After the devastation
July 27, 2009 1:29 PM   Subscribe

How long did it take for radiation to fall to safe levels in Hiroshima after August 1945?

We learn from many sources that Hiroshima was rebuilt after the war, but how long would radioactivity continue to affect the area in dosages high enough to affect humans?

(Or maybe the question does not compute, and the city was simply reinhabited and rebuilt oblivious of health hazards?)
posted by yclipse to Science & Nature (16 answers total) 6 users marked this as a favorite
 
I've been told that the biggest threat is radioactive iodine 131, which has a half life of around 8 days. After a couple of months, it would be all but gone.
posted by Ctrl_Alt_ep at 1:41 PM on July 27, 2009




If nuclear fallout lasts thousands of years, how did Hiroshima and Nagasaki recover so quickly?

The bombs that destroyed Hiroshima and Nagasaki produced their share of residual radiation, but it didn't stick around long, for two reasons. First, both bombs were detonated more than 500 meters above street level so as to wreak maximum destruction (surrounding buildings would have blocked much of the force of ground-level explosions). That limited surface contamination, since most of the radioactive debris was carried off in the mushroom cloud instead of being embedded in the earth. There was plenty of lethal fallout in the form of "ashes of death" and "black rain," but it was spread over a fairly wide area.

Second, most of the radionuclides had brief half-lives--some lasting just minutes. The bomb sites were intensely radioactive for the first few hours after the explosions, but thereafter the danger diminished rapidly. American scientists sweeping Hiroshima with Geiger counters a month after the explosion to see if the area was safe for occupation troops found a devastated city but little radioactivity. Water lilies blackened by the blast had already begun to grow again, suggesting that whatever radioactivity there had been immediately following the blast had quickly dissipated.

posted by KokuRyu at 1:43 PM on July 27, 2009


It's also my understanding that the bombs that fell on Hiroshima and Nagasaki were considerably weaker than your average nuclear bomb of today, no?
posted by EmpressCallipygos at 1:48 PM on July 27, 2009


You may find the wiki article on Operation Crossroads (the nuclear tests in Bikini Atoll after WWII) interesting. These explosions were the first that weren't air bursts and left lots of lingering radiation behind. I think it was these tests that produced the image we have in our minds of a nuclear bomb creating an uninhabitable radioactive wasteland.

Baker was the first nuclear explosion close enough to the surface to keep the radioactive fission products in the local environment.[2] It was not self-cleansing. The result was radioactive contamination of the lagoon and the target ships. While anticipated, it caused far greater problems than were expected.[51]

The Baker explosion produced about two pounds of fission products,[52] equivalent in radioactivity to hundreds of tons of radium.[53] These fission products were thoroughly mixed with the two million tons of spray and seabed sand that were lifted into the spray column and its cauliflower head and then dumped back into the lagoon. Most of it stayed in the lagoon and settled to the bottom or was carried out to sea by the lagoon's internal tidal and wind-driven currents.

A small fraction of the contaminated spray was thrown back into the air as the base surge. Unlike the Wilson cloud, a meteorological phenomenon in clean air, the base surge was a heavy fog bank of radioactive mist that rolled across all the target ships, painting their surfaces with fission products.[54] When the mist in the base surge evaporated, the base surge became invisible but continued to move away, contaminating ships several miles from the detonation point.[55]

posted by TungstenChef at 2:19 PM on July 27, 2009 [1 favorite]


It's also my understanding that the bombs that fell on Hiroshima and Nagasaki were considerably weaker than your average nuclear bomb of today, no?

Hiroshima was a fission bomb based on U-235. Nagasaki was a fission bomb based on Pu-239. They both rated in the neighborhood of about 20 kilotons.

Most of the bombs in the current US arsenal are fusion weapons with yields in the range of 450 kilotons, give or take.
posted by Chocolate Pickle at 2:25 PM on July 27, 2009 [1 favorite]


Threadjack: Does the same apply to White Sands?
posted by NoraReed at 3:59 PM on July 27, 2009


Basically, the bombs used weren't the long-lasting salt-the-earth types of nuclear weapons, but were designed to cause as big an initial explosion as they could?

Pretty much all bombs are designed to make a big (or at least efficient) explosion. Nobody ever built (or admitted to building anyway) a cobalt or other salted bomb that would be "long-lasting salt the earth".

Even neutron bombs weren't designed to "salt the earth," just to deliver a massive prompt dose of neutrons over a relatively short distance, so you can kill that tank column without killing everyone in the village a few klicks away. Presumably the neutrons would have induced radioactivity in the area, though?
posted by ROU_Xenophobe at 5:10 PM on July 27, 2009


One other factor that lowered residual radiation at Hiroshima, but vastly increased initial misery, is that much of the city was wooden construction, and even after most of the buildings within 1km of Ground Zero were knocked down by the A-bomb blast, the rubble burned for several days. The resulting smoke plumes further dispersed fallout into the wind, resulting in contamination of a wider area, at lower intensity.

