Re: So basically....
It's the "Son of SL-1"
There are (unfortunately) a couple of significant similarities...
a) small, low power reactor that's portable
b) a single control rod can make it go critical
If there is a way of doing a safety shutdown that does not involve "that one control rod" it would certainly be a LOT safer This is because the fission products in a fission reactor are often "physically bigger" than the original material. Reason: higher density for uranium, lower density for "what it splits into". basic physics.
Basically, the fission process causes 'fuel swelling' as the fuel is burned off. This could cause 'blistering' and/or 'swelling' of the fuel material to an extent that it impedes control rod motion, depending on the design.
And that "single control rod" can EASILY get stuck as the fuel changes its shape over its lifetime. This means you lose control of the reactor. Oops.
To compensate for this, designers need to make sure that you have rod channels that resist the effects of swelling and blistering, as well as having "some other means" to shut the thing down.
NASA needs to work with the U.S. Navy on this (if they aren't already), as well as General Electric and Westinghouse and other military contractors that know how to build small reactors for submarines.
For a bunch of reasons, the reactor core needs to be very compact, as compact as possible. This not only reduces the fuel loading, but it improves the temperature coefficient. Small reactors tend to have negative temperature coefficients, which means that as temperature goes up, reactor power goes down. this makes it stable. Very large reactors tend to have positive temperature coefficients, which means you can't change power levels very fast, and have to constantly monitor it and tweek it to keep power levels stable, because an increase in temperature makes power go UP [in a transistor they call this 'thermal runaway']. Large reactors, however, can have a much larger fuel load so (in theory) you could go longer without refueling. Additionally, a large reactor could be fueled with 'fuel pellets' inside of a permanent tube structure, rather than alloyed fuel material that has to be completely changed out, making the refueling process a lot simpler.
So large reactors have their advantages. they're just WAY harder to control.
In this case the small reactor would be perfect. Down side, it would very likely have 'alloyed' fuel, meaning that it would be subject to swelling and blistering that could cause the single central control rod to stick, resulting in a potentially uncontrollable reaction. The end result could be a partial meltdown, but probably would just overheat everything and damage it beyond repair. Or, it might just slowly drop in temperature until it shuts itself down. There's no telling. Any of these are possible for a number of non-obvious reasons, caused by changes in power demand [among other things], and despite the highly negative temperature coefficient, it's still possible [following a large power transient] for a stuck rod to cause power levels to go unstable and either melt it down right away. or temporarily shut it down and make it go into a 'cyclic' uncontrolled shutdown/restart mode that results in an unrecoverable power transient that completely destroys the thing. Not 'boom', just melt.
So, yeah. That single control rod is a big concern.