CHAPTER TWENTY-FOUR
PLUTONIUM
MANHATTAN PROJECT SCIENTISTS, ENGINEERS and their corporate partners had overcome myriad obstacles since they began their journey. After the war, Pentagon officials would calculate that they had filed several thousand patent applications for new devices, new methodologies, new technology to clear a path to the atom bomb. None of those obstacles posed a greater challenge than detonating a plutonium bomb.
The gun type detonator that triggered the U235 bomb wouldn’t work with plutonium. The thousandth of a second required for the projectile and target to assemble was too slow. Fortunately, Oppenheimer had a backup. Some of his scientists had been experimenting with an idea for a faster detonation technology called “implosion.” It was a long shot, so it had gotten little attention. Now it was Oppenheimer’s only option. It had to work.
It was far more complex than the U235 detonator: a high-performance racing machine versus a Model T. Thirty-two high-energy explosive charges encompassing the outer shell of the spherical bomb would fire at precisely the same moment. Inside the bomb, the shockwaves from those explosions would compress a grapefruit-sized plutonium sphere into a dense, golf-ball-sized critical mass. The compressed mass would, in turn, squeeze an initiator, which would spew out millions of neutrons, accelerating the nuclear chain reaction and leading to an explosion.
The dynamics of the implosion had to work to perfection. If not, the plutonium mass would deform like a squeezed balloon. Neutrons would escape, and the chain reaction would fizzle out. 209
In the fall of 1944, Oppenheimer and General Groves pulled out all the stops, racing to develop the new detonator. They asked Fermi, who was at the Met Lab in Chicago, to relocate to Los Alamos. They scoured the country looking to recruit additional mathematicians and explosives experts. General Groves relaxed the tight security at Los Alamos so that British scientists, whom he had previously barred from the top-secret site, could be brought in. And he arranged to have 600 members of the Army’s Special Engineering Detachment reassigned to help with the exhaustive testing that went into designing the bomb. 210
Finally, after months of experimentation, they came up with plans for a working detonator. If all went well, the plutonium bomb would be even more powerful than the U235 bomb. 211 But manufacturing the precision components proved more difficult than anticipated. The first shipment of parts for the explosive mechanism failed at an alarming rate, and a second contractor supplying the firing circuits was late on its deliveries. 212
THE CONVOY, THREE LARGE panel trucks sandwiched between a lead and rear escort car, arrived early in the morning. Each vehicle had two guards “armed with shotguns, revolvers and a submachine gun.” 213 Their destination: a vault dug deep in the side of a mountain in Hanford, Washington. As the armed guards watched, soldiers emerged from the vault carrying dozens of wooden boxes and set them gently on racks inside the trucks. Each box contained a stainless-steel flask. In that flask, 80 grams of a bluish-green slurry: plutonium.
From Hanford the five-vehicle convoy traveled 700 miles to Fort Douglas in Salt Lake City. There they met a second convoy of soldiers who transferred the precious cargo to their trucks. The first returned to Hanford; the second headed for Los Alamos. Neither convoy knew the full particulars of the journey, where it originated and where it ended.
As Hanford’s three giant nuclear reactors disgorged more and more plutonium, the convoys grew more frequent. 214 By July 1945 they were delivering almost enough plutonium to produce three bombs a month. Oppenheimer’s long struggle to develop the implosion detonator was almost over. Would it work? Oppenheimer was confident that the U235 bomb with its relatively simple detonator would. But the plutonium bomb with its precision firing system was always a serious concern. As far back as January 1944 Oppenheimer and Groves had begun discussing ways of testing the bomb. 215 That test was finally scheduled for July 16, 1945.