After Hiroshima was annihilated on August 6, 1945, the Japanese knew better what was going on, and a commando raid on the F-31 “Fat Man” implosion weapon assembly hut on Tinian was organized immediately. Philip Morrison and Robert Serber were directing the complicated work on F-31, and the hypodermic tube, used to monitor the subcritical activity in the bomb core, had just been installed at mid-morning on August 8. Two segments of the spherical aluminum bomb casing, Y-1560-6 and Y-1560-5, were being bolted together. The atmosphere was getting tense in the hut, and a few of the team members took a break outside, trying to rest under a tree. Looking out to sea, they suddenly noticed an odd-looking ship, approaching about a mile off, to the north. It was diesel-powered, painted completely black, about 150 feet long, with the deck five feet above the waterline. It was devoid of markings but was flying a tattered American flag. Swimmers were diving off the deck, at about 100-foot intervals, and making for shore. By the time the ship had passed the assembly hut, at least 30 swimmers were in the water, with more peeling off the deck. A security guard on the embankment opened up with a machine gun, firing over the heads of the assembly techs and aiming for the bow.
It was strange that they tried this stunt in broad daylight. Had they been delayed by several hours and missed their insertion schedule? The ship hove a hard right and headed out to sea, picking up what few swimmers it could. Clearly, a desperate attempt to sabotage the next A-bomb had failed.[29]
As a demonstration of the overwhelming strength of the Allied invasion force bearing down on Japan, dropping a uranium bomb on Hiroshima was unsurpassable. The mechanics of the A-bomb explosion have been thoroughly studied, and here is a summary:
The nuclear fission explosive uses the fact that a uranium-235 or plutonium-239 nucleus can split into asymmetric fragments when it encounters a loose neutron. This unusual reaction releases about 200 MeV of energy, which on the atomic scale is a great deal. Also emerging from this mini-explosion are two extra neutrons. These neutrons, traveling at high speed, crash into other nuclei in the tight matrix of a bomb core, which consists of a metallic mass of the fissile material. The first reaction thus accelerates into two reactions, and each generation of reaction leads to twice as many subsequent reactions. In fewer than 90 such generations, every nucleus in a 50-kilogram uranium bomb core will experience the fission stimulus, and the combined reactions release the energy equivalence of exploding a million tons of TNT high explosive. Given the speed of the flying neutrons, the size of a bomb core, and the response time of a uranium nucleus, these 90 generations take place in about one millionth of a second. The short time in which this much energy lets go provides the condition for a hell-on-earth explosion.[30]
Most of the energy from this explosion, 85 percent, is released in the form of heat. The heat radiates as light energy, from infrared to ultraviolet. The remaining 15 percent of the energy release is radiation of nuclear origin, but only five percent is immediately involved. Residual radiation, ten percent of the bomb’s energy, is released on a falling exponential rate over thousands of years after the instant of detonation.
The World War II bombs, the only nuclear devices ever used as weapons so far, were airbursts, detonated at about 1,900 feet above the ground.[31] The air surrounding the bomb instantly heated to incandescence. This feature is called “the fireball.” This rapidly expanding sphere translated a percentage of the thermal energy into blast energy, or a destructive wave of compressed air moving outward at high speed, capable of knocking over concrete buildings.
The first thing hit by this airwave was the ground directly underneath the bomb, or “ground zero.” This was a hard thump, and it resulted in an earthquake-like shock energy traveling outward through the ground. The total energy from the detonation was thus distributed as 50 percent blast and shock, 35 percent thermal radiation, 10 percent residual nuclear radiation, and 5 percent initial nuclear radiation. The scientists had not been wrong in predicting small damage due to nuclear radiation, but they had been way off in considering the damage done directly and indirectly by the intense thermal energy. The burns that injured many survivors of the A-bombs were not caused by gamma or beta rays, but by light. Simply being caught standing behind a light-shield when the bomb detonated could be life-saving, providing you weren’t struck down by the shield as it was blown away seconds later in the air blast. The temperature at the center of the explosion was far outside human experience, probably millions of degrees, approaching the conditions in the center of the sun, and the air pressure produced was on the order of millions of pounds per square inch. Everything flammable within 12 miles caught fire. Some people were vaporized in the fireball, tens of thousands were crushed in the air blast, and tens of thousands more were severely burned by the flash of light. The death-toll would eventually reach about 83,000 people, as some would die decades later from radiation-induced cancer.
The heat and initial nuclear radiation portions of the event were over in about 60 seconds, but the bomb effects continued to develop for 6.3 minutes. The rapidly expanding fireball created a large vacuum in midair, and as the heat dissipated, air from the surrounding territory started to be sucked in. The blast thus blew air both ways: first outward, a pause, then inward, back toward ground zero. This effect is called the “afterwind.” Meanwhile, the residual heated air rose in a strong updraft, like a hot-air balloon. Solid material on the ground, now pounded to dust, was drawn up into the rising column, making a dirt-cloud.
In thirty seconds, the cloud reached a height of three miles. When the ever-rising cloud reached an altitude where its density matched that of the surrounding air, at the base of the stratosphere, the cloud started to spread out horizontally. The sight of this feature became an icon, a dreaded emblem of the atomic age — the mushroom cloud.
On August 9, 1945, the Strike Centerboard operation, carrying the Fat Man plutonium implosion device in a B-29 named Bock’s Car, dropped the second weapon on Nagasaki, and World War II was over except for the shouting.[32]
To develop these science-fiction-level devices into things that could fall from an airplane required a crash program of unprecedented speed and complexity. Not only was the nuclear reactor invented, prototyped, powered up, and operated for three months, but a huge reservation was built in Washington State so that several reactors could be run 24 hours a day at high power, experimental reactors were built and operated in Tennessee and Illinois, massive plutonium and uranium purification plants were built and run, and risky physics experiments were conducted in New Mexico, all without a single fatal accident or even a radiation injury. Thousands of people worked on this project, some in hazardous conditions and most without a clue as to what they were building. The effort was constantly plunging ahead into the unknown, and the potential for disaster was always close; but due to heightened vigilance and a touch of luck, nobody got hurt. There are no atomic accidents in the Manhattan Project on which to report, right up until the last bomb was dropped. There were, however, some close calls that could foretell later problems.
29
The only record I can find of this action is in a book written by team member Harlow W. Russ,
30
This one-megaton bomb is a theoretical device used by Samuel Glasstone in his definitive work,
31
The airburst tactic did two things: it maximized the radius of destruction, and it minimized the resulting radioactive fallout. The explosions kicked up a lot of dust, but the only radioactive material to be spread off-site was the bomb itself, which was about 9,700 pounds of metal. The bomb debris consisted of fuel that failed to fission, fission products, and various metals in the structure of the device, a portion of which were neutron-activated to radioactivity in the explosion.
32
Atomic bomb trivia: Stenciled in black on the bright orange nose of the Fat Man bomb were two things: a profile of a fat man with a capital F at his back, and the cryptic inscription “JANCFU.” It meant “joint army navy and civilian fuck-up.” The assembly team had been unable to use the heat-tempered armor plates for the bomb’s airframe. The plates had been warped in the process of hardening them, and in desperation they resorted to using the mild steel plates left over from the pumpkin series of practice bombs. Once they got it all together, they realized that one of the safety plug sockets had been wired wrong. With no time to take the thing all apart and re-wire the socket, they modified the safety plug to match the error, so this bomb was not completely built-to-prints.