If I remember right, something like a safety factor of 30. So not really as big as people would like!

More detail in this great summary: http://large.stanford.edu/courses/2015/ph241/chung1/

And the Los Alamos paper is really good reading:

http://www.sciencemadness.org/lanl1_a/lib-www/la-pubs/003290...

The summary talks about a safety factor of 1000 on an extremely unreasonable scenario of 1MeV of average temperature, falling down to 10 in an incredibly more unreasonable scenario of 10MeV temperature.

I don't know the actual probabilities, but I do really expect a few atoms to get over 1MeV inside a fission bomb (as the reaction emits ~5MeV of energy). But they will almost certainly not hit each other. I also don't see how any atom anywhere can reach 10MeV.

I'm trying to find where I remember the factor of 30 figure from. Maybe it was the equivalent calculation for hydrogen fusion in the ocean? I'll keep looking.

It's well within the 1000 - 10 range. May be an overall approximation somewhere on the paper.

But I was just summarizing it. Looks like the kind of calculation one does when he's entirely convinced it's impossible, but needs to verify anyway. I liked reading it, and I was surprised they didn't take meteors into account.

My eyes! Is there a more readable scan anywhere?

I was really surprised when I read the quote, they were worried about a runaway fusion reaction! I had heard this story before but always thought they were worried about "burning" nitrogen in the chemical sense, ie. creating nitrogen oxides. It seems doubly ridiculous to worry about fusion when we can't even get a fusion reactor to work.

It wasn't a ridiculous thing to worry about at all.

Yeah, I'm with the scientists on this. They had built the first bombs only a couple years after the first nuclear pile (a sustained nuclear chain reaction), and it was clear what they were trying to do was massively in excess of that. Taking a moment to make sure Earth didn't become a nuclear pile would be great.

n.b. That first nuclear pile was in December 1942, which is way later than even I thought it was. This stuff was happening fast.

didn't really mean it was ridiculous for them to think about, just that it seems so now in retrospect.

We can get fusion to work in H-bombs so it might not be that silly.

True, but H-bombs don't really produce runaway chain fusion reactions afaik. The conditions exist only momentarily due to the heat and pressure from the fission devices, then quickly subside and the fusion stops.