They tried this sixty years ago, with the SL-1 reactor and other efforts. Long before small computers became available.
SL-1 underwent a runaway criticality incident that killed three people. One got pinned to the ceiling with a control rod through his groin. That kind of took the wind out of the small military reactor research effort.
SL-1 used fuel that was weapons-grade, or close to it; so do submarine reactors.
“They tried this sixty years ago, with the SL-1 reactor and other efforts.”
Yes, they used a simple rope for control of Chicago Pile 1.
But that was 1941!!! On the south side of Chicago surrounded by a university and many houses/apartments.
https://en.wikipedia.org/wiki/SL-1
On January 3, 1961, the reactor was being prepared for restart after a shutdown of eleven days over the holidays. Maintenance procedures required that the main central control rod be manually withdrawn a few inches to reconnect it to its drive mechanism. At 9:01 p.m., this rod was suddenly withdrawn too far, causing SL-1 to go prompt critical instantly. In four milliseconds, the heat generated by the resulting enormous power excursion caused fuel inside the core to melt and to explosively vaporize