Does Weapons-Grade Uranium Pose a Health Risk When Handled?

Dear Dr. Zoomie – I was watching The Man in the High Castle and there was a bit about weapons-grade uranium posing a health risk to people around it. Is this true?

I’ve been watching this series too and I just watched that episode myself. I have to admit I was happy to see one of the “bad guys” who was concerned about the health of the public. On the other hand, I was disappointed that the writers (and researchers) got this part wrong. The short version is that uranium – even highly enriched uranium – is simply not very radioactive. I can confirm this from personal measurements – I’ve made radiation dose rate measurements on depleted uranium, natural uranium, and enriched uranium and none of them are very radioactive. Here’s why:

There are a couple of ways to approach this question. The easiest one is to do a calculation using something called a gamma constant – the gamma constant tells us the radiation dose rate from a given activity of a radionuclide at a given distance. For U-235 (the isotope of uranium used to make nuclear weapons) the gamma constant is only about 0.176 mR/hr from 1 curie of radioactivity at a distance of one meter). So now we just have to figure out how many curies of U-235 there are in a nuclear weapon.

The Little Boy nuclear weapon used highly enriched uranium – about 64 kg (141 pounds) of it. This is significantly more than what’s used in modern weapons, but it was our first one and the designers still needed to figure out some of the tricks we use today. Since the uranium in The Man in the High Castle was also intended for a country’s first nuclear weapons we’ll assume they were using this same amount of uranium. OK – this gets us a weight, but we need an activity to use the gamma constant. So a little more work is in order.

Every nuclide also has what’s called a “specific activity” – the amount of radioactivity (in curies or in the international unit of Becquerel). This is the amount of radioactivity in 1 gram of that radionuclide. For U-235 the specific activity is 91 microcuries per gram, so 1 kg will have a thousand times as much, or 91 mCi and the 64 kg in Little Boy would contain 5.8 Ci – to make the math easy, let’s call it 6 Ci.

So – if 1 Ci gives a dose rate of 0.176 mR/hr at a distance of 1 meter, 6 Ci will produce a dose rate of 0.176 x 6 = 1.056 R/hr at a distance of 1 meter. The actual dose rate from the Little Boy bomb was probably lower than this because it wasn’t 100% U-235, and the other nuclide present (U-238) has an even lower specific activity. But let’s use a dose rate of 1 R/hr at a distance of 1 meter – again, to make the math a little easier.

Now we’re to the final part – what health effects do we expect to see from this level of radiation exposure?

The lowest radiation exposure that’s been shown to cause short-term health effects is about 25 rem – this will cause your blood cell counts to drop for a few months due to damage to the blood-forming organs. It takes about 100 rem to cause radiation sickness, about 400 rem to give someone a 50% chance of death (without medical treatment), and nearly 1000 rem to be fatal. With a dose rate of 1 R/hr at a distance of 1 meter this part’s easy – it’ll take 25 hours of exposure to cause a change in blood cell counts, 400 hours to give a 50% risk of death, and 1000 hours to cause death. At a speed of 60 mph it takes about 50 hours to cross the US – not even enough time to develop radiation sickness. And that’s for a person sitting for that whole time at a distance of 1 meter from the uranium – a person in the next row (or the next seat) further away would receive only half (or less) that amount of radiation.

So – when we put all of this together it seems fairly safe to say that even a person sitting right next to all that uranium on a cross-country trip wouldn’t even get radiation sickness. Which means that The Man in the High Castle is overstating things a bit – nuclear weapons are bad when they explode, but they don’t give off dangerous levels of radiation.

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