The idea is that you would have pieces of plutonium or U233 close enough that if left to their own devices they should go supercritical in a fraction of a second. However, in between the pieces you have a layer of magical darkness which can absorb any radiation without generating any waste heat.
Importantly magical darkness can respond to a variety of stimuli and be given complex preprogrammed instructions, which it will then carry out at light speed. This is crucial as it allows the magical darkness to modulate neutrons on an otherwise impossibly short timescale. Magical darkness is physically intangible and can occupy the same space as any material which is fully or semi-transparent.
So say you have 30 lbs worth of plutonium pieces closely packed together, or embedded in a semi-transparent matrix which can carry magical darkness within it. This setup should go supercritical rapidly: So you have the darkness pull back to briefly allow a portion to go supercritical, before moving so that the neutrons can force another now unshielded section to go supercritical, while tamping down the first portion so it goes back to merely critical.
Another much simpler, but I suspect lower output idea is having the magical darkness repeatedly pull back so the reactor goes supercritical for a split second, then tamp things down and repeat the process in a cyclical manner such that the average output is a bit below whatever temperature your reactor materials can handle.
The idea being that you could turn an arrangement of plutonium and neutron reflectors sufficient to go supercritical into a scalable power source. Modulating the neutrons to stop the feedback loop once you reach your desired level of power output.
Is this notion of moving a region of super-criticality around extremely quickly within a reactor to increase its effective output actually plausible?
Since any radiation absorbed by the magical darkness is wasted, the fuel efficiency of such a reactor would be terrible, but that's not a concern in this scenario. Similarly don't be too worried about nuclear contamination in your answers, since the people of this world are about as resilient as those fungi that live in the remains of Chernobyl's reactor.
Such a reactor has very obvious applications in that it lets you make reactors that weigh considerably less than 22 tons. Similarly if you used the hot fuel to directly generate thrust like Project Pluto then ought to allow you to make a 50 lb W54 warhead sized nuclear thruster. Given magical darkness can totally shield you from radiation this seems like it could even allow for flying nuclear powered exosuits. Also people would obviously use the magical darkness for radiation shielding and not just to absorb neutrons within these reactors.
I also find this idea fascinating because it seems like in theory you should be able to come up with a way to do this in real life in order to get super tiny reactors.
For instance by having some neutron reflectors and some neutron poisons in a vacuum, being moved around magnetically extremely quickly. So as to repeatedly get the reactor to go super-critical for brief periods, making its average output nearly as hot as you can handle. Though making such a super-critical reactor safe seems inherently very difficult.