This is the best summary I could come up with:

In chemical rockets, hydrogen had to be mixed with an oxidizer, which increased the total molecular weight of the propellant but was necessary for combustion to happen.

Fuel rods made with uranium 235 oxide distributed in a metal or ceramic matrix comprise the core of a standard fission reactor.

This reaction is kept at moderate levels using control rods made of neutron-absorbing materials, usually boron or cadmium, that limit the number of neutrons that can trigger fission.

The hotter you make the exhaust gas, the more you increase specific impulse, so NTRs needed the core to operate at temperatures reaching 3,000 K—nearly 1,800 K higher than ground-based reactors.

Then there was the hydrogen itself, which is extremely corrosive at these temperatures, especially when interacting with those few materials that are stable at 3,000 K. Finally, standard control rods had to go, too, because on the ground, they were gravitationally dropped into the core, and that wouldn’t work in flight.

Los Alamos Scientific Laboratory proposed a few promising NTR designs that addressed all these issues in 1955 and 1956, but the program really picked up pace after it was transferred to NASA and Atomic Energy Commission (AEC) in 1958, There, the idea was rebranded as NERVA, Nuclear Engine for Rocket Vehicle Applications.

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