U-238 as Radiation Shield

An old CBS story includes this sentence: “Even in stainless steel casks lined with lead or depleted uranium to absorb the radiation, the nuclear waste will still be so hot and so dangerous it will have to be moved with remote-controlled machinery.”

My first thought was "Wha?? Depleted uranium?!?" I am familiar and comfortable with providing shielding using lead, but not uranium. Depleted uranium, U-238, is radioactive. Granted its half-life of 4.5 billion years is much, much longer than that of U-235 and 234, but it’s still emitting alpha particles and radon gas. How can adding something that is radioactive help to absorb and shield radiation?

The key is thinking about the different types of radiation. U-238 is an alpha emitter. Alpha particles are a major problem if you manage to ingest/inhale/inject them, but otherwise they aren’t a big issue. They can’t get through skin or a plastic bag. Smaller, more energetic forms of radiation – beta particles, gamma rays, X-rays - are a much bigger threat for passers-by. Wikipedia has a cute little comparison chart comparing the shielding ability of different materials, and U-238 comes out looking 5 times better at shielding gamma rays than lead. Their source appears to be a book called Nuclear War Survival Skills. Fishy.

Apparently, the heavier the nucleus is, the better the absorption of gamma and X-rays. Pb is used because it is the heaviest element that’s common enough to be reasonably cheap. I am curious about when designers would choose to use U-238 or BaSO4 instead.