Let’s just skip directly to the worst-case scenario, like in Japan, where failure of the coolant system caused the reactor to overheat uncontrollably. In terms of what would happen to a pebble bed reactor, this means that there’d no more helium coolant. So, okay, as you might expect, the reactor would start to get really, really hot. As nuclear fuel heats up, the uranium atoms start to move faster, making it harder for them to absorb extra neutrons and split, reducing the reactor’s power. This is what’s called negative feedback, and while it takes place in all reactors, the low fuel density of the pebbles magnifies it in a PBR. As the PBR continues to heat up, the negative feedback gets stronger and stronger until at about 1600 degrees Celsius, the core stabilizes at an “idle” temperature. This temperature is a solid 400 degrees short of what it would take to cause any damage to the fuel spheres or reactor vessel, which are made of a special kind of super strong graphite.
The upshot of all this is that a pebble bed reactor can have the entirety of its supporting infrastructure power down, blow up, get flooded, get stolen, run out of gas, or otherwise fail, all while the entire staff is on vacation, and the only thing that happens is that the PBR will warm up to its idle temperature and… Stay warm. No meltdowns, no explosions, no radiation leaks. The reactor will just sit there and radiate the heat it produces until you cool it back down or take the fuel out. This scenario was tried once, in a prototype PBR in Germany: they shut off the coolant and removed the control rods and watched, and nothing bad happened. A later inspection of the reactor and fuel pebbles showed no damage.
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