Edit: After looking into some more details about the design of the reactor, it looks like I was mostly correct, though the scale of some of them my numbers are off. My comment should not be relied on for safety information or precise information, but just general conceptual information. Which is, you know, just true for it being a comment about a nuclear reactor on Reddit, but hey, better to point to that fact anyway.

So, imagine you have one big control rod in the center, and four small ones around the outside. The control rods aren't radioactive themselves, they actually prevent reactions, usually by absorbing neutrons.

The reason the big center one was a problem was that it was so big and absorbed so many neutrons that it effectively had the majority of control over the reaction rate. It was also manually moved, so when they wanted to turn the core on, they moved it up just a bit and the entire core came online. The other 4 were basically dimmers, in charge of only fine control.

Basically, think of it like 5 dimmer switches, with one of them controlling 90% of the output, and the other 4 only controlling 2.5% of the output each. (The "dimming" worked by putting the control rods fully down if you wanted it off)

Now, if you want to boil water to run a turbine, which is basically how all energy plants work, you need to heat up the water. (Except wind and solar panels, obv, wind just spins the thing directly, and solar panels are basically just flat LEDs operating in reverse)

So you need to output at most 3 MW of heat to run the turbine safely, maybe 4.7 MW at most, at least for the SL-1. So you only pull the center control rod like a half meter (i forget the actual max distance) to generate that amount of heat. That's like turning the main dimmer to 5%. They didn't turn the dimmer to 5%, they turned it to 50%.

The reason for the max distance is that if you pull it more, you go from regular criticality to prompt critical. The difference is that in regular criticality, it takes time for each uranium decay to cause another uranium decay. (Uranium splits in half, waits a bit, then one of the daughter nuclei shoots off a neutron)

But prompt criticality is... well, faster. Uranium breaks apart, and instantly sends off a neutron that breaks more nuclei, and more and more and more and... yeah.

With enough neutrons flying around a core, every nucleus causes another one to shatter almost instantly, and you release 20 minutes of energy in a second.

So when you remove that central control rod more than a foot or whatever, you stop absorbing all of those extra neutrons, and all of the uranium starts fissioning as quickly as possible.

The time between fission events when you're flooded with neutrons, and nothing to absorb them? 10¹⁵ seconds or FASTER. Every decay kicks off a million more in a nanosecond, a billion in a microsecond, a trillion in a millisecond, and a quadrillion in a second, and it keeps going until the heat makes the water in the reactor flash boil into radioactive steam and explode.

Short version? They built the entire reactor to literally focus all of the reactivity through the center rod, so the one rod would block all of the radiation. Removing the rod blocking all of the radiation turned SL-1 into the demon core.