Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: John Reid on 17/08/2009 11:30:02
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John Reid asked the Naked Scientists:
Dear Chris,
I have been unable to get through to you during your weekly talk with Redi so I hope this does get through to you.
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I have in recent years I have become totally absorbed with many issues relating to Einstein's theory and having read much including Stephen Hawkins A brief history of time and of course David Bodanis E=mc^2
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NOW can you tell me and possibly many of your listeners:
The conventional nuclear bombs as fell on Japan were Plutonium fuelled and caused serious radioactive fall out. I presume this is because of the Uranium/Plutonium component. What would the radioactive fallout from a hydrogen bomb be considering that hydrogen is not radioactive to start with.
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John D.M. Reid
What do you think?
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www.progressive.org/images/pdf/1179.pdf
The above URL tells you how to build your own H-Bomb but it might not be wise to leave it on your computer too long.
Hydrogen bombs are triggered by fission bombs of at least the size of those that were used by the Americans against Japan hence the radioactive fallout from unconsumed Plutonium will be just as great.
The proportion of casualties caused by radioactivity is quite low most people are either blown to pieces or burnt to death!
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Why no further replies, too sensitive a subject ?
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Oh, I was under the impression the question was answered..
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As you say, most natural hydrogen is not radioactive (i.e. excluding a small natural tritium content. But, during a fusion reaction both ionising and non-ionising radiation (e.g. neutrons, gamma rays, x-rays and alpha and beta particles) will be produced. Some radioactive isotopes (e.g. tritium) will also result from the reaction.
More signifcantly, if a fusion bomb is exploded close enough to ground level a significant quantity of surface soil, water, plants etc.. will be lifted and activated by the intense radiation. This can give a very wide variety of radioactive isotopes which will be manifest as 'fallout'.
Tests have shown that a fusion bomb exploded high enough produces less fallout, but syphrum says, the fission trigger can be significant on its own.
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Weellll... it's not just the fission trigger that's the problem but the fission secondary that's used in most 'hydrogen' bombs. What we think of as 'hydrogen' bombs are mostly two stage fission-fusion-fission devices where the great majority of energy is released by fission of the secondary. The primary fission trigger supplies the energy to induce fusion in the fusion fuel but the fusion energy is then used to initiate fission in a much larger amount of additional U235/238. What is thought of as 'clean' hydrogen bombs are really just ordinary fission-fusion-fission bombs where the uranium secondary tamper is replaced with one of lead, so you get no secondary fission. These 'clean' versions are naturally much weaker than the 'dirty' versions.
As batroost says though, detonating a 'clean' fission-fusion bomb close to the ground will still produce a lot of fallout.
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I'm fairly sure that at least some of the "hydrogen" in an H bomb is tritium anyway so it is radioactive. Also, I think that one aspect of the bomb's design is a thick layer of depleted uranium acting as deadweight to contain the initial explosion in order to improve transfer of energy to the fusion fuel.
That tamper becoes intensely radioactive fallout too.
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The atomic bomb that detonated over Hiroshima used Uranium-235, while the Nagasaki bomb had Plutonium-239. The half-life of U-235 is 700 million years, while that of Pu-239 is 24,000 years. In other words, once on the ground, they will be there for a very long time.
Makes you wonder how they rebuilt, drink water, eat plants.
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The half-life of U-235 is 700 million years, while that of Pu-239 is 24,000 years.
Makes you wonder how they rebuilt, drink water, eat plants.
The half-life of K-40 is around 1.3 billion years and U-238 is around 4.5 billion years.
It may seem counterintuitive, but the longer the half-life, the safer the element is (as far as emitting radiation is concerned).
The really dangerous isotopes are:
- Radon (4 days): Found in basements of houses built on certain rocks
- Iodine 131 (8 days): Actively absorbed by the thyroid from the diet
- Tritium (12 years): Participates in many biological processes
- Cesium 137 (30 years): Water soluble, and imitates bone-building chemicals
The concentration is also important; there is far more K40 in the environment than U238, and produces a much higher radiation dose for most* of us; but we still eat bananas and Brazil nuts.
*Some areas of outback Australia have high natural concentrations of U238; for people living on these soils, U238 would be the dominant source of radiation.
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I am under the impression that even if a bomb somehow only used D+T fusion, the neutrons produced could interact with nearby stable isotopes to produce unstable (some of them intensely radioactive) isotopes. Plus, as Bored pointed out, any unreacted tritium, is certainly radioactive. With a half life just over a decade, one might expect significant issues over the course of several years, but tritium is often highly mobile (tritiated water moves with the water cycles, tritium gas would probably burn to make water during the explosion, but even if it did not, it would dissipate even faster).