What happened to the radiation that was supposed to last thousands of years in Hiroshima (1945)?

As this item makes rather clear, radiation is not nearly the threat assumed so long as it takes its natural form.  Concentration is our enemy.

Thus nature will work at diluting and removing any available material and dumping it in the ocean happens to be a sensible thing to do in most cases.

Our difficulty comes from our ability to detect even very low safe levels easily.  Then the press gets excited without making any case for the reality of the risk.
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What happened to the radiation that was supposed to last thousands of years in Hiroshima (1945)?


C Stuart Hardwick, works at Writers and Authors

Updated 3d ago · Featured on Quora's Twitter · Upvoted by William Murphy, Professor of American history. 



If you were expecting Hiroshima to be uninhabitable for thousands of years, you are (understandably, given the deplorable state of science education) making a whole bunch of errors in your understanding of radiation.



First of all, radiation isn’t magic death cooties. You are radioactive (traces of unstable potassium in your bones). My kitchen is radioactive (traces of unstable uranium in my granite countertop). If you have smoke detectors in your home (and you should) there’s a good chance they are radioactive (americium—quite, quite radioactive, but harmless unless you eat it or inhale it).



When it comes to radiation, the type, intensity, and duration of exposure mean everything. 


The Little Boy bomb contained 64 kg (141 lbs) of highly-enriched uranium. What that means is that the government sorted through many, many, many trainloads of naturally occurring uranium ore to separate out the isotope needed for bomb-making. In nature (on Earth) uranium is mostly U238, which has a half life of 4.5 billion years. This extremely long half life means the energy it releases as radiation is spread out over far longer than the age of the universe, and is therefore harmless to life (it would be nearly harmless anyway, because it emits alpha particles that can be stopped by as little as the dead outer layer of your skin). U238 is too stable for use in making bombs. For bombs, you need uranium with much more U235, an isotope with a half life of a mere 700 million years. This means (roughly) that U235 is about 600 times more radioactive than U238, making it so intensely radioactive that… 



… you can hold it in your hands with no ill effects at all (the gloves are to keep the metal clean and prevent any dust from making its way into the technician’s lungs or mouth—which would be dangerous).



Uranium is a naturally occurring ore that is more dangerous as a chemical toxin than for its weak radiation. When enriched to 80%, U-235 is weapons grade stuff—far more radioactive—but still not harmful unless ingested. 

So, okay. What makes the stuff so dangerous? Well, when you put too much U235 in close proximity and under the right circumstances, you can create a chain reaction in which neutron release astronomically speeds up the decay of the atoms, making it astronomically more radioactive, making all that atomic energy come out astronomically faster. This can give you a lethal dose in a few seconds, or boil water to run a turbine, or go boom—all depending on how tightly and how quickly the atoms all come together. 

The Little Boy bomb was little more than a lab experiment stuck in a cowling and hung under an airplane. Only about 1.5% of the uranium fissioned. The remaining 64 kg (141 lbs) went up in the mushroom cloud and spread across the Pacific ocean. Oh no! What have we done to mother Earth??? 

Not a lot, actually. The ocean already contains uranium. This is Earth, after all, and it’s a rocky planet, and the ocean contains the runoff from the mountains and the soup from hydrothermal vents. Every 20 cubic kilometers of unadulterated seawater already contains the same amount of uranium spilled by the bomb. The ocean contains roughly 1.332 billion cubic kilometers of water, so it already contains 66,600,000 times the amount of uranium released by the bomb. Put another way, the bomb had zero impact on the amount of uranium in the environment. Zero. Zilch. Nada. 

But what about the 1.5% that actually fissioned? That’s your nightmare poison, right? Well, yes. Much of it transmuted into a cocktail of highly radioactive scariness, however: 

Not all isotopes are equal. After an atomic bomb goes off, the isotopes that hurt people are those with short half lives, not long ones. Isotopes like Niobium-95, Cerium-141, Barium-140 and in particular, Iodine-131 are extremely dangerous because they have half lives of only days. They release all their radiation quickly, so it can do a lot of damage—especially Iodine-131 which can be taken up by the body and transported to the thyroid gland, and Strontium-89 which can be taken up by bones. These fission products are truly monstrous—but they don’t last long. In weeks, they are no longer a reason not to enter the area unprotected. In a year, they are gone. That leaves longer-lived isotopes like Strontium-90 and Cesium-137, both with half-lives of about 30 years. These pose a long term cancer risk, but by now, they are basically gone too. The only effect they impose on today’s world is mucking up highly-precise scientific measurements. 

So what’s this thousands of years business? Hysteria and misinformation, that’s what. 

I do not, by this answer, mean to downplay the horrors inflicted by the bomb or to imply that radiation isn’t dangerous. It can be, but it can also be extremely helpful. Consider that Japan, first victim of nuclear warfare, entered World War II mostly over control of oil supplies in its South Pacific region of influence. After the war, nuclear power fueled a robust, peaceful economy. Now, Fukushima has the Japanese spooked. They are thinking of retreating from nuclear power. And if they do, it will be a mistake. 

Even after Fukushima, the total number of members of the Japanese public killed by the peaceful application of nuclear energy remains 0. Meanwhile, 20,000 Americans die each year due to lung cancer caused by radioactive radon, most of which is dug up and spewed out the smoke stacks of coal-fired power plants. If Japan abandons nuclear instead of upgrading to the newer safer designs now available, they will have to get their power at least partly from coal or natural gas. If they do that, for the first time since the bomb, radiation will start killing large numbers of Japanese.* 

The point is, we don’t need to blindly fear nuclear energy. We need to respect it, understand it, and hold those who wield it to a high standard of public scrutiny. Ignorance is what we need to fear. 

*In fairness, it probably already is. A significant number of lung cancers are caused by radioactive polonium contained in cigarettes. Polonium is another naturally occuring breakdown product of uranium (alongside radon) and for some reason, tobacco plants soak the stuff up, making it far more dangerous than it is in the environment at large.