system
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human
[Fallout] Radiation Dissipation and Vaults

Two questions here.

1) The Control Vaults (And select Test Vaults) were designed to reopen after only 20 years. At first that sounds pretty reasonable, but fast forwared to 2277, 200 years after the bombs fell. Some areas are still irradiated. The sky even has a radioactive green tint to it in the D.C. Area. What gives? Was it all part of the experiment or was the planet really habitable after 20 years?

2) About the Vaults that were intended to stay closed longer, or even forever in the case of 101; Could a Vault really continue to provide for citizens and remain secure for that long? Let's say 500 years, even. Vault 101 had been closed for 200 years and I'd assume they'd begun to have at least some problems, right? Vault 13 suffered a breakdown severe enought that the inhabitants were forced to open the Vault. Is that a fluke, one time deal or would that have happened to every Vault eventually?
gpt
1) It's reasonable to assume Vault-Tec wasn't aware of China's actual nuclear capability. They probably assumed that, in the post-resource wars world, China wouldn't have been able to deliver such a massive and apocalyptic nuclear strike on US soil. So 20 years was a nice round number to reassure the paying general public that they'd be safe but still able to walk out of the vault within their lifetime. Plus, the attacks on DC were especially vicious because it's the seat of the American government. It's likely vast stretches of North America look closer to what we saw in California or even the Mojave. Less craters and green radiation, but still total devastation and other post-apocalyptic horrors.

2) Long-term vaults would have had, by necessity, more storage for replacement parts and other maintenance needs. They might have had slightly different construction to allow better access to the life-sustaining machines to ensure they can be reliably maintained for centuries. Of course, people assigned to live in these vaults would have some EXTENSIVE education programs to make sure as many people as possible knew how to fix their machines. Also, it's probably not expected for these sealed vaults to actually remain functional forever. Just for a few centuries max. By then either the company would not exist anymore or nobody would care about the experiment as all the people inside slowly (or quickly, depending on what failed) died.