Thin Man (nuclear Bomb) - Design Issues - Predetonation

Predetonation

The feasibility of a plutonium bomb had been questioned in 1942, when James Conant heard on 14 November (indirectly via Wallace Akers - then Research Director of ICI) that James Chadwick had recently concluded that plutonium might not be a practical fissionable material for weapons because of impurities. He consulted Lawrence and Compton, who acknowledged that scientists at Chicago and Berkeley had known about the problem since October, but could offer no ready solution. He advised Groves, who assembled a special committee of Lawrence, Compton, Oppenheimer and McMillan. They concluded that any problems could be overcome by requiring higher purity. Du Pont (who were considering taking over all the plutonium project, not just the chemical separation process) were advised, but still had strong doubts about the project.

In April 1944, experiments by Emilio G. Segrè on the newly reactor-produced plutonium from Hanford showed that it contained impurities in the form of the isotope plutonium-240. Plutonium-240 has a far higher spontaneous fission rate and radioactivity than the cyclotron-produced Pu-239 isotopes on which the original measurements had been made, and its inclusion in reactor-bred plutonium appeared unavoidable. This meant that the background fission rate of the plutonium was so high that it would be highly likely the plutonium would predetonate and blow itself apart in the initial forming of a critical mass. The distance required to accelerate the plutonium to speeds where predetonation would be less likely would need a gun barrel too long for any existing or planned bomber. The only way to use plutonium in a workable bomb was thus implosion — a far more difficult engineering task.

The impracticability of a gun-type bomb using plutonium was agreed at a meeting in Los Alamos on July 17, 1944. All gun-type work in the Manhattan Project was directed at the enriched uranium gun design (Little Boy), and almost all of the research at Los Alamos was re-oriented around the problems of implosion for the Fat Man bomb.