The Challenges of Plutonium
“Fat Man” Plutonium Bomb
The Challenges of Plutonium
J. Robert Oppenheimer, who served as director of the Los Alamos laboratory during World War II, recalls the challenges of designing an atomic bomb that would use plutonium as the primary fissile material. To learn about the bombings of Hiroshima and Nagasaki, please see "Bombings of Hiroshima and Nagasaki."
Narrator: In the spring of 1944, J. Robert Oppenheimer considered resigning as director of the laboratory. Designing a bomb that would work with plutonium in time to be used in the war seemed impossible. Oppenheimer reorganized the lab to double down on designing the plutonium bomb. As he explains, this was one of the most challenging parts of the project.
J. Robert Oppenheimer: Well, I think the set of problems connected with implosion was the most difficult. It required very new experimental techniques, and it was not a branch of physics which anyone was very familiar with.
This was both from a theoretical, from an observational, and from a practical point of view quite an adventure. It was still a very reasonable opinion that one of the many things that were needed to make it work was not completely in order on July 16 .
The doubts which then existed were not of the metaphysical quality [laughter]. We had always had this in mind as a possibly more effective and more sensible way to assemble the bomb.
“Fat Man” Plutonium Bomb
Richard Rhodes, author of “The Making of the Atomic Bomb,” explains why Manhattan Project scientists could not use plutonium for a gun-type bomb. He also describes how the implosion method for the plutonium “Fat Man” bomb worked. To learn about the bombings of Hiroshima and Nagasaki, please see "Bombings of Hiroshima and Nagasaki."
Narrator: The “Fat Man” bomb was much more complicated than Little Boy. Author Richard Rhodes explains why Manhattan Project scientists couldn’t use plutonium for the gun-type bomb, and how the implosion method worked.
Richard Rhodes: There was a crucial point in the program at Los Alamos in the summer of 1944 when it was realized that plutonium—of which you needed a lot less than highly enriched uranium to make a bomb—was so fissile, that if you tried to use the standard design that was going to be used for the uranium bomb, which was basically a three-inch navy cannon about six feet long with one piece of uranium attached to the muzzle, and another piece was a sort of bullet fired up the barrel to mate with the other piece to make a critical mass and start the bomb going.
If you tried to do that with plutonium, they discovered that summer, this piece would melt down before it even got up to the other end of the barrel, even if it were fired at three thousand feet per second. That’s how reactive plutonium was.
Then they realized that they were going to have to invent an entirely new technology, one that would involve taking a solid ball of plutonium that was just subcritical, squeeze it with high explosives to almost double its normal density, thereby pushing the atoms of plutonium closer together and making the chain reaction possible, making, if you will, the critical mass much smaller because it’s denser.
Physics professor and Manhattan Project expert Bruce Cameron Reed describes why the “Fat Man” atomic bomb’s 32 explosives lenses had to be triggered simultaneously for the bomb to work as designed. To learn about the bombings of Hiroshima and Nagasaki, please see "Bombings of Hiroshima and Nagasaki."
Narrator: The implosion design looked like a soccer ball on the outside, with 32 lenses fitted to form a sphere. The lenses are made of a mixture of explosives, and surround a baseball-sized core of plutonium. As physics professor Bruce Cameron Reed explains, detonating all the lenses at exactly the same time was crucial to produce an effective explosion.
Bruce Cameron Reed: Consider a spherical lump of plutonium about the size of a softball sitting in my hand. To trigger the nuclear explosion, this has to be compressed to a higher density. Then the issue becomes, how do you compress a metal maybe to half of its initial volume? This requires an enormous amount of pressure.
The technique they developed for achieving this was to essentially wrap it in segments of explosives in a three-dimensional assembly. Think of sort of pyramid-like chunks of explosives that would fit together like a three-dimensional jigsaw puzzle, which when detonated would blow inward to crush this thing.
Given the speed of the explosive, they all had to trigger within a microsecond. There were 32 segments surrounding that core altogether. If the implosion is say a little off-centered, the core might shoot out one side, and you get a much less efficient nuclear explosion.
Narrator: If any of the explosives lense charges were misfired by even a microsecond, then the bomb could be a dud.
Nobel Prize-winning physicist Val Fitch recalls working on the timing apparatus electronics for the “Fat Man” bomb. Nuclear archaeologist John Coster-Mullen explains why it was so challenging for Manhattan Project scientists to engineer simultaneous detonators. To learn about the bombings of Hiroshima and Nagasaki, please see "Bombings of Hiroshima and Nagasaki."
Narrator: Val Fitch was a young recruit assigned to the plutonium or “Fat Man” bomb. Without modern electronics or computers, Fitch and his colleagues had to figure out how to simultaneously detonate the lenses around the plutonium core. Fitch went on to become a Nobel Prize-winning physicist.
Val Fitch: We were detonating explosives in such a way as to produce a shockwave, a spherical shockwave going inward to compress, in this case, plutonium to a critical point. Timing of all these explosive lenses is all-important. I was very much involved in developing the timing apparatus, measuring when the shockwave passes a certain point. We developed the electronics for doing that, and also made some of the measurements.
Narrator: The challenges of the task were enormous, as atomic bomb expert John Coster-Mullen explains.
John Coster-Mullen: When you think back on it now, that whole design, it was pie in the sky. I call them “garage bombs” or glorified science fair experiments. They didn’t know if any of this was going to work. This was all a big, giant experiment. Each of these individual components had to work perfectly. The primary thing were the detonators all going off within a microsecond of each other.
The fact that they got it down to a microsecond, which is a millionth of a second, simultaneity between these things—you look back on that now, and it’s absolutely, stunningly remarkable that they were able to do this.