You Might Want to Know: How have we kept our nuclear weapons from going off by accident? Part One
You Might Want to Know: How have we kept our nuclear weapons from going off by accident?
Part One
Nuclear weapons are very reliable. If you want them to go off, chances are good that they will. Very good.
But we don’t want any going off by accident. If one of the megaton-range nuclear bombs we keep on our airbases for bombers went off by accident, the airbase would disappear. In the radioactive ruin, we could well be wondering: was this an attack or an accident? Anyone who might have been able to tell us that it had been an accident wouldn’t be there anymore.
What are some of the ways our weapons designers have found over the years to keep our nuclear weapons from going off by accident?
With Little Boy--the bomb we dropped on Hiroshima on August 6, 1945—the worry was what could happen if the B-29 carrying it, the Enola Gay, crashed on take-off. The B-29s hadn’t been introduced that much earlier. They were crashing a lot.
By current standards Little Boy was a tiny nuclear bomb in yield but it was still a nuclear bomb. If it had gone off in a crash, it would have destroyed the base we had just built there on Tinian Island and everything on it that wasn’t in a bunker a mile or more from the accident.
Little Boy worked by shooting two shaped pieces of U-235 fissile fuel together. The two pieces had been put into the opposite ends of a big gun tube. Behind one of the pieces was some cordite, a conventional explosive. When the cordite was set off, the piece in front of it shot into (around it, actually) the other piece. Supercriticality was achieved, and about a millionth of a second later, the nuclear explosion began.
If the Enola Gay had crashed and the cordite had gone off, the whole bomb would have gone off, in the same way it did over Hiroshima on August 6. To keep this from happening, the cordite was left out of Little Boy until the Enola Gay had taken off safely and was on its way to Japan. A scientist from Los Alamos was on the Enola Gay. He then installed the cordite.
Fat Man, the bomb dropped on Nagasaki three days later, had a different fissile fuel, plutonium, which required using a different design. Instead of shooting one piece of fissile fuel into another, Fat Man used conventional explosives to implode and compress a grapefruit-sized ball of it to the point of supercriticality. When Bockscar, the B-29 that was to take Fat Man to Japan, took off from Tinian, the conventional explosives were already in the bomb. What was kept out of the bomb this time was the fissile fuel—the grapefruit of plutonium. After the bomber got airborne, a scientist from Los Alamos who was on Bockscar put the grapefruit into Fat Man.
The bomb wasn’t dropped in Nagasaki quite where we had planned, but it worked fine.
The crash of a bomber wasn’t the only kind of accident to worry about, we learned. In the spring of 1957, one of the big B-36 bombers that we had designed and built after World War II just to carry atomic bombs was coming in low to land at Kirtland Air Force Base, south of Albuquerque, New Mexico. The bomber dropped a Mark 17 hydrogen bomb it was carrying. By accident. The Mark 17, our first mass-produced hydrogen bomb, was very heavy, twenty-one tons, four times what Fat Man had weighed. But its design yield was fifteen megatons, seven-hundred fifty times what Fat Man had yielded when it destroyed the city of Nagasaki.
When the Mark 17 hit the ground, the conventional explosives in it detonated and blasted out a crater the size of a swimming-pool. No fifteen-megaton nuclear explosion occurred, however, because the fissile fuel was being carried separately in the bomber. Good thing we were still doing that, at least sometimes. A fifteen-megaton explosion would have atomized the B-36 and destroyed Kirtland AFB and nearby Albuquerque. The radioactive fallout would have made a mess of points east, more than a third of the United States possibly. For quite a while. Years.
We might have wanted our leaders to tell us about the accidents like this one and others we were having with nuclear weapons, but they weren’t going to if they could help it. They didn’t want us worrying and asking questions about what was going on. We learned about this accident years after it happened. People started going to the site to look for souvenirs.
Next: How have we kept our nuclear weapons from going off by accident? Part Two