It’s unlikely they’re that close to the edge - the purpose of the bomb is to move it as quickly from sub-critical to critical so that as much material as possible can fission before the entire device ‘explosively disassembles’ itself - and therefore stopping the reaction.
The more naturally reactive the material, the harder this is to do.
Usually that’s done by having everything as far from critical as possible until it is already mashed together/at maximum density from the explosive lense, then use a neutron initiator to kick off the actual reaction.
Early weapons (gun type) were very bad at this, so yield/material efficiency was terrible. It’s estimated only 1.5% of the uranium in little boy had a chance to fission when it exploded for instance.
Even the first implosion weapons were far better. Fat man had an estimated 17% efficiency.
Davy Crocket style weapons were only possible because of ever more increasing sophistication on this front.
The total weight of a Davy Crockett warhead was only 4x the critical mass of plutonium would normally be at STP, for instance, and most of that is ‘overhead’.
The more naturally reactive the material, the harder this is to do.
Usually that’s done by having everything as far from critical as possible until it is already mashed together/at maximum density from the explosive lense, then use a neutron initiator to kick off the actual reaction.
Early weapons (gun type) were very bad at this, so yield/material efficiency was terrible. It’s estimated only 1.5% of the uranium in little boy had a chance to fission when it exploded for instance.
Even the first implosion weapons were far better. Fat man had an estimated 17% efficiency.
Davy Crocket style weapons were only possible because of ever more increasing sophistication on this front.
The total weight of a Davy Crockett warhead was only 4x the critical mass of plutonium would normally be at STP, for instance, and most of that is ‘overhead’.