I asked a question about this a while ago:
https://www.lesswrong.com/posts/e7CfiKCSW3xngSaEy/how-large-is-the-fallout-area-of-the-biggest-cobalt-bomb-we
Overall my sense is that cobalt bombs are not practical. See also this discussion by Anders Sandberg and Carl Shulman 8 years ago:
https://www.lesswrong.com/posts/dnSNAPWXs5aM3LqKa/mini-advent-calendar-of-xrisks-nuclear-war?commentId=ZSeswMvx9t49JYuNx
They seem similarly skeptical about cobalt bombs.
I asked a question about this a while ago:
https://www.lesswrong.com/posts/e7CfiKCSW3xngSaEy/how-large-is-the-fallout-area-of-the-biggest-cobalt-bomb-we
Overall my sense is that cobalt bombs are not practical. See also this discussion by Anders Sandberg and Carl Shulman 8 years ago:
https://www.lesswrong.com/posts/dnSNAPWXs5aM3LqKa/mini-advent-calendar-of-xrisks-nuclear-war?commentId=ZSeswMvx9t49JYuNx
They seem similarly skeptical about cobalt bombs.