The safety of the system comes from it's design. It doesn't rely on anyone, or any mechanical process cooling it.
> The pebble-bed reactor is designed so that this effect is relatively strong, inherent to the design, and does not depend on moving parts. This negative feedback creates passive control of the reaction process.
From the Wikipedia article on the PBR that was linked in a neighbouring comment.
It should, at least if built correctly, be impossible for it to get into a run-away state. Even if it lost all power, and everyone walked away, it should not melt down.
> The pebble-bed reactor is designed so that this effect is relatively strong, inherent to the design, and does not depend on moving parts. This negative feedback creates passive control of the reaction process.
From the Wikipedia article on the PBR that was linked in a neighbouring comment.
It should, at least if built correctly, be impossible for it to get into a run-away state. Even if it lost all power, and everyone walked away, it should not melt down.
[1] https://en.wikipedia.org/wiki/Pebble-bed_reactor#Safety