[gascan]

I fully understand direct exposure can be lethal nearly instantly.

It looks like what I didn't understand is radiation-blocking of a material (lead, concrete, water) is actually a decay function (half of the radiation makes it through X feet) rather than an absolute function (no radiation makes it through X feet).

In other words, given an infinitely radioactive source, you would require an infinitely thick lead barrier to protect you.

Really, I should have guessed given everything else about radioactivity is the same way.

[mirimir]

Right, it's just like transparency for visible light.

But many of the fission products in neutron-exposed uranium emit high-energy gammas. And even lead is somewhat transparent.

Also, from what Google tells me, those "infinity rooms" at Hanford are probably contaminated with plutonium-239. That's an alpha emitter, and alphas (helium-4 nuclei) are pretty easy to stop.