If anyone wants a lay explanation of how a reactor could possibly be meltdown-proof, here it is: Nuclear reactors generate energy by giving off heat in a controlled way. The reactor core fissions and emit heat, and the cooling systems carry away that heat to do useful work. Older designs required the cooling to remove excess heat. This cooling apparatus was typically a pump, powered by the plant’s own output, with backup diesel generators in case the plant failed. If the power went out AND the backup generators failed, the cooling system would stop, heat built up in the reactor core, and things exploded. This is what happened at Fukushima: the backup diesel generators got flooded by the tsunami and didn’t come online. (The above is a simplification, obv) In the pebble bed reactor, the fuel and its configuration are constructed so that as temperatures rise, it becomes less efficient and gives off LESS heat. Thus, if unexpected external events cause a disruption in the cooling system (e.g. the generators break), the reactor’s own overheating causes it to slow down, which passively reduces the reactor’s output to a lower equilibrium until things can be fixed. This is actually a bit like how human bodies work: if you exercise (e.g. run) really hard, you give off heat. When you overheat, you don’t explode, your muscles just stop working and you are forced to stop until you cool off. If you have better cooling, e.g. running upright dissipates heat, but also cold drinks or even ice packs, you can keep on running for as long as you have fuel. So the cooling elements of “meltdown-proof” reactors are designed to carry away the heat for useful work, but if the cooling elements stop, the reactor fuel gets too warm to fission efficiently and slows down its own heat generation.