In response to:
The Secrets of the Bomb from the May 25, 2006 issue
To the Editors:
In “The Secrets of the Bomb” [NYR, May 25] Jeremy Bernstein writes that at the Manhattan Project at Oak Ridge, Tennessee, most of the separation of uranium-235 from uranium-238 was accomplished by means of electromagnetic fields. My understanding is that no significant amounts of the fissionable isotope uranium-235 were ever obtained by this method. In any case, the uranium-235 used in the Hiroshima bomb was produced by the diffusion of gaseous uranium hexafluoride through miles of nickel tubing.
Dana Professor of the History of Science (Emeritus)
Jeremy Bernstein replies:
I would like to thank Professor Palter for his letter. It gives me the opportunity to present more information about uranium enrichment. There are several ways in which the enrichment of uranium has been attempted. Some, like the use of lasers, are still experimental, and some have been made industrial.
At Oak Ridge four kinds were tried. Centrifuges were tried and abandoned because they were too complicated and unreliable. That left thermal diffusion, gaseous diffusion, and electromagnetic separation. In thermal diffusion liquid uranium hexafluoride is introduced into a cylinder which has its inner and outer walls maintained at different temperatures. The lighter isotope uranium-235 tends to concentrate near the hot wall. In gaseous diffusion, gaseous uranium hexaflouride is forced through tiny pores in a membrane. The lighter molecule travels faster than the heavier one and more of them pass through the membranes.
The final method uses electromagnetic separation. Here the uranium molecules are ionized so that they carry an electric charge and then they are guided and separated by electric and magnetic fields. This is similar in spirit to how a cyclotron works. It is not surprising that the device used at Oak Ridge was called a “calutron”—after California—and that its inventor, Ernest Lawrence from Berkeley, also was the inventor of the cyclotron.
None of these methods by itself worked satisfactorily at Oak Ridge so it was decided to use them in sequence. The two diffusion methods enriched the uranium to over 20 percent and this stock was fed into the calutrons, which did the final enrichment to over 80 percent. So all the uranium used in the Hiroshima bomb was enriched by both the diffusion and electromagnetic methods.