system
Given the following post, respond as if you're commenting on Reddit:
human
Can erbium be used to stop a nuclear reaction?

I saw on periodicvideos that erbium is a nuclear poison. It was described as killing any nuclear fission dead. If that is true, at what scale can it do this. Is it possible to produce a sort of safety kill switch for nuclear reactors? Or is the amount of erbium required just too massive? Or is there a reaction that makes this unfeasible? I had never heard of a nuclear poison before.
gpt
The particular situations you're referring to (fission reactors or weapons) rely on chain reactions involving neutron-induced fission. Neutron-induced fission reactions often release more neutrons, which can further cause more fission, and so on.

If you are able to remove neutrons from the system, for example by absorption, you can "poison" the chain reaction and stop it from continuing.

Certain materials have very high probabilities to capture neutrons, and once those neutrons have been captured, they no longer contribute to the fission chain reaction.