The uranium-based atomic bomb design, “Little Boy,” was so foolproof there was no question that its use would result in a full nuclear explosion. The only problem was producing the purified U-235, but it looked as though a full bomb load would be ready by August 1945. The configura­tion was straightforward. A stack of nine rings of U-235, 6.25 inches (15.9 cm) in diameter, were to be shot down a 6.5-inch (16.5-cm) gun barrel using four two-pound (1.8-kg) bags of cordite canon propellant. The four — inch (10-cm) hole in the center of the stack of rings would slide perfectly over a stack of six U-235 disks bolted to the end of the gun and centered in the bore. The stack of rings and the stack of disks would come together quickly as the rings reached the end-of-travel in the gun barrel, slamming into a tungsten-carbide anvil, with the two combined uranium shapes forming the equivalent of four supercritical masses. Instantly, the nose of the bomb would convert into a ball of superheated, radioactive plasma,

"Little Boy" Nuclear Weapon

6.5-inch (16.5-cm) gun tube

U-235 Target rings Tamper plug

U-235 projectile rings

Projectile tungsten-carbide disk

Polonium-beryllium Rod holding initiators target components

Gun breech

© Infobase Learning

“Little Boy” brought two masses of uranium-235 together to form a hypercritical mass, using a gun barrel.

1.25 miles (two km) in radius, as the uranium underwent unimpeded, prompt fission on the high end of the neutron energy spectrum. There was no need to even test this version of the atomic bomb.

The plutonium bomb, Fat Man, to be detonated by the implosion method, was another matter. There were too many unknowns, too many bits and pieces of technology that had never been tried before, and there was too much surplus Pu-239 not to test it. After several configuration changes in early 1945, the final production design, designated Model 1560, was frozen by April 3,1945. At the center of the 10,265-pound (4,656-kg) bomb was a 14-pound (6.4-kg) ball of plutonium, to be crushed into a hyperdense, hypercritical mass. The ball, or core, 3.62 inches (9.2 cm) in diameter, was surrounded by a shell of U-238, to act as an inertial weight to keep the core together for a few microseconds as it fissioned, sur­rounded by a thin layer of boron-10 to hold down spontaneous neutrons

reflecting back into the core, surrounded by a thick layer of aluminum, to hold it in place. The chemical explosives surrounded the metal core pieces in two layers. The inner layer was 32 close-fitting segments, made of a slow-burning explosive called baratol-70, precision cast like parts of a plastic puzzle. Surrounding the baratol layer were another 32 segments of a fast-burning explosive called Composition B. On the inner surface of each segment of the outer explosive was a depression, backfilled with baratol. These were the explosive “lenses” that would direct the detonation into a shrinking spherical shock wave, imploding the core and starting the fission. Each outer segment of the explosive was equipped with two Model 1773 bridge-wire detonators, all wired to go off at once.

A replica of the bomb to be dropped by airplane over Japan was assem­bled, having all parts except the outer steel armor plating, and was tested in the desert of New Mexico, early in the morning on July 14, 1945. The test was code-named Trinity, or TR.

The test site was a lonely patch of desert named Alamogordo. On it was erected a 60-foot (18-m) steel tower, bought as surplus from the U. S. Forestry Service, and the bomb was winched into position on a wooden platform at the top. Scientists from the lab were invited to the test, and they could watch from a spot 10 miles (16 km) away from the blast. They were cautioned against looking into the darkness at the point where the explosion was expected to occur, because it was expected to be an unusually bright flash. Most looked in the opposite direction, some just closed their eyes, some looked through welder’s goggles, and Richard Feynman (1918-88), an American theorist from MIT and Princeton, decided to sit in a truck and look through the windshield. He reasoned that the ultraviolet rays from the light would be shielded from his eyes by the glass. Oppenheimer, scientific director of the project, watched the 25-minute countdown at the 10-mile point, while General Groves observed from a more discrete 17 miles (27 km). There was a pool of bets on the strength of the blast, ranging from a bet that nothing would hap­pen to a bet that the atmosphere would catch fire and the entire world would be destroyed.

The countdown had to be stopped at 20 minutes, as a rainstorm blew across the test site, with lightning. There was fear that a lightning strike would set off the bomb, but the storm left, and at 5:10 a. m., the countdown resumed. At 5:29:45 a. m., the first atomic bomb exploded. It lit up the darkness like instant noon. Feynman, staring directly at the blast, was temporarily blinded as the brilliant white light overloaded his retinas. The

light was visible on the horizon 150 miles (241 km) away, and the shock wave rattled windows at a distance of 200 miles (322 km). Performance of the gadget exceeded most expectations, with an energy yield equivalent to 20,000 tons (18,143.7 mt) of TNT, or 84 trillion joules.