Why is Hiroshima habitable now when the half life of uranium 235 is 70 million years?
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answers:
Raymond say: U-235 does not exist in pure form, not even inside a bomb -- it is always mixed with something else. Back then it was with Uranium-238. This applies to the Hiroshima bomb (the Nagasaki bomb was plutonium).
If you could find a small piece of U-235, you could hold it in your hands. It would feel warm but (as long as your skin is healthy) you could hold it for a few minutes with very little risk. It is an alpha emitter (it emits particles made up of 2 protons and 2 neutrons) which are stopped by a small layer of air, a simple sheet of paper or the surface of human skin (as long as the skin has no cuts or sores). Once the emitted particle is stopped, it is simply a nucleus of helium (chemically neutral). Some atoms of U-235, when hit by neutrons, will split (they will fission) into two smaller nuclei AND emit a couple of neutrons. It MAY happen that these neutrons could hit another atom of U-235, causing it to split. However, there is lots of room in an atom for the neutrons to completely miss the nucleus.
It is when you pack lots of U-235 (mixed with U-238) together, you may reach "critical mass": this means that it is impossible for a released neutron to miss all the other atoms (there are just too many, packed in the small volume). It WILL hit one, which will, in turn, release more neutrons that cannot avoid hitting other atoms... ans so on, in a chain reaction.
It is the chain reaction that releases all the energy of the bomb.
Once the bomb has exploded, the remaining uranium (whether it is U-235 or U-238) just sits there, emitting alpha particles, and you can "shiled" that radiation with... a sheet of paper.
The "radiation danger" of a bomb is usually from the by-products of the splitting of atoms. The resulting atoms may be radioactive isotopes of otherwise common elements, such as strontium (absorbed by the bones) and iodine (absorbed by the thyroid). THAT is what causes radiation poisoning.
A bomb blows itself apart well before all the uranium is used up. Typically, in the older bombs, less than 10% of the uranium participates in the chain reaction. The rest gets blown away and gets mostly pulverized. As long as the powder remains suspended in the air, survivors will breath it in and, once inside the lungs, there is no skin to protect the lungs from the alpha particles. You get lung cancer (and it can get inside other organs, depending what you do as a survivor, for example drink water or use the water to clean open wounds.
Once the remaining uranium has settled down and, more importantly, once the more dangerous by-products have "half-lived" themselves down to acceptable levels, you can go in (with proper gear) and clean up the regions of higher concentration.
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Jim Moor say: Any remaining U235 was returned to nature, mostly in wind currents towards the USA.
The real problem was all the radiative by-products of splitting U235 into things like Strontium which can be absorbed into our bones.
The most radiation a body receives is from our own bones naturally.
Second is rocks and concrete naturally.
The third is the air, esp Radon.
The body can handle a lot of radiation, it's not a fragile machine.
That said, the bomb was exploded in the air to have the most 'heat' damage and direct radiation deaths instantly. Only part of the remaining radiative byproducts made it to the ground in the city.
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akihiko say: Nagasaki is healthy due to Pu. Nuclear bomb was first exploded in US
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Ronald 7 say: The Japs are tough people
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Alexander say: Besides the fact that most of the U-235 in the bomb fissioned into other elements and isotopes posing less danger from radioactivity, contaminated structures and objects in the bomb zone can be removed and buried elsewhere.
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Will say: Most of the radiation was cleaned up and the remaining is minor, within acceptable levels. Plus the bomb used on Hiroshima was quite weak compared to what was developed during the Nuclear Arms Race.
What you're thinking of is uranium itself. When atomic bombs go off, the element used no longer exists because it self destructed. However new radioactive elements can be left behind. Such as Americium after the Ivy Mike thermonuclear bomb test.
Pripyat is uninhabitable because the radioactive isotopes themselves were what were released, sending lethal doses of radioactive particles flooding the area by a means of the explosions, steam, and smoke.
Look at the Nevada Test Site (NTS) and the Pacific Proving Grounds. They were the locations of dozens of tests. You can go to Bikini Atoll and scuba dive to see the wreckage of the ships that were used in Operation Crossroads and you can go on a tour around the Nevada Test Site to see the many craters left behind and structures that were built to see what could withstand the heat and shock waves. The radioactivity has weakened over the years.