"... Pretty much all bombs are designed to make a big (or at least efficient) explosion. Nobody ever built (or admitted to building anyway) a cobalt or other salted bomb that would be "long-lasting salt the earth". ..."

Warheads targeted on hardened targets like missile silos and underground bunkers were often setup to detonate near or below ground level, to maximize blast pressure and heat against the thick concrete blast walls protecting these targets. The result of nuclear surface blasts is generally to create a lot more fallout, and local residual radiation, than air blast bombs, which create destruction through massive blast and heat waves transmitted through the atmosphere, as happened at Hiroshima. Hence, the somewhat alarming findings for surface bomb explosions from Bimini Atoll, as quoted upthread by KokuRyu.
posted by paulsc at 8:53 PM on July 27, 2009


Threadjack: Does the same apply to White Sands?

Trinity Site, which is located on White Sands Missile Range, is open to the public for people to take happy smiling family photos in front of ground zero twice a year. Cheerful military docents bearing Geiger counters explain that only alpha radiation is present.

posted by yohko at 9:01 PM on July 27, 2009


Response by poster: It was interesting to learn, as I was trying to get an answer to this question before posting, that Hiroshima was hit dead-on by a typhoon about five weeks after the bomb fell.
posted by yclipse at 4:49 AM on July 28, 2009


Fission tends to split a uranium nucleus into two not-quite-equal neutron rich fragments, and a couple of free neutrons. Just from the italicized mass numbers in the nuclear wallet cards, essentially all of these fragments fall between zinc (30 protons) and europium (63 protons). Most of the lifetimes are measured in seconds. A few happen to be stable or nearly stable.

A radioactive contaminant affects humans if it is both long-lived and biologically active. The thyroid exchanges iodine pretty rapidly, so iodine-131 (half-life 8 days) can deliver a big dose to locals in the first few weeks after exposure. After the Chernobyl accident there was concern that strontium-90 (half-life 30 years) would displace calcium in European milk. The other long-lived isotope people talk about is cesium-137 (half-life 30 years). I don't know that cesium really acts that much like potassium or sodium, but Cs-137 is particularly copiously produced in fission.

ROU_Xenophobe: a nuclear weapon liberates something like a kilogram of neutrons. Neutrons on a biological system stop mostly on hydrogen, making (stable) deuterium. I think the long-lived radiation from a "neutron bomb" would come mostly from the fallout, rather than from activation.
posted by fantabulous timewaster at 6:13 AM on July 28, 2009


a nuclear weapon liberates something like a kilogram of neutrons

That seems unlikely considering that there's only about 10 kg of Pu-239 in a fission bomb. (Depending on the design.)
posted by Chocolate Pickle at 1:07 PM on July 28, 2009


Well, if every Pu fissions, and each fission turns one free neutron into two, the mass of neutrons released is 1/239th the mass of Pu. So 10kg completely fissioned Pu-239 would make about 40 grams of neutrons.

If each neutron carries 200 MeV of heat, releasing a gram of neutrons would make a five kiloton explosion. There've been megaton bombs. Grams to kilograms of neutrons is the right scale.
posted by fantabulous timewaster at 1:38 PM on July 28, 2009


Most of the energy release is in kinetic energy carried by the atom fragments, not by the neutrons. A lot of the rest is x-rays and gamma rays. The neutrons represent only a small percentage of the energy release.

Also, the fission rate isn't 100%. And no fission bomb can produce megaton yields. 30-40 kilotons is about the limit.
posted by Chocolate Pickle at 2:41 PM on July 28, 2009


If a particle at rest splits in two, the daughters have equal momentum, and so the lightest particle has the largest p^2/2m. The x-ray and gamma ray energy per fragment is less than about 8 MeV; if a daughter nucleus is more excited than that, it can usually emit another neutron. The neutrons really do carry the bulk of the heat in a fission reaction.

Neutrons also carry away most of the energy in d-t -> n-alpha fusion, but only about 14 MeV per reaction. So fusion weapons release ten times more neutrons per kiloton than fission weapons.

Coming back to the question: even though there are a boatload of neutrons coming off a nuclear weapon, they see biological materials as equal parts hydrogen, carbon, oxygen, maybe nitrogen. Most of the capture is on hydrogen. Since H-2, C-13, O-17, and N-15 are all stable, neutrons don't impugn biological material with long-lived activity the way they do heavier materials.
posted by fantabulous timewaster at 3:25 PM on July 28, 2009


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