what sort of chain reaction are you referring to?
As we speak, there are submarines from multiple countries lying in wait with enough nukes to take out most of humanity; they are (were?) instructed to launch if central command is taken out, like a world-ending dead man's switch. These go on 3-6 month missions, their only limit being food for the crew, and they will survive all out nuclear war just to pop up and finish the job if needs be.
Or in the case of the UK, if Radio 4 Today programme goes off air....
I think that fears of all countries using nuclear weapons on each other at the same time is frequently inflated with the fear governments will just become kill everyone for no reason or will behave madly.
There is plenty to fear even when we keep everybody rational, let's stick to that
Now I wouldn't do it, but nobody is going to let me near that kind of position.
Soviets had two notable examples: Submarine captain refusing to launch a nuclear torpedo despite clear orders to do so: https://www.nationalgeographic.com/culture/article/you-and-a...
And then one man has saved us all by refusing the launch nuclear strike on US despite Soviet satellites confirming multiple launches from US heading to CCCP. He decided to wait for radar confirmation first despite everyone above him(and next to him) approving the counterstrike. If he followed his training we'd all be dead now.
https://en.wikipedia.org/wiki/1983_Soviet_nuclear_false_alar...