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billrussell42 say: The dangerous byproducts of a nuclear explosion are not U235, that is all used up in the explosion.
The dangerous ones (and there are lists available) are short lived very active isotopes.
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JosephV say: Hiroshima has recovered into a bustling manufacturing hub with a population of 1.1 million people and only the natural background radiation is present as in other normal places.
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megalomaniac say: It depends on how one defines "habitable". I didn't test it, nor look at the results, but I would be willing to bet that the background radiation there is more than elsewhere.
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say: Because radiation isn't as dangerous as people think it is.
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Tom S say: This will explain it to you: https://zidbits.com/2013/11/is-nagasaki-...
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Fred say: From my understanding the fact the bomb was exploded up in the sky and not on the ground meant there was way less radiation left around after the explosion. The survivors of the blast freaked when a while after the blast black rain fell being caused by the smoke from the burning city. This rain was radioactive and did affect those who it fell on. Likely as the radiation that landed on the ground was cleared away or washed away by rain over the weeks and the city did not suffer badly in the long run for those who chose to rebuild.
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Andrew Smith say: U235 is not the problem.
Because the half life is so long the amount of radiation emitted per year is negligible.
That is why natural uranium sources are not a threat.
Many of the radioactive elements decayed very quickly giving an extremely high dose of radiation but then vanishing in seconds, minutes, hours, days or a few years.
The ones that have the greatest risk are those that have half lives measured in a few decades.
These are moderately radioactive but their radioactivity has not decreased a lot in between.
If there are too many of these then the area would remain uninhabitable for hundreds of years.
As an example. Take 1 unit of uranium with a half life of 7 * 10^ 7 years.
This gives 1.4 * 10 ^ -8 units of radiation per year.
Now take a different element with a half life of just 70 years. and the same quantity of it.
it would give an amount of radiation that is 1.4 * 10 ^ -2 which is ONE MILLION TIMES greater than the radiation from the same amount of uranium.
it would take 20 half lives ( 1400 years ) to reduce to be the same risk as the uranium was on day 1.
Now take something with a half life of 0.07 years ( about three days) and take the same quantity of it.
Initially the radiation would be 10 ^ 10 times greater than the radiation from the uranium.
but after 30 half lives the radiation would be 9 times greater than the uranium ( this is however only a bit under 90 days )
after another few weeks the radiation from that active source is considerably less than that of the uranium.
So it is entirely negligible after a year.
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Who say: there aint any U235 in hiroshima (nagasaki was a plutonium bomb - PU238)
the 1/2 life of U235 is irrelevant
U235 was the "fuel" for the bomb so it got destroyed when the bomb went off
(the bomb went off cos the U235 atom splits into smaller atoms ("daughter products") releasing lots of energy when it does - so there is no U235 atom left - Its those daughter products that may be radioactive (there are several possible daughter products - some more radioactive than others)
(note
EVERY atom bomb (including hydrogen bombs) after hiroshima used plutonium not uranium cos it was easier to "make" PU238 from U238 (plutonium is man made) - than refine U235 from U238
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say: I heard there's a very small part where they still can't go.
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Dixon say: Not really my area but I would guess several reasons.
If something has a long half life, that basically means it is pretty stable and doesn't give off much radiation per mass per time interval. The dangerous stuff is the stuff with a short half life which is therefore emitting a lot of radiation while it exists.
When a source of radiation is spread over a large area it is locally much less radiation that when it was all in the same place. It's like chlorine in a swimming pool, you wouldn't want to sit in 5 litres of chlorine but in a whole pool it's fine.
Presumably the whole point of a nuclear bomb based on U235 is that it breaks down into other elements and releases binding energy, so it isn't even U235 any more anyway. Although I expect it doesn't all get converted.
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say: the US did about 125 above ground nuke tests , the Russians have done a similar amount , plus the UK have done a few , and france , china , Pakistan , Israel , south africa , india , and NK . fukishima has been leaking the equivalent radiation of a Hiroshima blast every day for 6 years now . the world is awash in radiation . we are doomed
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