You Might Want to Know: How have we kept our nuclear weapons from going off by accident? Part Two
How have we kept our nuclear weapons from going off by accident? Part Two
At first, we used the Separated Components strategy to keep our implosion bombs from going off by accident. If you kept the fissile fuel separate from the high explosives, you’d always be able to keep the nuclear explosion from happening. Only after you’d loaded in the fissile fuel and armed the weapon could accidental nuclear detonations happen.
But in 1957, we had started to deploy some nuclear bombs that that had been streamlined so they could be carried outside bombers and even by airplanes that weren’t bombers. To permit this, we had devised a way the critical components could be assembled by a mechanical screw-drive machine while the airplane was in the air.
If the bomb weren’t used and the pilot wanted to come back and land, the screw-drive machine would have to disassemble the components before the landing. Turned out this part didn’t always work. The screw-drive also added bulk.
So we’d gone ahead and started to make “sealed pits,” pits that had all the components for the bomb already assembled inside a sealed “physics package.” Hmmm. What was going to keep those things safe? Especially since we had also figured out how to set off our implosion bombs with only two detonators on the high explosives instead of the more than thirty we’d had on Fat Man.
We thought of a couple of things that we hoped would do it. One was to design a pit that would be “one-point safe.” That meant that if the high explosive inside the sealed pit were detonated at just one point, no significant nuclear yield would result. There’d be some, but it wouldn’t be “significant.” “Significant” was defined as a nuclear yield that was the equivalent of four pounds of high explosive.
Some radiation would get spread around, of course.
Beginning soon after World War II, we had developed some ways of doing “mechanical safeing.” That is, we would put different mechanical devices on bombs that we hoped would keep them from being armed until we wanted them to be.
When we got the big megaton-range bombs, we realized we would have to attach parachutes to them to give our bombers time to get far enough away from the explosion not to be burned up or blasted out of the air. One mechanical safeing device we devised kept the bomb from being armed if the parachute hadn’t deployed.
In 1961, over Goldsboro, North Carolina, a B-52 that was carrying two Mark 39 thermonuclear bombs broke up in the air. The two 3.8 megaton bombs, both of which had the fissile fuel in them already, fell out. On one of them, the parachute deployed.
Both bombs hit the ground. Neither one exploded. Not even the conventional explosives.
The crash of this bomber couldn’t be hidden from us nor the fact that its bombs had fallen out. People knew about this one. Could we take the fact that the Mark 39 bombs hadn’t detonated as evidence that they were safe? Some of us might have thought so. But in 2013, we learned that a report done in 1969 that we didn’t know about had said that on one of the bombs, three of four safeing mechanisms had failed. All that had kept it from detonating was the one that remained.
Another accident prevention thing we’d gotten to work on was developing a kind of high explosive that would be “insensitive.” Here we were looking for something that would be less likely to go off in a crash or if, say, someone accidentally dropped the nuke on a runway or in a storage facility, or if the bomb were hit by a bullet or shrapnel, something like that.
We succeeded in this and by 1979, we were using Insensitive High Explosive in most of our new nuclear warheads. We’d also be putting it in some of our old ones but not all of them. That would have been too big a job, too expensive, it was thought. By 1978, we’d manufactured thousands of warheads. If you had a warhead that was manufactured before 1978, it probably had regular High Explosive in it. Don’t drop it.
We also wouldn’t be using IHE in the new warheads for our missiles. That was because by weight, IHE contained only two-thirds of the energy of HE, so you would need more of it to get the same punch. In your missiles, you don’t need any extra weight .
Finally, we developed safety devices that were electrical, designed to keep the detonators on the high explosive from being set off at the same time unless certain conditions had been met. This was called Enhanced Electrical Isolation and it was complicated. Codes would have to be entered to make it possible for the detonators to receive the electricity that would detonate them.
The power source for the electricity that would go to the detonators could also be made to collapse on a sudden impact, like in a crash, or in the presence of high temperatures, as in a fire.
EEI was first introduced in 1977 in our B-61 tactical bomb. After 1991, all our warheads had EEI on them. That’s what we’ve been told.
And there you are. To keep our nuclear bombs from going off by accident, we had developed all these methods: separable components, mechanical safeing, one-point safety, insensitive high explosive, and enhanced electrical isolation.
The only method we aren’t using at all today is separable components. That’s the one we’ll use again when these weapons are dismantled. That’s when they will become completely safe.
Next: How long would it take an Intercontinental Ballistic Missile to get from here to there? Or there to here?