How long would it take for the effects of a nuclear weapon to wear off?

Question

I would like to know how long it would take for the plant life in an area to recover from an explosion from a nuclear bomb exploding.

Parameters for questions:

• The bomb is a 100 megaton nuclear bomb

• The area is 50 square miles

• Let's say for simplicity the bomb was on the ground and it suddenly exploded and the entire area was wiped of life, so the ground was barren and irradiated

• Assume no intervention from humans

  I would like to know how many years it would take when the area would look unaffected, like the explosion had never occurred. (Meaning plant life has returned)

  Additionally, I would also appreciate knowing how long it would take so that any effects from the bomb have a minimal effect on plants and humans (Like radiation shortening lifespan or the likes)

Thank you in advance

Answer 1

o.m.'s answer is pretty good, but does neglect one detail of the original question - this is a 100 megaton bomb.

The fireball alone would have a radius of 8km, which would sweep out an area greater than the 50 square miles stated in the initial question (about 60 square miles, in fact). It would result in a crater 10 miles wide. The depth is somewhat more problematic, as there have never been any ground-burst tests in the thermonuclear range (atoll tests don't really count because of the water all around). I don't think it's unreasonable to estimate that at least at ground zero you could expect it to carve out a space a kilometre deep.

Fallout would be astonishing, since all the dirt and rock that used to be in that crater would be scooped up in the enormous mushroom cloud and distributed over an absolutely enormous area (that is, in fact, one of the reasons that Tsar Bomba was limited to 50 MT). Depending on prevailing winds, you could see an area as large as the eastern seaboard of the United States rendered uninhabitable for decades.

But even after the radiation had died down to levels that were within an order of magnitude of the background radiation for the region, I don't know if the results of a ground-burst full-yield 100 MT nuclear detonation would ever fade to the degree where the area would look "unaffected". In that power range, it's comparable to a meteor strike, so it would take thousands or tens of thousands of years of erosion and deposition before the crater was worn down and the bedrock eroded and re-covered with the soil and plants scythed from it by the blast.

(All back-of-the-envelope calculations regarding radius and fallout spread provided by NukeMap, a fantastic resource for this sort of thing.)

Edit:

For comparison there's Lake Bosumtwi. It is the middle of an impact crater that is approximately the same size as this 100 MT bomb's hypothetically would be. It has also been around for approximately 1.1 million years. The area is green and fertile, yes, but geographically, any event of that magnitude will leave the area changed effectively forever.

Answer 2

Any amount of radiation is bad for your health. On the other hand, there is natural background radiation on Earth, so the question would be when do the weapon effects become so low that it takes a systematic scientific study to tell the difference?

• It depends on the design of the weapon. Salted weapons were designed to produce extra fallout. Ground bursts also produce more fallout than air bursts.
• Bikini is still considered too unhealthy to live. But pictures of the plant life look normal to the layman.

So less than a century for a slightly bigger bomb, if significantly increased cancer risks are ignored.