Technically the reactors could be encased in materials proven to be unbreakable at these lower altitudes.... then the only danger would be high-altitude crashes in which case the reactor parachuting would be an answer... except you can't guaranteed the parachutes would be in working condition after a _________ miles up in the air. A normal engine failure could be rescued from, but, say, a mid-air collision at high-altitudes between two planes (regardless of how unlikely it may be) may leave the reactor rescue and parachuting systems in no condition to guarantee a safe ejection.

What materials are proven unbreakable in a 500 mph aeroplane-meets-hard-surface collision?

Also, what about a surface-to-air missile?

Pebble reactors anyone? http://en.wikipedia.org/wiki/Pebble_bed_reactor

I'm not gonna pretend to be an expert on reactor safety, but I do note that the article mentions that a pebble bed reactor is kept inside a big concrete building designed to withstand aircraft crashes.

I assume this means that an aircraft crash involving a pebble-bed reactor _not_ inside a big concrete building could be bad. The second layer of containment (two-meter thick walls) also sounds pretty difficult to make fly.