Yet more evidence why a smuggled nuke is far down on my list of terror worries:

Detroit — Federal agents briefly stopped several runners in the Detroit Free Press/Flagstar Bank Marathon when one of them apparently triggered radiation sensors inside the Detroit-Windsor Tunnel.

Agents on the U.S. side of the tunnel underneath the Detroit River determined that the sensors were set off by a medical radioisotope like that injected into the blood system for stress tests, Ronald Smith, spokesman for U.S. Customs and Border Protection, told the Detroit Free Press.

We've been worried about such things since at least the 1950s, so I'm quite certain there are plenty of safeguards against that. It's the stuff we can't imagine that always gets us.