This is all somewhat complicated. Most modern designs claim that if you just switch everything off, the reactor is safe. This is either because having no machi

This is all somewhat complicated. Most modern designs claim that if you just switch everything off, the reactor is safe. This is either because having no machinery around the reactor working still makes it safe, or the few critical components have tons of redundancy, and everything is designed to be “broken”. 300MW is what the reactor produces with the moderators retracted. You put them in place and the reaction stops immediately. So you don’t need to cool 300MW passively. What you do have is the residual heat, and fission products decaying away and giving more heat. I’m not sure what power output that equates to, but far, far less than 300MW, and decreasing over time too as shorter-lived fission products go away. Some major advantages of small reactors in this context: - the ratio of volume (r^3) to surface area (r^2) is better so it’s easier to cool - there is simply less bad stuff in any one reactor so the tail risks are smaller