Uranium correction

OK, now I see where that quote of "4 billion years" of radioactivity
for depleted uranium comes from. U-238 is _not_ stable, as I had
recalled; it decays with a half-life of 4.51 billion years. The less
stable U-235 has a half-life of 710 million years, within an order of
magnitude. I guess what's more important about U-235 is that it's
readily fissionable. Real-life "depleted uranium" does of course still
have a small amount of U-235 in it.

I've heard various things about the dangers, or lack thereof, from
U-238. It's actually used as a counterweight in the tail of at least
one Boeing aircraft model, according to an engineer I knew who worked
for Boeing in the '70's (their calcs for the ideal CG of the plane had
been a bit off, so they compensated with a compact, dense, hunk of
uranium). Unlike plutonium, uranium is not chemically toxic, but
breathing a lot of dust containing it is not a good thing, I'm sure.

JdL