Break Out the Potassium Iodide!

Or rather, don't. XKCD explains relative radiation doses. It's interesting to read, and to compare the differences among Three-Mile Island, Fukushima, and Chernobyl.

Clearly, TMI was nearly a non-event by comparison. The maximum external dosage was only 1 mSv, one-third the dosage from a single mammogram. The lowest radiation dose clearly linked to increased cancer risk is 100 mSv, so if you got that maximum external dosage near TMI for 100 years, you'd just reach this threshold. It's pretty clear that containment worked at TMI.

Next take Fukushima. All the facts are not yet in, but it's clearly worse than TMI: a one-day dose 50km away was already measured at 3.6 mSv. But — probably — no actual reactor fuel has been exposed to the atmosphere, which implies that the radioactive materials causing this dose have a short half-life and will quickly decay. There is reason for caution here, but this is almost guaranteed to be no Chernobyl.

It has been over 20 years since Chernobyl. And according to XKCD's chart, you can still get a 6 mSv dose in one hour on the grounds of the stricken reactor. That means spending a single day there, unprotected, would increase your risk of cancer. Spending two weeks there could very well kill you with a 2.1 Sv dose.

You could safely go on a camping trip to TMI today. Not so Chernobyl. The jury is still out of Fukushima, but my bet is that in 20 years it'll be much closer to the safer end of this spectrum.