“Fat Man” Atomic Bomb

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 [1945].

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.

Author Richard Rhodes, archaeologist John Isaacson, and Manhattan Project chemist Gordon Knobeloch explain why plutonium could not be used for a gun-type atomic bomb design. They also describe the crucial plutonium research conducted by Emilio Segrè and his team at the Pond Cabin in the Pajarito Canyon. To learn about the bombings of Hiroshima and Nagasaki, please see “Bombings of Hiroshima and Nagasaki.”

Narrator: Emilio Segrè conducted research on the newly discovered element plutonium in this most humble log cabin known as Pond Cabin. Richard Rhodes, John Isaacson, and Gordon Knobeloch discuss Segrè’s discoveries and their implications for the Manhattan Project.

Richard Rhodes: Plutonium was a manmade element discovered by Glenn Seaborg and his colleagues in 1941. When work at Los Alamos was just beginning, the Italian physicist Emilio Segrè was assigned to research the physical properties of plutonium.

John Isaacson: Ashley Pond was the director of the Boy’s Ranch School here at the Los Alamos town site. I guess it was in the early 1920s, had a small sort of hunting camp dude ranch down in Pajarito Canyon. It wasn’t a very good business deal, and never really got off the ground.

But there were a number of buildings in the Pajarito Canyon. During the Manhattan Project, they used every existing building on the Pajarito Plateau for something, because there was hardly anything up here. The Pond Cabin was used by Segrè and his grad students to do spontaneous fission rate calculations.

Rhodes: Segrè worked for a year in Pond Cabin with very limited amounts of plutonium that had been made in a cyclotron. Then, when plutonium began to flow from the big production reactors in Hanford, Washington, he discovered that it was contaminated with another isotope of plutonium, plutonium-240.

Gordon Knobeloch: And that’s bad, because plutonium-240 has the unfortunate capability of undergoing spontaneous fission. It doesn’t need a neutron to hit it to start it off. The presence of these early neutrons would have caused the gun-type of plutonium to fizzle, pre-detonate and not give a proper yield.

Rhodes: A risky and uncertain alternative was to develop an implosion weapon, where a plutonium core is surrounded by explosive lenses. Inward-moving shockwaves from the explosion compress the plutonium to form a critical mass, assembling it so rapidly that it didn’t have time to pre-detonate. But these were new technologies, and the challenge of making such a design was daunting.

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.

Author Richard Rhodes and archaeologists John Isaacson and Ellen McGehee describe the work that went on at the V Site to assemble the “Gadget” nuclear device before the Trinity Test. 

Narrator: The “Gadget” nuclear device detonated in the Trinity Test was assembled at the V Site. Richard Rhodes, John Isaacson, and Ellen McGehee describe that site.

Richard Rhodes: Only in the spring of 1945, after hundreds of experiments, were the scientists finally able to confirm that the plutonium implosion design would probably work. They weren’t certain; therefore, they knew they needed to test that bomb.

The High Bay Building at the V Site is where the plutonium device was assembled that was tested on July 16, 1945. The test took place in the desert of southern New Mexico at a place now known as Trinity Site. The bomb’s code name was the “Gadget,” and at six feet in diameter, it was an imposing structure.

John Isaacson: They assembled the high explosives components of the Trinity device in this High Bay building that was sort of set off from the rest of the high explosives area surrounded by a high fence where you couldn’t look in. This was the most secret of secrets at the laboratory.

Ellen McGehee: There was an I-beam in the building and a hoist where they could suspend the device while they were doing assembly work. With assembly, it means not only put the high explosives together but also position the detonators.

They weren’t using scotch tape, but it was like duct tape. It was interesting the way that they actually put the device together. Not as sophisticated as you would think. But it worked.

The Manhattan Project built the Concrete Bowl at Los Alamos to capture the plutonium in the event that a test of the implosion device fizzled.

Narrator: This 200-foot diameter “Concrete Bowl” was to catch the plutonium if a test of an implosion device was a dud. In spring 1945, scientists were far more confident that the “Gadget” would work, and they moved the test to the desert over 200 miles away.

Richard Rhodes, John Isaacson, and Los Alamos National Laboratory archaeologist Ellen McGehee explain the challenges of designing and testing the implosion bomb.

Richard Rhodes: In August 1944, Oppenheimer expanded and reorganized the laboratory to focus on implosion research. With the new demand for technicians to deal with this new research, the Army gathered together some 1200 military recruits who had some kind of technical training. These young men and women came to Los Alamos to form what was called the Special Engineer Detachment.

Many of them were assigned to the new X-division—X for explosives—which was run by a former Cossack in the White Russian Army and Ukrainian nationalist, Harvard professor, chemist, explosives expert named George Kistiakowsky.

The concrete bowl is testimony to the uncertainty of the plutonium bomb design.

John Isaacson: They realized early that taking a large percentage of the world’s plutonium and testing the weapon, if it didn’t work, they had to have some way to recover the plutonium. They just spent a billion dollars producing it. If they just took the plutonium and blew it up with a bunch of high explosives and it didn’t go critical, they had to have some way to get it back.

One of the ideas they had was, they would build a huge sort of concrete dish or bowl, fill it full with water, put the tower where they would test the Trinity device on it in the center of this big pool of water. And then, if it didn’t produce the kind of explosion they were looking for, the explosion would sort of suck up the water out of the bowl, engulf the material.

Ellen McGehee: And it would come down within this large sort of water reservoir, and then you could drain that down to a central area and recover that material.

Author Richard Rhodes and archaeologist John Isaacson discuss how the Fat Man atomic bomb was assembled in the Quonset hut at Los Alamos. To learn about the bombings of Hiroshima and Nagasaki, please see “Bombings of Hiroshima and Nagasaki.”

Narrator: Before the “Fat Man” bomb was shipped to Tinian Island in the Pacific, some assembly of the bomb’s explosive components was done in the Quonset hut. Richard Rhodes and John Isaacson describe the hut and the work done there.

Richard Rhodes: The Trinity Test on July 16, 1945, was an extraordinary success. The bomb yielded 18,000 tons of TNT.

It still had to be fitted into the bomb bay of a B-29 and that required a special type of release, different from the release that had been used typically in that aircraft. This Quonset hut is one of the places where that work was done.

John Isaacson: The implosion device that was shipped to Tinian Island and then used at Nagasaki, the Fat Man device was assembled in the Quonset hut within this high explosives area.