Over the last few years, your Lofty Duo has had an inordinate amount of interest in the Manhattan Project. If you were to draw a Venn diagram of our many overlapping interests in this historical event, it’s likely that somewhere in the shaded region at the center of the diagram would be a man named Henry Cullen. Henry was Anna’s grandfather. In his professional life, he was a Pullman Conductor on the Santa Fe Chief. The stories that Henry told about his train dropping off men with foreign-sounding names and accents in-the-middle-of-nowhere New Mexico are a part of Anna’s family lore.
That middle-of-nowhere spot was Lamy, New Mexico, situated about ten miles south of Santa Fe. During the years 1943-1945, the Lamy railway station was the disembarkation point for thousands of American scientists, engineers, soldiers, and their families as they made their way to the heart of the Manhattan Project: Site Y, more popularly known as Los Alamos. Site Y was one of the thirty locations that made up the Manhattan Engineer District, an administrative organization for the atomic bomb project that was created within the Army Corps of Engineers.
The military director of the Manhattan Engineer District was General Leslie M. Groves, who received the assignment to manage the Manhattan Engineer District as a result of his success with building the Pentagon. As Groves contemplated the necessity of moving so many valuable technical people around the country, he became concerned by the possibility of airplane crashes. As a result, trains like the Santa Fe Chief became the primary mode of cross-country transportation for the people working on the Manhattan Project. If it weren’t for the General’s fears, it’s unlikely that Henry Cullen would have crossed paths with so many individuals who were in the process of changing the course of history.
Henry Cullen’s outsider-looking-in stories about the then secret world of the Manhattan Project have given rise to a number of projects here at Lofty Ambitions. We’ve made trips to Santa Fe and Los Alamos numerous times. We’ve visited a number of atomic-themed museums. And we’re academics, so we’ve turned what we learned into conference papers and presentations. Doug is also using parts of Henry’s story in the novel he’s writing this summer.
As we mentioned earlier this month, over the next year, we’re going to be taking a look at the last year (August 1944-1945) of the Manhattan Project. Our starting point is a sequence of events that led to a massive reorganization of the laboratory at Site Y seventy years ago in August of 1944. That reorganization centered on a new design, a new model for the atomic bomb called implosion. This new design was necessary in order for the project to make use of the element plutonium, about which we’ve written. To understand this shift in August 1944, it’s helpful to keep in mind how the Manhattan Project scientists had initially thought they might go about designing an atomic bomb.
Hungarian physicist Leo Szilard is the scientist credited for first recognizing the possibility of using the energy released by the splitting of an atom—the process of nuclear fission—to create a weapon. In the late 1930s, much of the research in the area of nuclear fission was focused on the radioactive element uranium.
In uranium, the fission process begins with the absorption of a neutron (a subatomic particle with no electric charge, and one of the three constituents of atoms along with electrons and protons). This new neutron introduced to the uranium atom adds to the protons and neutrons in the nucleus, a process that excites the atom and makes it unstable. As a result of this instability, the uranium atom breaks apart into lighter elements (krypton and barium), three more neutrons, and energy.
However, this set of byproducts is the result of the fission in a specific uranium isotope, U-235. Naturally occurring uranium has two isotopes: U-235 and U-238. The element uranium has 92 protons in its nucleus. Isotopes are alternative configurations of a chemical element that differ in the number of neutrons in the nucleus. U-235 has 143 neutrons in its nucleus, and U-238 has 146 neutrons. The number after the chemical symbol—235 or 238—indicates the total number of protons and neutrons for that isotope (e.g., U-235: 92 + 143 = 235).
The nuclear fission that described above for U-235 releases three new neutrons. Each of those neutrons can then go on to fission more uranium atoms. As this process repeats cycle after cycle, it produces what is known as a chain reaction. In nuclear engineering, a controlled chain reaction is a nuclear reactor, a machine that can be used to generate power. An uncontrolled chain reaction is a weapon, and that was the goal of the Manhattan Project. Get that fission started and let it run wild.
U-238, the other naturally occurring isotope of uranium, has a nuclear reaction that generates only a single new neutron. So, one neutron is needed to cause fission, and one neutron is produced by the fission. That’s just not enough to sustain a chain reaction. So the Manhattan Project needed U-235.
Naturally occurring uranium, however, is found in an isotope mix that is 99.3% U-238 (which the scientists and engineers didn’t want) and about 0.7% U-235 (which was what they did want). They worked as best they could with this situation of separating out the isotope they wanted. As their work proceeded, though, they wondered whether plutonium might be used instead of uranium. As they began to think about how plutonium might work, they realized that the bomb design under development for uranium wasn’t suitable for using plutonium.
So while the Manhattan Project continued to pursue a weapon that used uranium, they refocused efforts on plutonium and began developing another design.
For the next post in “Countdown to the Cold War,” click HERE.