiently, nuclear electric propulsion systems accelerate spacecraft for extended periods and can propel a Mars mission for a fraction of the propellant of high thrust systems.
Illustration of a Mars transit habitat and nuclear propulsion system that could one day take astronauts to Mars.

and

Nuclear thermal propulsion technology provides high thrust and twice the propellant efficiency of chemical rockets. The system works by transferring heat from the reactor to a liquid propellant. That heat converts the liquid into a gas, which expands through a nozzle to provide thrust and propel a spacecraft.

In the link it is clearly stated that a nuclear reactor will be used. A reactor provides energy and is used to generate electricity all over the world.  They are trying to use the power of a reactor either with small thrusts, or by creating steam for large thrusts.
They are talking of going to Mars, not Andromeda, feasible use of nuclear power.
