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.