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When Nuclear Bombs Are Tested How Long Does It Take For The Radiation To Disappear

How long does it take for the Nuclear Radiation to go away?

It depends which element you are interested in.
Following the Japanese accident there were 2 main
elements released into the air.
Iodine 131 ... this one has an 8day halflife so it is pretty well gone
after 3 months
Cesium - 137 This has a 32 year halflife so it will be around for a couple hundred years.
This means some areas may need cleaning up of Cesium-137

The radiation in the nuclear fuel itself can last for thousands
of years....this is the biggest problem to deal with.

How long will lethal radiation last after a nuclear war?

The Seven/Ten rule, namely that for every sevenfold increase in time, radiation is reduced tenfold, gives an estimate of about two weeks for radiation to fall to generally survivable levels (and if you’re careful about radiation protection, even 48 hours may suffice, depending on location and the amount of radioactive fallout).However, the exact answer will always depend on the circumstances:How heavy was the bombardment?Are we talking about locations downwind from the points of detonation or not?Were nuclear facilities damaged in the attack, releasing radioactive materials into the atmosphere in far greater amounts than a nuke?Was it a groundburst or an airburst?Each of these factors and more affect radiation decay rates, so the Seven/Ten rule is more of a guideline than an actual, hard rule.For further reading on the subject, you can try the Nuclear War Survival Guide: The Dangers from Nuclear Weapons: Myths and Facts

What are some negative uses of nuclear radiation?

Household smoke detectors are slightly radioactive
because they use americium to ionize air,
but the radiation is at such a low level it's not harmful
and cannot pass through even a few inches of air.

Nuclear reactors that are generating electrric power
are the source of waste products that are hard to store safely,
and even hard to transport safely. There has been a lot of
controversy about whether nuclear power is "worth it",
because of the difficulty of storing the waste products.

Another problem related to nuclear reactors is the
possibility of malfunction. In the Japan tsunami of
March, 2011, several nuclear reactors were badly damaged,
and harmful levels of radioactive matter were ejected into the air
and spread out over many miles. Excess radioactive matter
derived from this incident has even been found in the USA,
but not at levels harmful to anyone's health here.

You may or may not be old enough to remember that the
president of the Ukraine, Viktor Yushchenko, was poisoned by
polonium, a radioactive element. This was probably a murder
attempt by Russian spies. Yushchenko did not die (the dose
was too small), but was very disfigured by the poison.
I think this happened about six years ago.

Hiroshima nuclear bomb radiation?

A few facts to clear the air

Both the Hiroshima and Nagasaki bombs were fission weapons. Modern nuclear bombs are fusion weapons. Fusion actually releases less alpha, beta and gamma radiation than a comperable sized fission weapon. But, fusion weapons actually use a fission trigger and this releases considerable quantities of these radiation forms.
The primary energy release from both types of weapons, however is in the form of heat.

The Hiroshima bomb was a uranium bomb using an impact trigger. The Nagasaki bomb was a plutonium bomb using an implosion trigger. Impact triggers are not as efficient so more of the fuel in the Hiroshima bomb did not fission and this was spread across the area by the explosion. Essentially, the Hiroshima bomb was much dirtier.

Not everyone in Hiroshima died either in the explosion or subsequently from radiation poisoning or other radiation related diseases. Approximately 200,000 were killed in the explosion, mosty from the heat flash. There have been another 200-300,000 "excess deaths attributed to the effects of the bomb (incidently, even the highest estimated death count from Hiroshima is less than the verified fatalities from the fire bombing of Dresden).

Not everything was destroyed. In Hiroshima today, the remains of a large concrete building that largely survived the blast stand as a memorial. Other concrete buildings also survived. Hiroshima, like most Japanese cities of the time was largely constructed of wood and paper, these of course burned to the ground.

Now, sixty years later the only remaining effects of the bombing at Hiroshima is a slightly higher leukemia rate.

There is of course no scientific data to support the contention that it takes "hundreds of years" for the radiation from a nucear explosion to decrease to safe levels.

Incidently, I've been to Hiroshima and walked directly below the detonation point, and I don't glow in the dark.

If a place on earth is bombed with nuclear weapons, how long will it take for the radiation levels to fall back to normal limits?

If a place on earth is bombed with nuclear weapons, how long will it take for the radiation levels to fall back to normal limits?It depends on the type of weapon and the altitude of the detonation.Nuclear fission (splitting) produces much more radioactivity than nuclear fusion. Fusion weapons (= thermonuclear weapon = hydrogen bomb) however use fission to achieve the temperatures needed to start the fusion. The more energy is derived from fusion, the “cleaner” the bomb is. The infamous 15 megaton Castle Bravo test was about 75% fission and therefore very dirty. The 50 megaton (!) Tsar Bomba derived only 3% of it is energy from fission, a much “cleaner” design. The second factor is the altitude of the detonation. All material contacted by the ionizing radiation will become radioactive. A low altitude blast, where the fireball touches the ground, will make the soil radioactive and then suck it into the mushroom cloud, spreading it around (fall out). At higher altitudes only the surrounding air will become radioactive. Dust will still be sucked into the mushroom cloud, but it is much less radioactive. The latter was the case in Hiroshima and Nagasaki. The Castle Bravo test — which was already dirty — was done at ground level and a whole island worth of radioactive soil was sucked into the atmosphere.

What would happen if a nuclear bomb were to explode in Outer Space.?

There would be no concussion because, in the vacuum of space, there are very few atoms around to transmit the physical force of the explosion. There would be an intense flash of radiation and the atoms of the bomb itself would be accelerated very rapidly away from the explosion. On earth, a nuclear bomb creates such an intense flash of radiation that anything flamable that is near by is heated to the ignition point and it bursts into flame (if there is oxygen around). Even without the oxygen, the air immediately around the explosion is heated to incredibly high temperatures and that creates a pressure wave (the concusion). The radiation from the bomb travels at the speed of light but the concussion travels more slowly.