Our first “Countdown to The Cold War” post appeared LAST WEEK, so you may want to start there.
In the vernacular of the Manhattan Project scientists and engineers, assembly is the process of transforming a subcritical mass of either uranium or plutonium into a supercritical mass, an uncontrolled nuclear chain reaction resulting in an explosion. In the earliest days of the project, most of the effort was spent on developing what was called the gun-type assembly method. This is essentially the act of slamming together two subcritical masses by firing one at the other. As a means of setting off an atomic explosion, this process has always struck the Lofty Duo as the equivalent of one of our very distant ancestors stumbling across two stones, banging them together, and wiping out the entire forest in which they lived.
The initial designs for a gun-type weapon were essentially navy cannons with one end containing a near-critical mass of fissionable to be shot at from the other end by a smaller mass of fissionable material. The first attempts were thought to require a ten-thousand pound, seventeen-foot long cannon. These designs were known as the Thin Man, after the Dashiell Hammett novel of the same name.
Scientists and engineers hoped that this design would work for both uranium and plutonium. While enriched uranium–enrichment being the process used to increase the proportion of desirable U-235 vs. undesirable U-238 in a given amount of uranium (see last week’s post)–had suitable physical properties for a gun-type weapon, the enrichment process was complex and expensive. During the Manhattan Project, electromagnetic separation, thermal diffusion, and, to a lesser extent, gas centrifugation were all used as enrichment processes. In fact, these processes of enriching uranium were so difficult that there were serious questions about whether enough uranium could be produced to build a bomb.
Plutonium, on the other hand, could be produced by transmuting–transmuting being changing one element or isotope into another–uranium in nuclear reactors (atomic piles at the time). Once produced, its purification and separation could be handled chemically, as opposed to the complicated means necessary for uranium. Plutonium is a fiendish metal to manipulate, and its been called the most dangerous substance known to humankind. In the early days of the Manhattan Project, it was also in short supply. As more of it became available in April 1944 and subjected to experiment, scientists at Los Alamos, particularly physicist Emilio Segrè and his group, discovered that reactor-produced plutonium (as opposed to previous plutonium samples which had been created in cyclotrons) suffered from an alarming problem.
As Segrè and his group discovered in their Forrest Service cabin deep in Pajarito Canyon, the plutonium produced in atomic piles has two isotopes: Pu-239 and Pu-240. The presence of the second isotope, Pu-240, caused the plutonium that Los Alamos was receiving to undergo spontaneous fission. In nature, fissionable elements can also undergo nuclear reaction known as spontaneous fission. This process is a somewhat different process than when nuclear fission is artificially induced through the use of a neutron. Richard Rhodes in his Pulitzer Prize Winning tome, The Making of the Atomic Bomb, gives a footnote definition of spontaneous fission: “a relatively rare nuclear event, differs from fission caused by neutron bombardment; it occurs without outside stimulus as a natural consequence of the instability of heavy nuclei.” Spontaneous was not what the Manhattan Project wanted in its nuclear material.
The unplanned for nuclear reaction was occurring to such an extent that, as two subcritical pieces of plutonium were brought in proximity to one another, the assembling mass of plutonium would be subject to pre-detonation. In short, the plutonium produced in Hanford’s reactors couldn’t be used in a gun-type assembly method. So the scientists and engineers needed to figure out what kind of bomb assembly would work if they wanted to use plutonium.
It was relatively quickly realized that, in order to make use of plutonium and to avoid pre-detonation, the subcritical mass would have to be assembled fast. Very fast. The only method that was available to Los Alamos was implosion. We’ll discuss that and its implications for the Manhattan Project next in our “Countdown to The Cold War.”
In the meantime, for more on uranium, plutonium, and fission, see our post called “Uranium & Plutonium & Fission.”