It's not much of an answer but: because that's how it was designed.
Reactors are very complex machines with so very many things that effect the way they operate.
Running a reactor is like balancing a stick by its tip. There are things that make it more reactive and things that make it less reactive. One of those things are the control rods (but they are not the only mechanism)
The people who designed the RBMK reactor decide to design the reactor to have more "powerful" control rods. The graphite ends are just a way to assert more control over the reactor.
This design decision made it possible to put the reactor in to an unstable state.
There probably exist other reactors where Nuance of their design allows the same thing but we don't hear about them because those operating regimes are never used because they are not safe. I would asume most have safeguards (like the rbmk has) to prevent it.
It's worth mentioning that just because it's posible to break a thing by using it incorrectly doesn't mean that the thing is poorly designed.
axloo7 t1_jc2z7z5 wrote
Reply to Why were the control rods in the reactor featured in the HBO series 'Chernobyl' (2019) tipped with graphite? by Figorama
It's not much of an answer but: because that's how it was designed.
Reactors are very complex machines with so very many things that effect the way they operate.
Running a reactor is like balancing a stick by its tip. There are things that make it more reactive and things that make it less reactive. One of those things are the control rods (but they are not the only mechanism) The people who designed the RBMK reactor decide to design the reactor to have more "powerful" control rods. The graphite ends are just a way to assert more control over the reactor.
This design decision made it possible to put the reactor in to an unstable state.
There probably exist other reactors where Nuance of their design allows the same thing but we don't hear about them because those operating regimes are never used because they are not safe. I would asume most have safeguards (like the rbmk has) to prevent it.
It's worth mentioning that just because it's posible to break a thing by using it incorrectly doesn't mean that the thing is poorly designed.