Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jack_ on 19/03/2019 10:54:40
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Johnny has asked:
I would think Caesium-137 (with a 30-year half-life) would be about 1,000 times more radioactive than Plutonium. What am I missing?
My understanding of the concept of "half-life" is that it is the time that, statistically, half of the atoms of a radioactive substance will decay (of course they may or may not decay into other also-radioactive elements).
So when I hear "the plutonium in nuclear waste has a half life of 24,000 years" it seems like that means "this stuff decays very, very slowly". Since the number of atoms decaying is small over any given time-span (that's why it takes so long for half of them to decay) doesn't that mean that the amount of radiation being emitted is commensurately small?
Have you got any suggestions?
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I would think Caesium-137 (with a 30-year half-life) would be about 1,000 times more radioactive than Plutonium
It is (usually).
About 1000 times more active- there is another factor. Because 137Cs atoms are lighter than 239Pu you get more of them in a kilogram.
So you get more decays per second than you would if you had the same number of atoms.
However I think the big problem here is that plutonium comes in a number of different isotopes
239Pu has a half life of, as you say, about 24000 years.
But 238Pu has a half life of about 80 years. and would be roughly as radioactive as 137Cs
The relevant calculation is shown here
https://en.wikipedia.org/wiki/Plutonium-238
And there are other Pu isotopes with shorter half lives that would be more active.
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Other aspects affect the lethality of the radiation:
- Cesium is absorbed into our bones - right alongside the stem cells in the marrow which produce our blood cells at a high rate. These stem cells are very susceptible to radiation. (As boredchemist pointed out, I was thinking of radioactive Strontium)
- Plutonium is chemically toxic like many heavy metals, in addition to the radiation damage
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Pu is also a bone seeker, and arguably a lot more dangerous.
Different radiations have different lethality in vitro, so for a given dose (the amount of energy deposited per unit mass of absorber) gamma radiation has a "radiation weighting factor " wR of 1, and alpha radiation has wR = 20. However the latter figure is a consensus value as the experimental evidence is poor. I think the value for α should be closer to 200, which would then explain why UK childhood leukemia clusters were only found around Sellafield, Dounreay and Burghfield (the only sites where plutonium is machined as bare metal and discharged into the environment) and Capper Pass (where polonium was discharged to the atmosphere in significant quantities). It would also explain the high incidence of birth defects in Kerala (environmental thorium dust) compared with Colorado where the same nominal background doserate is γ, not α radiatoin.
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Cesium is absorbed into our bones - i
No it in't
" The biological half-life of caesium is rather short, at about 70 days."
from
https://en.wikipedia.org/wiki/Caesium-137
I think you have muddled it with 90Sr
I think the value for α should be closer to 200, which would then explain why UK childhood leukemia clusters were only found around Sellafield, Dounreay and Burghfield
Yes, but it wouldn't explain the other data.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1059986/
https://www.nature.com/news/2011/110506/full/news.2011.275.html