Carroll Quigley - Tragedy and Hope - A History of the World in Our Time

By April 1944, it seemed clear that 95 percent U-235 could not be obtained before 1946 even if the gas-diffusion and electromagnetic plants were run in series instead of parallel, with the latter starting off with 20 percent U-235 from the former instead of both trying to process natural uranium from scratch. At that point, Oppenheimer discovered that Philip Abelson (who had originally discovered how to make uranium hexafluoride) had been working for the navy, trying to make enriched U-235 to be used to propel a nuclear submarine. He was using thermal separation, one of the two methods (the other was centrifuge) that the Manhattan District had rejected in 1942. Thermal separation was based on the fact that a liquid mixture in a container with a hot wall and an opposite cold wall will tend to separate; the heavier liquid will tend to accumulate near the cold wall, will cool, and sink, while the lighter liquid will tend to gather near the hot “wall, get warmer, and rise. Abelson, who knew nothing of the work of the Manhattan District, or of the successful nuclear pile at Chicago, was working at the Philadelphia Navy Yard where he had 102 vertical, double concentric pipes, each 48 feet long, in which the inner pipe was heated by steam, the outer pipe was kept cool, and the ring-shaped space between the two was filled with a uranium liquid mixture whose two isotopes tended to separate from each other. From the top of these pipes he hoped to be able to draw one-fifth ounce a day of 5 percent U-235 by July 1, 1944.

Groves grasped at this straw, and on June 27, 1944 signed a contract for a thermal-diffusion plant at Oak Ridge to be ready in ninety days. The new plant, which eventually cost over $15 million, was 522 feet long, 82 feet wide, and 75 feet high, and was to contain twenty-one exact copies of Abelson’s plant (2,142 tubes in all); it would yield U-235 enriched to a few percentage points to be fed into the inadequate gas-diffusion plant. It began to produce in March 1945. By placing the three separation methods in sequence and working night and day to improve the efficiency of all three, it began to look as if U-235 for one bomb might be available in the second half of 1945.

These disappointments with U-235 naturally turned men’s hopes to the plutonium being made at Hanford. When the first giant pile went “critical” there on September 27, 1944, it shut itself down after a day and then restarted itself again after another day. Frenzied study and consultation with the smaller piles at Oak Ridge and at Chicago finally revealed the unexpected production, within the pile, of a neutron-absorbing isotope, Xenon 135, with a half-life of 9 hours; the pile started itself again when this decayed, and thus stopped draining neutrons from the uranium fission process. This problem was eventually solved by greatly increasing the uranium tubes in the pile.

All through this worry, Los Alamos was having problems with the trigger mechanisms. Experiment and calculations eventually showed that the critical mass of U-235 was less than 11 pounds, about the size of a small grapefruit, if it were properly compressed and in spherical shape. To achieve this, two mechanisms were conceived, known as the “gun” and “implosion.” The “gun” was designed to create a critical mass by shooting a lump of U-235 at high velocity into a subcritical mass so that the combination would be over the critical mass. The resulting shape, however, was so unspherical that it was calculated that the ‘whole amount of U-235 necessary for the gun trigger bomb would be almost twice the ideal critical mass. This increase from about 11 to about 21 pounds of U-235 per bomb would extend the date on which the bomb was ready by weeks, since the output of U-235 was so small.

The second trigger, called “implosion,” planned to make a hollow sphere of U-235 or plutonium which was critical in total amount but kept subcritical by the hole in the center. This metallic sphere would be crushed together into the space in its center to make a critical mass there by the explosion of twenty or more crescent-shaped pieces of TNT which surrounded the sphere. The difficulty was that all the surrounding TNT had to explode at the same instant in order to ram the nuclear material together at the center; any lag would simply bulge the nuclear material erratically and prevent the achievement of critical ‘mass. All the ordnance experts, including Captain Parsons, of the United States Navy, in charge of this part of the work at Los Alamos, were convinced that such accurate timing of TNT explosion, with two dozen pieces exploded within a millionth of a second, would be impossible.

This brought up another crisis because Glenn Seaborg (Nobel Prize, 1951) and Segre predicted and then demonstrated that the PIutonium-238 which they were seeking from the Hanford piles spontaneously changed itself, at a slow rate, into its isotope Plutonium-240. Since Pu-240 was a spontaneous fissioner, this impurity would prematurely explode the target mass of plutonium in the gun-type trigger, since the inefficiency of the gun mechanism made it necessary to have the target mass so large (perfectly safe with U-235, but suicide with Pu-238 if there was Pu-240 in it also). The plutonium, therefore, had to be used with an implosion trigger, and, if that could not be devised, the $400 million cost of the Hanford plant had been practically thrown away.

Fortunately, George Kistiakowsky, chemistry professor from Harvard and a great authority on explosives, came to Los Alamos, and by the spring of 1945 had worked out an ignition by which all the TNT would explode within a few millionths of a second. This saved the plutonium scheme, but it was clear that this material would hardly be available in a bomb amount until late summer of 1945 and that there would not be enough to test the implosion trigger on it, if it were to be used in the war.

By July 1945, everyone concerned with the bomb was working around the clock, and a few had begun to fear that the war would be over before the bomb would be ready. On the other hand, a group of the scientists, led by Szilard who had instigated the project, were beginning to agitate that the bomb should not be used against Japan. Their motives have been questioned since, but were both simple and honorable. They had pressed for the atom bomb in 1939 because they feared that Germany was working on one and might get it first. Once the defeat of Germany ended that danger, many scientists regarded continued work on the bomb as immoral and no longer defensive (since there was no chance of Japan’s developing one). No one in July 1945 realized that all the significant information about making the bomb, notably the relative merits of different kinds of uranium, methods of plutonium separation, and the two kinds of trigger mechanisms, had been sent on to the Soviet Union, chiefly from Klaus Fuchs and David Greenglass by way of Harry Gold and Anatoli A. Yakovlev in June 1945. Even today American “security” agents are trying to keep secret these facts which have been fully explained in easily available technical publications.

For many years after 1945 the American people were kept in a state of alarm by stories of “networks” of “atomic spy rings,” made up of Communist Party members or sympathizers, who were prowling the country to obtain by espionage what the Soviet Union was unable to achieve by its own efforts in scientific research and industrial development. These stories have been spread largely by partisan conservatives and Right-wing neo-isolationists, by the periodical press and other entertainment media who make money out of sensationalism, and by the publicity agencies of the Federal Bureau of Investigation (whose chief purpose, for more than a quarter-century, has been to depict J. Edgar Hoover as the chief, if not the sole, defense of our country against subversion).