Sorry, I know y'all are probably getting a lot of questions re: Fukushima, but I had a very specific one and no-one has been able to answer it.

I am specialised in medical radiation, and have been explaining the risks to workers in this accident, but I need to know what are the rough proportions of alpha, beta (+/-) and gamma radiation flying around.

If I have this right, most of the decays in the uranium cycle are alpha or beta+ and beta-. Some isomeric transitions (and obviously the beta+ decays) will release gammas. Obviously there will be a small amount of xrays too, from Bremsstrahlungs and such.

But what are the percentages of this in general uranium fuel material (I guess 'spent' fuel is the real problem here, not fuel undergoing chain reaction)? Is 50% of the radiation alphas? Is 80% gammas?

It makes a huge difference from a biophysics perspective as alphas are easy to protect against (for the workers) but gammas are unshieldable. Beta- decays are in between, pretty easy to deal with.

Anyone know the uranium fuel cycle well enough to give a rough estimate here?

EDIT: Added some thoughts in my response to Bars, who helped a lot. Any other thoughts that follow on from those insights?


3 Answers 3

  • Te-132 - beta
  • I-132 - gamma
  • I-131 - betta & gamma
  • Zr-95 & Nb-95 - beta
  • Ba-140 & La-140 - beta


That's from wikipedia & http://www.matpack.de/Info/Nuclear/Nuclids/

  • $\begingroup$ thanks very much for that, I am not sure why I could not find that trawling wikipaedia myself. At the bottom of the page en.wikipedia.org/wiki/Nuclear_fission_product it shows the decay methods. Unfortunately they are all gamma and beta decays (all the alphas are really on the super massive element decays I guess). The betas wont get through the suits, but at least 40% according to that graph will be gammas, for which shielding will be largely ineffective. Cheers again $\endgroup$
    – SoulmanZ
    Mar 17, 2011 at 10:30
  • $\begingroup$ in fact, that probably isnt clear enough still. Cs-137 for example decays via a 500ish keV beta decay, and then a 622 keV gamma decay (isomeric). So only half of that energy is penetrating radiation (the gamma). Iodine-131 is even better, with 360 keV gamma and 600 keV beta, or 33% penetrant radiation. I suspect this should mean the workers on the ground are only exposed to a fraction of the current 'reading' on their dosimeters. Anyone have further thoughts? $\endgroup$
    – SoulmanZ
    Mar 17, 2011 at 10:42
  • $\begingroup$ final thought - doses are being given in weighted units (mSv/hr) but I am fairly sure this is inaccurate. Have they really weighted the exposure to account for absorption factors or have they just gone "1 rem = 1 Sv"? I can't imagine their dosimeters ignore all alpha and beta decays, despite the fact they are relatively harmless to the workers. $\endgroup$
    – SoulmanZ
    Mar 17, 2011 at 10:54
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    $\begingroup$ @SoulmanZ, I've found this on a page about Chernobil disaster. Yes, I guess alphas are not here at all, but they surely exist. I doubt dosimeters can detect alphas, as the ones I've seen had geiger tubes shielded by some 1mm of material, which is impossible to penetrate for alpha. High-energy Beta's are not 'safe' and can shoot through body. Also found this awesome picture: en.wikipedia.org/wiki/File:Table_isotopes_en.svg $\endgroup$ Mar 17, 2011 at 13:07
  • $\begingroup$ Cheers again Bars, just wanted to correct that betas need to be very high energy to have any chance of penetrating a suit, air gap and then the dead skin layer to get to active tissue. Most betas are below this threshold. Also, I did get an answer elsewhere about detectors - most (like the ones we use at work) are simple gamma detectors, but the high quality ones you hope they would have on the ground, and the ones the US would be flying over the top can detect betas and gammas. All interesting stuff. Still hopeful the situation in Japan will sort out without major problems $\endgroup$
    – SoulmanZ
    Mar 17, 2011 at 22:01

The thing you have to worry about most (unless you're close to Fukushima) is not the radiation by the reactor itself but the radioactive isotopes of Cs and Iodine, which when consumed expose body cells to direct and no longer shieldable radiation. That's why Iodine pills are distributed (I'm not a doctor, but one shouldn't take some unless instructed to) to saturate the Thyroid so the radioactive particles don't stay.

  • $\begingroup$ sorry tobias, I am a doctor and thats why I was asking the question. Cs and I are not the problem per-say, the problem is they are radioactive, so you are ingesting alpha emitters. This means the skin can't protect you, and you get high doses to gut, thyroid (I), bone (strontium) and others. As mentioned above I want to know not for the effects down the line, but because things like cesium, if only generating alphas, are not a risk to workers. They are protected by suits. I just need to know at any given time, if there is say 3mSv/hr surrounding fuel, what % is gamma, what is alpha or beta etc $\endgroup$
    – SoulmanZ
    Mar 17, 2011 at 9:02
  • $\begingroup$ I should have said they are beta emitters, not alpha emitters. The rest of the statement stands. $\endgroup$
    – SoulmanZ
    Mar 17, 2011 at 11:13
  • $\begingroup$ @SoulmanZ: I'm afraid in that case I can't add anything to what BarsMonster already stated, sorry, I'm no expert in this area $\endgroup$ Mar 17, 2011 at 11:26
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    $\begingroup$ Nevertheless this mix up of "radiation" and "radioactive material" is very common in news and source of most confusion. $\endgroup$
    – Georg
    Mar 17, 2011 at 11:47
  • $\begingroup$ @Georg: exactly my point $\endgroup$ Mar 17, 2011 at 12:11

It is impossible for anyone here to tell you that with any degree of certainty. What's emitted depends completely on the state of the system. What materials are contained in it, and at what relative concentrations for example. How it's shielded (the shielding itself will get excited and emit both particles and X-rays).

The main danger from any radiological incident like this isn't the direct radiation at all (unless you're in the direct vicinity) but fallout from radioactive products dispersed through the atmosphere. We can likely expect for example some of that in the form of rain as the water used to cool down the cores evaporates and rains down again later. This will of course be massively diluted depending on distance and time.

At any distance from the source, even gamma radiation will be low enough that it's no threat. What most laymen never seem to grasp (in no small part no doubt due to scare campaigns designed to do just that) is that radiation at low doses is NOT a problem. My guess is that the people now panicking in Tokyo (and even the US west coast) aren't getting any more exposure than they'd get during a dental X-ray, at most.


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