Reactors
Reactor (“critical” fission reactors) - site of fission of atoms to create neutrons, producing heat. the heat passes into water or a gas in order to turn steam turbines which turn electrical generator shafts.
Cooling
Cooling system: pumps circulate coolant through the reactor’s cores. These pumps are either powered by the plant itself, the grid, or by other means. In the event of a total loss of electricity (the plant itself + the surrounding grid), onsite backup diesel generators are continuously fueled in order to power the cooling system.
without cooling, cores can reach unsafe temperatures in hours or days.
after fission reactors are shut down, (radioactive) decay heat is still generated in high quantities immediately after, dropping off significantly over a week, but continues at a low rate for years.
“Meltdown”
Melting/collapse of a reactor core. happens when damaged and/or overheated. has potential to release radioactive materials to the environment.
ref: Arnie Gundersen and Mat Stein - Solar storms, EMP and grid vulnerability