How dangerous is a chunk of weapons-grade uranium?

Originally posted to Bad Astronomy on Jan 25, 2006.

What sort of radiation does "weapons grade" Uranium put out?

The reason I ask is because of the recent sting in Georgia (former Soviet Georgia, not the American State) where someone was trying to sell weapons grade U that he had in a pastic baggy. I was curious if he would have taken much of a dose, or if it was something like Alpha Particles that can be stopped pretty effectivly by skin.

Short answer: not much of a dose, in the example given.

Long answer: U-235 has a slightly shorter half-life than the more common U-238: 770 Myr vs. 4500 Myr. Neither isotope produces a large number of decays per second. Very rough calculations I just did on a napkin indicate that, in a kilogram of U-235, about 1.1E8 atoms of U-235 will decay each second. That corresponds to a radioactivity of about 3 millicuries for the U-235 alone.

Let us consider only the U decays for a moment. The U atom emits an alpha particle with an energy of about 5 MeV. The alpha particle is easily stopped by collisions with atoms in a few cm of air, or a sheet of paper, or a layer of dead skin. In this view, holding a chunk of U-235 is no big deal. A good place to read about health safety and radioactive decay, by the way, is

Open Source Radiation Safety Training, Module 1: Radiation Properties

If one computes the energy of all those alpha particles emitted in a single second from a kilogram of U-235, one finds a power of about 0.088 milliwatts. One would not expect a chunk of U-235 to feel warm, on this basis, let alone be dangerous.

However, the U-235 decay chain -- as described nicely in a Wikipedia article --

doesn't stop with the first alpha particle emitted by the U-235 atom. The first decay product, Th-231, decays by emitting beta rays (electrons). The electrons are more penetrating, in general, than alpha particles. If you look at the entire chain, you'll see additional alpha emitters and beta emitters. To a rough approximation, the number of decays from each product in the chain should be of the same order of magnitude as the number of U-235 decays each second. Some of the decay products could produce millicuries of energetic electrons; those would be the most dangerous of the radioactive products emitted by the chunk of U-235, but I don't know if even they would present any real danger. As an anecdote, my high-school chemistry teacher would occasionally carry a coin-sized piece of uranium in his pocket, and he lived to a ripe old age.

The total energy of all the particles emitted by a single set of decays all the way down the chain adds up to roughly 46 MeV, which is about 9 times as much as the original alpha particle emitted by the U-235 atom itself. If we add up all the energy emitted by these particles in one second from the 1-kg chunk of U-235, it still comes to only about 0.8 milliWatts. That is _still_ not going to feel warm to the touch.

On the other hand, radioactive isotopes with much shorter decay times -- such as Pu-239, which has a half-life of only about 2.4E4 years, a factor of about 10,000 shorter than U-235 -- can emit a significant amount of energy from a 1-kg chunk. If we wave our hands and estimate that Pu-239 produces 10,000 times the power of U-235, then a 1-kg chunk would yield something like 8 Watts of power. That would probably feel warm to the touch. In fact, one might guess that as a rule of thumb, chunks of heavy metal which feel warm to the touch should be avoided ...