build a radiation detector out of house hold materials:
A "Pith Ball: Radiometer
Take a glass pickle jar, 12-16 oz size
clean it out and crumble some gypsum (the white stuff in dry wall sheets) about 1/2 inch deep on bottom
Bake it/heat it (to work, the inside of the jar must have very lttle or no humidity)
cut a hole through the lid, and “jam” a nail throughm so that the point comes out of the top
tie some cotton thread to the “head” of the nail so that about two 1.5 inches lengths of thread dangle.
Tie some very dry styrofoam chuncks (about the size of a pencil eraser) onto the end of the threads, so you have two balls of stryofoam hanging (think of twin cherries) from the end of the nail. (which is jammed through the lid)
put the lid on, and stick a 3/4 inch or so ball of foil on the point.
static electricity, when applied to the foil ball on the top of the jar will make the styro balls repell each other. When ionizing radiation hits the foil ball, the balls will move closer together, or jerk farther apart. If the balls start to really dance around, you are in a fairly hot area.
It depends on what you mean by “dangerously radioactive” - you can tour the NTS. Also, it is allegedly still used to store radioactive materials and perform sub-critical testing.
Bikini Atoll had its landmass rearranged by a large number of nuclear tests, including the US’ biggest acknowledged “oops” - Castle Bravo - which turned out to be the largest known detonation by the US, due to overshooting its design output by 3 to 4 times due to an incomplete undersanding of the properties of the naturally-ocurring lithium used. And yet the area was declared safe for the natives to return to, until it was discovered that the plant life was concentrating Cesium to harmful levels.
And, by the way, Chernobyl has turned into a bit of a tourist attraction:
I went to the Trinity site in 1993. It had been 47+ years since the very first nuclear explosion. We were told that there was more radiation in a commercial flight (at altitude) than there was residual radiation from the nuclear bomb.
The biggest determinant as to the radioactivity of any given place has to do with what kind of burst would be used.
Air bursts (hundreds or thousands of feet in the air, depending on the bomb yield) will only have the fallout that is formed from the bomb itself. Not a whole lot, in other words. What there is will be pretty hot, but it’ll also be pretty high in the air, making for not much immediate fallout, but a lot more regional and global fallout.
Surface bursts will essentially suck a LOT of dirt and other assorted crap into the fireball, which will be irradiated, and which will fall back to earth much closer to ground zero than that of an airburst. On top of that, it’ll be HIGHLY radioactive as well.
Castle Bravo was a surface burst- that’s a big part of why it was so dirty; had it been detonated at say… 5000 feet, it might not have been nearly so bad.
Look at it this way… how long after the bombings did people live in Hiroshima and Nagasaki? Sure, there are some radiation related issues among the survivors, but mostly due to prompt radiation, not fallout. Both attacks were air bursts, BTW.
After about three or four years, the biggest concern isn’t acute radiation sickness from ambient radiation; it’s ingesting and absorbing radioisotopes that will lead to suppressed immune systems, birth defects and various forms of cancer. Within a generation of the war there would probably be few places where you would actually get sick and die from radiation, but possibly many places that would be unhealthy to eat the locally grown food or drink the water. And even most of that would diminish after about three or four half-lives of Strontium-90 and Cesium-137 (~100 years). I’m uncertain how radioactive ground zero of a surface strike be, but unless there are piles of concentrated nuclear material (reactor waste, etc.) radiation wouldn’t be much of an issue. Although possibly you could have half remembered concerns about radiation devolve into superstition about some areas being “cursed”.