2) Fallout's complete treatment of radiation is...unscientific. Uranium, the primary component in modern nuclear weapons, has a half-life in millions of years
You're selling uranium short, it's 700,000,000 years. In any case, Pu-239 is the usual fissile material, with a half-life of "only" 23,000 or so years.
But that's not what causes radioactive contamination levels to drop in an area.
There's several stages.
1) Settling of fallout.
Fallout, or airborne radioactive elements, typically dirt and debris irradiated and thrown into the atmosphere, begins to settle rapidly. Wind will carry it a long way, but in a few years (or months, if the blasts are airblasts rather than near-ground detonations) almost all of the material will be on the ground.
2)Washing-out
Once the radioactive material is on the ground, it's not going to be anywhere near as dangerous.
Alpha radiation, which is the most irradiating, can be blocked by a sheet of thick paper. It's basically harmless,
unless it gets inside you (see: airborne fissile materials), because it can't penetrate your skin. This is why workers at nuclear powerplant disasters wear masks.
Gamma radiation is far harder to stop. It'll go through six feet of lead easily, but crucially, it's very unwilling to irradiate in the sense of "make radioactive", because it's literally gamma-ray electromagnetic waves. As a result, only persistent emitters are dangerous - irradiated food is irradiated with this, and that's not radioactive, after all.
Beta radiation is the serious one in the long-term. This can be stopped by thin sheets of most metals, but can irradiate the body through clothes, skin, etcetera.
3) Fadeout
Luckily, as wind and rain occur, particles on the ground will be buried, washed into the soil, and into waterways (Not so lucky for water-based life!). It will remain dangerous for decades at a minimum - chernobyl was a tiny release, and it's still not safe to live long-term in pripyat. But then again, it's been less than fifty years. By the 200-year mark, the chernobyl area will probbably be safe, unless you feel like rubbing your face on the inside of that big-ass containment dome they're building on it to prevent new release of fissiles if the reactor has another hissy fit.
And even at chernobyl today, you can visit the power plant and look at the construction work from the nearby road, then leave, with no effect on health.
TL:DR Honestly fallout
over does it with the amount of radiation left. I mean, sure the DC area is a no-go, because that'll have been hammered so many times I doubt there's more than a crater left. But any city where there are buildings standing, is probbably habitable.
At least, if you
like cancer. Not that that's a serious worry for the average Fallout citizen with the all mutants, deathclaws, super-mutants, bandits, raiders, nazis, Super Nazis, and aliens after them.