We seem to have hit the limit with tests such as Tsar Bomba, but that is only because it’s not reasonable or necessary to push it any farther. But I’m wondering what the theoretical limit is. Is there a limit for a single device in which size would make an effective reaction impossible?
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A fission bomb has radioactive material which means it’s constantly losing some atoms to decay, but it’s typically slow enough that you don’t have to worry too much. I wouldn’t keep a plutonium core around the house as an accent piece, but brief exposure is no real issue. This decay is entirely spontaneous (no trigger)
The thing that’s serious is some of the expelled particles have enough energy if they hit another nucleus, they can kick-start the fission of that atom too. Normally, this is so rare that it doesn’t make a difference. Criticality is the term used to describe when enough mass of the core is in a small enough volume that the rate of this triggered decay is increased to a devastating chain reaction. More mass closer together increases the odds of a collision.
So the term critical mass is less applicable than critical density, but they are related. I have to think, though I don’t know if it’s been tested before, that you could make some sort of honeycomb structure or something that allows you to increase the mass without increasing the density. You could make this structure very very large. As long as it’s very close to the tipping point of critical mass, you can use conventional explosives to compress it to criticality, causing the chain reaction and explosion.
Eventually, though, one of two things will happen as you try to make it bigger and bigger. Either the mass of the object will cause its own gravity to overpower it’s structure and collapse under its own weight into critical mass. Or it could be that the structure gets so huge that there’s still enough *stuff* in the way that the ejected particles are still highly likely to collide with another atom eventually. This is less likely since unless there’s a perfect vacuum inside, the air will eventually slow down those particles until they don’t have enough energy to kickstart fission even if it does hit another atom.
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