Enriching Uranium

MIT History Professor David Kaiser discusses the discovery of nuclear fission in 1938 and its immediate impact on countries around the world.

Narrator: Just before the outbreak of World War II came the discovery of nuclear fission and the splitting of the uranium atom in 1938. In a flash, Germany, Britain, France, Italy, Japan and the United States entered the race to build an atomic bomb.

David Kaiser: It’s sort of amazing in hindsight how quickly this whole prospect of nuclear weapons seemed to unfold. Accidental discovery of nuclear fission happened in a German laboratory near Berlin in very late 1938. Within just a few months, Europe had descended into war. This was right as war was about to break out where the notion of releasing energy via slitting a nucleus was first sort of hashed out.

News of that spread very accidentally away from the continent and reached the United States partly because young researchers who would work with Niels Bohr in Copenhagen swore him to secrecy. He was scheduled to take a steamer, a boat ride from Europe to the United States. He arrived early in 1939. He couldn’t help but blab to everyone he saw.

John Wheeler: In January 1939, Bohr came to Princeton on a visit so we had a chance to resume our collaboration. That was the middle of January when I went down to meet him here at the pier in New York. Of course, he had been told by [Otto] Frisch. Frisch and Meitner had not wanted to tell Bohr until he got on the boat because they knew that he would be unable to keep the secret. He had to talk about a problem like this. He would have to talk with somebody. So here, all this pent up thought of his from the trip on the boat was discharged on me when I was there at the pier.

David Kaiser: Immediately, in Germany, in Britain, in the United States, in Japan as we now know, and in the Soviet Union, everyone realized there are weapons to be made here. And, this was not just another ho-hum scientific discovery. Everyone knew that this would have worldly implications within days and weeks.

We now know that every major power started its own nuclear weapons project within weeks of each other. Many of them stalled out. Many of them made very little progress. Others, like the Manhattan Project ultimately succeeded in building these real weapons. It’s an interesting thought experiment. What if nuclear fission had been discovered either five years earlier or five years later? Let’s say it was discovered ho-hum you know, in 1933. The world was still a dangerous place. Dramatic developments in Central Europe with the rise of the Nazis, but would that have been, “Drop everything and work on this?” It’s not so clear. Certainly, if it was discovered five years later, you know, who knows what world we would be living in today.

Physicist Philip Abelson was the first person to make uranium hexaflouride, a gaseous compound used to separate the fissionable uranium-235 from the heavier uranium-238.

Narrator: The principle behind separating uranium isotopes really is rather simple: molecules of a lighter isotope would pass through a porous barrier more readily than those of a heavier isotope. The fissionable isotope of uranium, U-235, the kind that works in a bomb, is a tiny bit lighter than U-238, the more common, more stable isotope. To create highly enriched uranium for a bomb, scientists needed a gaseous compound that could carry the uranium isotopes through a separation process with a series of barriers. Phil Abelson came up with an innovative process for creating large quantities of the gas, uranium hexafluoride, that could travel through the barriers.

Phillip Abelson: I ran a few solutions of the uranium, and I could see that water solutions of the uranium were never going to work. So I looked around to see what kind of a chemical substance of uranium might there be that was stable, and which would be a liquid itself. I saw that uranium hexafluoride might fit this. At that time, only a few grams of uranium hexafluoride had ever been made.

I could see that if I were going to conduct any experiments of uranium hexafluoride, I had to get the uranium hexafluoride. This could not be bought; nobody made it, there were only a few grams, so I must make some. I looked around to see how this should be made, and I devised a scheme for making uranium hexafluoride in good quantities. So I personally made the first several hundred pounds of uranium hexafluoride that was made in this country.

John Arnold, a chemical engineer who worked for Kellex, discusses the difficulties of working with uranium hexaflouride.

Narrator: Uranium hexafluoride, also known as “Hex”, presented several major problems. For one, uranium hexafluoride reacts very violently when exposed to water. It also is highly corrosive for most metals. Engineers had to be very creative to find effective ways to tame the “Hex.”

John Arnold: The corrosion was a big problem. We had to condition all the surfaces that this hexafluoride was exposed to otherwise we would get reaction. We had to be sure that we didn’t have a lot of moist air in them. We either flushed them out with dry nitrogen or got rid of the moisture before we put the fluorine in because the fluorine and moisture react very rapidly. Prior to that conditioning, we had to clean everything and get all the last bit of oil.

Almost invariably when we got a reaction to the hexafluoride, which was a gas under these conditions, we would get products which were solid. Then if it started to float around it would plug up the barrier.

Kellex President Percival Keith inspired chemical engineer J.C. Hobbs to design a special valve for the K-25 gaseous diffusion plant in Oak Ridge by giving him encouragement and support.

Narrator: Only a few of the project’s team members combined a talent for science and engineering with an appreciation for the arts. Dobie Kieth was such a man, and he used a great artist to inspire James Hobbs to produce a genius engineering solution.

Percival Keith: I use the story of the great bronze doors at the Palazzo de’ Medici in Florence. In Florence, at one small church, there are the most magnificent bronze doors that man has ever fashioned. I said to Hobbs, “Now look, there are going to be things that look small. They are going to look small to you. But if they are not solved, it cannot be built. You are an artist and you are a genius, and I expect you then to produce in your field that which is comparable to what the great bronze smiths did in Florence.” And by gosh he did.

Narrator: In June 1944, Hobbs’ revolutionary valve was approved and more than 5000 were installed in the K-25 gaseous diffusion plant at Oak Ridge. Since then, the valve has been used extensively, both at Oak Ridge and at other facilities for the production of special nuclear material.