It's a simple question, maybe it is obvious but I'm not a native English speaker and I don't know what this truly means...
What does that mean? I've already searched it on Google and couldn't find anything useful.
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In that context, "brainpower" means people capable of building nuclear bombs. I.e. more and more people in more and more countries are capable of building nuclear bombs, giving more opportunities for them to be used.
Another way of saying this is that they are more worried as countries like North Korea get closer to nuclear weapons.