You Might Want to Know: When it came to making the first atomic bomb, what was the hardest part?
The next four--briefer--entries are closely related. They will posted on successive days this week.
The next four--briefer--entries are closely related. They will posted on successive days this week. Back to one or two a week next week.
Part I When it came to making the first atomic bomb, what was the hardest part?
Convincing Congress and the Pentagon to spend the money wasn’t hard. We couldn’t tell them what the money was for. But we were at war with Germany, Italy, and Japan. Not hard to get money for a war effort. Even when you don’t know what you are spending it on. And they didn’t. Not even in the Pentagon.
Designing the bomb? That wasn’t hard either. At least not for the first of the bombs dropped on Japan, Little Boy. Little Boy used highly enriched uranium as its fissile fuel, and designing a nuclear bomb that used HEU was so straightforward that that the design wasn’t even tested before Little Boy was dropped on Hiroshima. The first atomic bomb dropped on Japan hadn’t even been tested.
Little Boy was basically a big gun barrel with a machined slug of highly enriched uranium at one end of it and rings of HEU at the other end. A charge of cordite, a conventional explosive was placed behind the slug. The atomic explosion over Hiroshima was produced by setting off the conventional explosive which shot the slug of highly enriched uranium into the rings of highly enriched uranium. Little Boy, that one bomb, yielded the equivalent of thirteen thousand tons of TNT, or, say, an attack by more than five hundred B-29 bombers carrying conventional bombs.
“I will now tell you the secret of making an atomic bomb,” Manhattan Project scientist Robert Williams told technical workers at Los Alamos three months after the bombs were dropped on Japan. “It is to take some fissionable material in several pieces, as pure as possible, and slap them together as quickly as possible.” This “secret” was reported in a press release from the Association of Los Alamos scientists on November 23, 1945. Not much of secret, you’d have to say, even if it hadn’t be reported out. Another Manhattan Project scientist, Luis Alvarez, wrote in his memoir,
With modern weapons-grade uranium, …terrorists, if they had such material, would have a good chance of setting off a high-yield explosion simply by dropping one half of the material onto the other half. Most people seem unaware that if separated U-235 is at hand it’s a trivial job to set off a nuclear explosion…. Given a supply of U-235,…even a high school kid could make a bomb in short order.
The hardest part when it came to making the Hiroshima bomb was getting enough of the highly enriched uranium to make the two chunks of it that would be shot into each other. In summer 1945 we had so little of it that we decided not to waste any in a test of the Little Boy design.
What made getting the HEU so hard? Uranium ore in the ground is usually less than 1.5% uranium. First you have to concentrate that ore to get rid of that 98.5% that isn’t uranium. That’s not so hard. We do that all the time with the ores of other metals.
The hard part comes because naturally occurring uranium, even if it is concentrated to 100% uranium, consists of two isotopes of uranium, U238 and U235. In pure uranium, only .7% of it is U235. Only U235 works as a fissile material in an atomic bomb.
An isotope of an element, any element, has a given number of protons in the nuclei of its atoms but a different number of neutrons. All uranium has 92 protons, but U238 has three more neutrons in its nucleus than U235 does. So think about it: how would you go about separating out the .7% of the uranium that is U235? You can’t use chemistry because U238 is for all practical purposes chemically identical to U235. The only difference between the two isotopes is the mass of three neutrons. Can you imagine a mass that small?
To be used in a bomb, uranium must be enriched to at least 20% U235. To make a bomb with uranium enriched to 20%, you’d need a shipload of it. For a bomb small enough to be delivered in an airplane, you would have to enrich it to 90%. That’s when the uranium gets to be “highly enriched,” or “weapons grade.”
The hardest part of making the first atomic bomb to be dropped on a city was producing enough highly enriched uranium for it.
Next: Part II-How did we enrich the uranium for Little Boy?