SciTech Tuesday: The Manhattan Project goes critical—Roosevelt approves funding for plants to deliver uranium, plutonium, and heavy water

A timeline of the events in the Manhattan Project’s history might be laid out in a logarithmic scale. As often happens in big projects things happen slowly at first and build, with progress coming more quickly, until a critical threshold (often visible only in hindsight) is passed. It could be argued that the critical point for the Manhattan Project was December 28th, 1942.

A fact that may not be known to many is that the project to build an atomic weapon began well before the U.S. entry into WWII. The letter from Albert Einstein and Leo Szilard to President Roosevelt warning of potential German development of a bomb using nuclear fission was delivered in August of 1939. Within 2 months Roosevelt had a Uranium Committee meeting to discuss the feasibility of an American effort. About 18 months later a government sponsored committee recommended more research, and while the British committee determined that building a bomb was possible. Based largely on the British report, in October of 1941 Roosevelt asked Vannevar Bush to investigate what it would take, and how much it might cost to build a fission-powered bomb. A month later the government sponsored committee declared such a project feasible, and a month after that was Pearl Harbor. In January of 1942 Roosevelt told Bush to begin development of the effort to build a nuclear bomb. Over the next several months plans bounced around between scientific committees debating the best plans for producing enough fissionable material. The primary arguments were between using uranium or plutonium (only discovered by Glen Seaborg in early 1941, but believed to be easier to bring to critical mass), and the methods to separate uranium isotopes to get pure Ur²³⁵. The basic structure of the Manhattan Project, with Groves directing it, Robert Oppenheimer leading the scientific mission, and the primary sites in Los Alamos, Hanford, and Oak Ridge, also developed in the last half of 1942.

So when Bush brought to Roosevelt plans to build facilities in December of 1942 the project was over 3 years old, and other than plans, there was not a lot to show for it. Roosevelt’s signature put an amazing effort in motion.

By the end of 1943 the three sites are fully staffed and mostly built. In early 1944, 200 g of Ur²³⁵ is shipped from Oak Ridge to Los Alamos, and models of bombs are built and tested. In August of 1944 Bush informs General Marshall that bombs will likely be ready by August of 1945. A month later Colonel Tibbets’ bomb squadron starts practicing with dummy bombs called ‘pumpkins.’

In early 1945 Los Alamos got its first plutonium from Hanford. Two weeks after Roosevelt dies (in other words late April of 1945) President Truman is briefed on the Manhattan Project. In July of 1945 the first atomic bomb is tested at Trinity. Just barely ahead of schedule, less than two years after Roosevelt approved the $2 billion project, two bombs are ready for use against Japan.

Another interesting thing to note about the project timeline, is the early moves by scientists and politicians to try to contain a potential arms race. This began in September of 1944 when Vannevar Bush and James Conant began to lobby for international agreements on atomic research, and continues with the Franck Report in June of 1945, written by scientists in the Chicago lab.

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Posted by Rob Wallace, STEM Education Coordinator at The National WWII Museum

all photos from the Wikimedia Commons.