I've heard a couple of scary stories from experienced accellerator physiscists about something called neutron clouds. Apparently, if you have an experiment like a fixed-target experiment that produces a lot of neutrons with the correct energy, they don't just dissipate or get caught in surrounding matter. Instead, they hang around due to their large half-life (~15 minutes). The rumor goes that they actually form clouds, that wander around the facility, and that in the early days of some CERN experiment, people didn't think about the effect, and got a nasty (although not accute) dose when they entered the collision hall just after shutting down the beam.

The description of the behavior of these clouds varies in different accounts. Sometimes they just pass through everything, but sometimes they're supposed to behave like a real gas, being held back by walls (but creeping through small openings).

* I can imagine this phenomenon is real, but how much of an issue is it in real experiments / nuclear facilities?
* Do the clouds really behave like a gas (I'd think the n-n cross section is not big enough to create pressure)? How do they behave wrt. walls?
* And in light of the recent nuclear waste transports in France and Germany: The waste emits a lot of gamma and neutron radiation, could it leave a temporary trail of low-energy neutron clouds behind?