Chain reaction, moderator and control

Neutrons leave fission very fast, at about 20,000 km/s. At that speed uranium-235 struggles to capture them. A moderator, light water, heavy water or graphite, slows them down through successive collisions to thermal speeds, where the probability of fission is hundreds of times higher. That is why the vast majority of reactors are thermal. Fast reactors do without a moderator and compensate with much more enriched fuel.

Power is governed by absorbing neutrons. Control rods, of boron, cadmium or hafnium, move in and out of the core; in many PWRs boric acid dissolved in the water is added as well. To shut the reactor down, all the rods are inserted in seconds.

What makes controlling something so fast possible are delayed neutrons: 0.65% of neutrons are not born in fission but seconds later, when certain fragments decay. That small delay gives the control mechanisms the time they need. A reactor that depended only on prompt neutrons would be uncontrollable, and that is exactly what happened at Chernobyl for a few seconds.