C Stuart Hardwick’s answer to What happened to the radiation that was supposed to last thousands of years in Hiroshima (1945)? – Quora

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.

Source: C Stuart Hardwick’s answer to What happened to the radiation that was supposed to last thousands of years in Hiroshima (1945)? – Quora

Go read the rest– it’s quite good.  And, sadly, one of those articles it’s a bear to get just ONE line from.

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