Does anyone know how long it took for the radiation to drop down to levels that were safe enough for folks to venture out of the vaults? On one hand, an entire vault became ghoulified because Vault-Tec designed a blast door to malfunction. On the other hand, it seems like raiders and people like the sorts who founded Megaton had coincidentally survived the blasts and were doing stuff topside right away.

I am designing a quest mod (it's the best thing to do while waiting for GECK ) and this lore will be very useful to me. Thanks.

Well, I have heard that it normally takes hundreds of years for an area to become livable again after a blast. However, it could be done in decades if the right tools and Science! were available to help with the process. As to how the wastelanders survived the nuclear holocaust, we may never know, as it never really goes into how it is possible. We can only assume that there were a few places that these people might have been when the bombs fell, that allowed them to not only escape the initial blasts, but the radioactive fallout as well. Perhaps those coin op fallout shelters might have done the trick in a few areas, despite most of them being filled with corpses later. We can assume that people survived initially in these structures, but then they had 200 more years to live before Fallout3 happened. So people were likely living in them as a house and such. But the truth be known, I have wondered this same question many times.

You've heard wrong. Hiroshima's nuked area is currently habitable.

For the real answer, The british government in the 70's and 80's prepared a few documents on suriving the apocolypse, called "Protect and Survive". These documents can be found via here:
http://www.cybertrn.demon.co.uk/atomic/

These documents dont mention directly, but allude to the "Rule of 7's"

A small quote from one of the pamphlets:


So roughly - If radiation is 100 at blast time:
In 7 hours - Radiation is 10
In 49 hours (7*7 = 2 days-ish) - Radiation is 1
In 14 days (2*7 = 14 days) - Radiation is 0.1

At this point according to the pamphlets it was probably safe to come out at this point for extended periods - unless told otherwise. The Pamphlets on making a shelter say to expect to remain in the shelter for this amount of time, and in the most radiation resistant part for the first 2 days. If needs be, you can come out for very short periods after 2 days, but you're looking at no more than an hour or two.

If you wanted to keep following, you'd be looking at 14 weeks for the radiation to get to 0.01... Keep multiplying by seven on one side, and dividing by 10 on the other.

The Pulaski shelters are still a bad idea though... Unless you just came from the supermarket, you'd struggle to survive that long in one.

Maybe the people who were in the area and received heavy doses of radiation became ghouls. The raiders moved into the area later from places not hit by nuclear bombs.

I'm also not sure just how effective they actually were to begin with, given that you sometimes find occupied ones with skeletons in them, which I must assume are the remains of people who tried to hide in them.

But in any case, if Fallout were using real science, I'd imagine that the radiation from the bombs would pretty much be gone after two hundred years, but it's not, obviously, and when the background radiation levels reached a survivable level I can't guess, but it's clear that people managed to survive outside of Vaults in some places, as while some of the people in the wastelands might be the descendants of the inhabitants of vaults which opened before, some obviously never got into vaults, some people probably managed to find places not hit directly and started expanding after the radiation levels in other places was safe.

Of course, some people didn't get killed in the blasts themselves, but still got hit by strong radiation, and became ghouls, I think one of the ghouls in underworld even explained her transformation as being caused by something like that.

What if nerve gas and germ warfare were also used, during the nuke-caust? Remember, there are portions of WWI battlefields still uninhabitable from mustard gas; there also is an island off Britain that rendered uninhabitable by WWII germ warfare tests.

Another possibility, could be a series of atomic wars; like WWI and WWII sequences.

It's been established that the nuclear holocaust lasted about 3 hours, in which civilization was destroyed. There were no secondary conflicts.. it only took one.

Also keep in mind that propaganda released by governments during the Cold War on surviving nuclear attack were sometimes... lies, developed to make people feel safe rather than be safe. The garbage about ducking under your desk or hiding in the crevice of a stree curb are complete fabrications. No desk could protect you from a nuclear blast -- unless it was all-encompassing and made up of several feet of concrete.

But radiation isn't a huge killer. In Hiroshima and Nagasaki many survivors who emerged minutes after the bomb went on to live full, mutation-free lives. At the time, scientists estimated it would take nearly a century for even plant-life to grow in the irradiated dirt. It took months.

But we're also talking about a global, all-destroying incident. Hiroshima and Nagasaki were able to be rebuilt because the rest of Japan survived. Irradiation naturally dropped off and dissipated into the atmosphere, the earth, water sources, etc, until it became essentially undetectable. However, those were just single (small) nuclear devices. Much more radiation would have been released in a global nuclear event and there would be little place for this radiation to naturally seep off to, leaving you with much higher levels of radiation for much longer (in laymens terms, if you have one person fart in a room it'll smell for a minute or so and then go away. If you have 20 people simeltaneously [censored] on the floor that smell is going to remain for some time longer).