For people who don't know this stuff, like myself a few years ago: When a uranium atom is hit by a neutron some of the will split immediately releasing new neu

For people who don't know this stuff, like myself a few years ago: When a uranium atom is hit by a neutron some of the will split immediately releasing new neutrons but some of them will go into an unstable state and then split some period of time later. If the instantaneous splits are enough to keep the reaction going that's called a "prompt critical" configuration and usually seen only in atomic weapons. If the neutrons released by both the immediate reactions and the delayed reactions are enough to keep the reaction going that's only "critical." Because there are many atoms in the reactor that have been hit by neutrons and are unstable but haven't split yet a reactor continues to release a lot of heat even when it's no long critical, somewhere on the order of 10% as much power as when it was fully on.