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Worldbuilding

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    $\begingroup$ "Critical mass" is a misnomer; what really matters is critical density. (Well, actually, critical neutron impact cross-section, but density is much easier to say.) You can cause a sub-critical mass to go critical by compressing it. It's a deeply implausible scenario for any number of reasons but its nuclear physics is hardly the worst offender. $\endgroup$ Commented Dec 25, 2021 at 23:21
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    $\begingroup$ @Cadence What really matters is the chance a neutron hits a fissile atom. That is both a function of density (reduces the chance it goes between atoms) and mass (more atoms in it's path to hit.) Note that causing detonation by compression isn't implausible--look at a Teller-Ulam H--bomb design. In addition to the trigger there's a separate plutonium rod inside the lithium deutride. It's compressed by the fury of the nuclear detonation inches away and detonates, setting off the now-compressed lithium deutride. $\endgroup$ Commented Dec 26, 2021 at 3:52