Oh, No! Not another “Uranium Dirty Bomb” Story!

According to a recent report from AP, Slovak police arrested people trying to sell highly enriched uranium to undercover agents. According to the police, the material, said to be about a kilogram of uranium, could be used for a dirty bomb. This is a replay of the Padilla case, the so-called “Dirty Bomber,” who was allegedly going to use uranium to make a radiological, or “dirty,” bomb. (The government later dropped reference to the dirty bomb but convicted Padilla on other charges.) I don’t think what the Slovaks have is actually uranium (see below) but, even if it is, dirty bombs are not the problem.

So, people, pay attention: Uranium is not a good dirty bomb material because uranium is not particularly radioactive. I guess because uranium is used to power nuclear reactors and nuclear bombs, people assume it must be highly radioactive. It is not. That is like saying that because coal is used to power furnaces, it must be hot. No, it is just coal, sitting there waiting to get hot when you burn it. But just being coal does not make it hot. The dominant isotope of uranium, U-238, makes up over 99% of natural uranium and has a half-life of about five billion years. That means that if I have a pound of uranium and let is sit and “decay,” or break down into lighter elements, for five billion years, I will have half a pound left. (For comparison, the Earth is about five billion years old.) The lesser isotope, U-235 is 0.7% and has a half life of almost a billion years. This is to say uranium decays very slowly, which is to say it is not very radioactive. One way to think of this is to imagine that, on the day you were born, you swallowed an ounce of uranium and then lived with it inside you until you died and you lived to be 100 years old. Well, in those hundred years, you have lived just two hundred millionths of the half life, so if half of the uranium will decay in a half life, you would expect about one hundred millionth to decay during your lifetime. (The actual math is a bit more complicated because the decay is exponential, not linear, but not far enough different to change the point.) Not a very efficient way to irradiate someone. This is not to say that uranium is harmless. It is very slightly radioactive. In places where the natural rock has uranium in it, houses have higher concentrations of radon and that does have a risk. It is a heavy metal so it is poisonous, just as lead, mercury, cadmium and many other heavy metals are poisonous, but there are far more dangerous radioactive materials in common use in industry that could be used as dirty bomb material.

When the Podilla case came out, FAS did some calculations on the quantities needed to make a dirty bomb using various radioactive materials. Note that to make a large “dirty” bomb using uranium, tons of uranium are required. The greatest danger from uranium from such a device would be having large chunks of uranium metal fall on your head.

As of now (Friday morning) I think the most likely explanation for the Slovak case is that some crooks were trying to scam someone. I don’t think the sample is actually uranium. The Slovak police released a photo of the radiation detector, which I got from the AP. The story from Slovakia makes no sense. You can see photos of the detector and get some specs here. As far as I can tell, it is only a little 2×2” hand held sodium iodide detector. One output on the detector reading is “Bq,” which stands for Becquerels, which is one nuclear disintegration or decay per second. Since the detector is just a hand held device and I don’t know how far the detector is from the source of radioactivity, there is no way it can measure the number of decays so I am guessing that what number really means the number of detected gamma rays. Anyway, I have a table here that tells me that the specific activity of U-238 is 12,445 Bq/g. So if I divide that into the number on the detector, 4.89 x 10^9, I get close to 400kg of uranium. I have to multiply that amount by some large number because the detector is seeing only a small fraction of the total radiation. In addition, the ratios of the U-235 and U-238 do not make sense for any conceivable sample of uranium. Obviously I can’t be sure unless I had the sample and a lab but my suspicion is that someone spiked some material with tiny amounts of some much more radioactive material specifically to fool this detector and hoped to sell it to some gullible person.

No Responses to “Oh, No! Not another “Uranium Dirty Bomb” Story!”

  1. kestasjk November 30, 2007 at 11:45 am #

    Thanks for that, I wish the media would consult an expert like yourself before posting these stories

  2. Passerby December 2, 2007 at 2:48 pm #

    Thanks for the post Ivan. I agree with you that the dirty bomb is a poor use for the U-235, especially if it had been separated to greater than 90% concentration. But then again, also as you alluded to, as a means of generating terror, the attempted sale at that price of such a quantity of pure U-235 is rather worrisome, especially because it demonstrates the active black market in pure U-235. Initially I take at face value the claims of the isotope ratio – as time passes if that doesn’t hold up, then the credibility of those making the claim will be tarnished. With regard to your calculation of the 400 kg of U-238, perhaps you might estimate the associated mass of the U-235 under the assumption of nearly 100% purity. The same analysis as you did for U-238 but for U-235: 2E5 on the meter and 0.8E5 for the Bqs/g, so 3 grams divided by the geometric efficiency factor between source and detector, which as you point out, we don’t know, but under some reasonable assumptions, the implied mass of U-235 seems comparable to that reported, approximately 0.5 kg.

  3. Marcello Corno December 3, 2007 at 10:14 am #

    From a different point of view Uranium is a really good “dirty” bomb material, on one hand, not being exceptionally radioactive, it can be handled more easily than much more radioactive materials, on the other hand it still is radioactive and most people associate it with “nuclear things”. A uranium “powered” dirty bomb would possibly do more actual damage with chunks of a heavy metal flying around, but the people reaction would be absolutely comparable with real, more effective bombs.

    The “terror” effect would be comparable, while being easier to obtain.

    For a while in Baghdad insurgents placed gas tanks near IEDs. the gas itself was harmless or just slightly irritating but the psychological effect of the IED augmented manifold.

    Thanks for posting, it’s always interesting to read informed thoughts.

    Marcello

    REPLY: This is an interesting point that I confess I had not thought of: What difference does it make how dangerous uranium really is? Dirty bombs are often called weapons of mass disruption rather than weapons of mass destruction because the damage they do might be more from social and psychological factors than actual physical damage. So if everyone thinks that uranium is dangerous, it might have much the same of effect as if uranium actually were dangerous. Certainly an interesting idea to consider.

  4. thermopile December 5, 2007 at 8:10 am #

    I agree that there is something fishy with the U-238 numbers. 4.89E9 Bq implies almost 400 kg of U-238. But the U-235 numbers are spot-on with natural uranium.

    The 2E5 Bq shown for U-235 implies about 2.5g (at 8E4 Bq/g). In the field, if the detector is held right on the sample (which can be done with the Inspector 1000), we usually multiply by 3 to account for the solid angle. This gives about 7.5g of U-235 — which implies 0.75% U-235 if the total sample is 1 kg, as stated in the report. 0.75% is synonymous with natural uranium, no enrichment.

    I can’t explain the very high activity seen for U-238. It’s hard to “mask” or spoof spectroscopic detectors – if you tried to sneak in Co-60, Ba-133, or another commonly available radiation source, the spectroscopic detector would identify it as such. The only “masking” isotope would be Th-230, which is a daughter product in the decay chain of U-238 and gives off some characteristic gamma rays that are energetic enough to measure. If you somehow got a quantity of Th-230 (which is hard because it’s not commercially available), you could just maybe fool the detector into thinking there is a lot of U-238 there. But that’s a long shot. The simpler explanation is “operator error.”

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