Nuclear Weapons

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

11.30.07 | 4 min read | Text by Ivan Oelrich

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.