I just read a great book on the Cold War. There was a section in there about how 25 percent of the B52 fleet would be airborne at all times, and with nuclear bombs aboard. This turned out to be not such a good idea because:
1) A plane was destroyed while refueling over Spain and four nuclear bombs fell to earth. I believe two were on land in Spain. One was in relatively shallow water, and the other was recovered by the Alvin submersible from 2500 feet of water. This created an international incident.
2) A plane crashed at an airfield in Germany. It burned. The high explosives in the bomb(s) blew up but the nuclear reaction was prevented by the safety devices. However, the radioactive material in the bomb was scattered all over the place. The US Government actually excavated quite a large tract of farm land surface and shipped it to the US for disposal.
3) Another plane crashed and burned at an airfield in Greenland, also exploding the high explosive and scattering the radioactive material. This caused an international incident with Denmark, whose treaties specifically stated that no nuclear weapons would be flown over their territory.
4) The most amazing one to me is the case where a plane crashed in North Carolina. Two 23 megaton bombs dropped to earth. One was recovered. Only one of the six safety devices to prevent a nuclear explosion worked. The others had failed. The second bomb was never recovered, and is believed to have landed in marshy farm land. Isn't that an amazing story? (23 megatons is 1,000 times as powerful as the Hiroshima bomb.)
After a number of these events, Robert Macnamara made one of his better decisions and decided that we really didn't need all those nuclear weapons in the air all the time.
The book, which I highly recommend, is:
http://www.amazon.com/Cold-War-Illustrated-History-1945-1991/dp/0316439533
