Indeed, in the second half of the 20th century, NASA proposed the concept of a "nuclear rocket," which was essentially "using a reactor to directly heat the propellant" and "ionizing the spent fuel after reaction to use it as propellant."
It's just that this concept never found much opportunity for application.
Mainly because...
The pollution is terrifying.
Spent fuel is something that can kill. Unlike most radioactive elements after a nuclear explosion that react instantly and cleanly, spent fuel can leave toxic residues for millennia.
Even if it's not directly expelling spent fuel, just expelling propellant directly through the reactor is quite scary enough.
However, if not using the reactor to directly heat the propellant but indirectly heating it, then the thrust-to-weight ratio of the reactor compared to chemical rockets holds no advantage.
But this is not a problem at all for "Argon."
