We’re continuing our review of the August 1944-1945 timeline of the Manhattan Project with a look at the month of January 1945. In Richard Rhodes’s award winning The Making of the Atomic Bomb, he says, “[T]he first stage of the enormous K-25 cascade was charged with uranium hexafluoride on January 20, 1945.” There’s an enormous amount of complexity that’s secreted away in that relatively compact phrase.
What was the outcome of that process?
One of the key scientific and engineering challenges of the Manhattan Project was producing fissionable materials. The process for doing that is known as enrichment. We’ve covered both the concepts of fission and enrichment in previous posts.
The enrichment problem was solved, and the process was carried out on an industrial scale at the Manhattan Project’s Oak Ridge site in Tennessee. The uranium produced at Oak Ridge was used in the Little Boy atomic bomb, which was used at Hiroshima on August 6, 1945. That marked the first use of an atomic weapon in warfare.
There were a number of enrichment technologies developed during the Manhattan Project, and one of the most effective was gaseous diffusion. The physical reason that uranium needs to be enriched is a result of the fact that only one of uranium’s two primary isotopes—U-235, not U-238—will sustain a chain reaction. Unfortunately for the bomb-makers, in nature, U-235 comprises less than 1% of naturally occurring uranium. In order to produce material for a bomb, often called fissile material, it is necessary to separate the U-235 from the more prevalent U-238. This process is fiendishly difficult, as the two isotopes are nearly chemically identical. But U-235 is the outcome they needed.
How did the process work?
One way to separate the two uranium isotopes is the process of gaseous diffusion. In his book The History and the Science of the Manhattan Project, physicist Bruce Cameron Reed describes gaseous diffusion in this way:
The basic idea is that if a gas of mixed isotopic composition is pumped against a porous barrier containing millions of microscopic holes, atoms of lower mass will on average pass through slightly more frequently that those of higher mass.
As Reed points out, the working material of the process is a gas. In this case, the working gas is uranium hexafluoride, or UF-6. UF-6 results from a series of chemical reactions starting with dissolving uranium ore, yellowcake, in nitric acid. This gas, eerily also known as “hex,” is a notoriously toxic and caustic gas. Its unplanned release caused a number of deaths in World War II and the ensuing Cold War. A small amount was released into the atmosphere last fall at a nuclear fuel plant in Illinois.
For the process of separating uranium, UF-6 must be pumped across barriers, or filters, made porous by microscopic holes. UF-6 reacts aggressively with a wide-range of materials, and this fact made producing the barriers a fantastically challenging engineering problem. It’s believed that the barriers were eventually constructed of sintered nickel, which doesn’t react with UF-6, but very little of the open literature about the Manhattan Project discusses the creation of the barriers in more than general terms. Reed hints that the process may well still be classified. Ultimately, the barriers were put to the test at Oak Ridge’s K-25 plant.
What was K-25?
K-25 was a massive industrial plant for the UF-6-based enrichment process. The plant’s name is derived from “K” for the Kellex Corporation, the plant’s operator, and “25,” a war-time shorthand for uranium based on its atomic number and mass.
Various numbers are reported, but it’s certain that the uranium used in the Little Boy bomb was enriched to greater than 80% U-235. Because of the tiny difference in atomic mass between U-235 and U-238, the gaseous diffusion cycle had to be repeated many times in order to produce such a level of enrichment of uranium. To do this efficiently, the builders of the K-25 plant organized the process into a serious of thousands of pumps and diffusion boxes called cascades. The size of this equipment and repeated process dictated an enormous factory, and K-25 was enormous in a number of ways.
Even before a number of the engineering details related to the gaseous diffusion process were worked out, General Leslie Groves ordered the construction of K-25. As a result, the facility was massively over-engineered. The plant design resulted in a U-shaped facility where the arms of the U were nearly a half-mile long. A number of sources declare that K-25 was the largest factory in the world when it opened. The plant’s construction started in June 1943, and it was completed at a cost of more than $500 million dollars. The total cost of the Manhattan Project is generally acknowledged to be in the neighborhood of $2 billion dollars, so it’s obvious that the cost of K-25 was roughly one-quarter of the whole project.
And then what happened?
The enriched uranium produced at Oak Ridge was used to build the Little Boy bomb, the first atomic weapon. Richard Rhodes said of K-25 that it was “the most advanced automated industrial plant in the world.” As he summarized, “It would proceed efficiently with only normal maintenance for decades.”
K-25 remained in operation until 1964. Years of decay led to the need to teardown the facility. The process of demolishing the facility began in December 2008 and concluded in December 2013.
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