1945

Trinity Nuclear Bomb

Robert Oppenheimer (1904–1967)

At its heart, a nuclear bomb exploits this simple principle: if 115 pounds (52 kg) or more of U-235 (uranium) comes together in a roughly spherical shape, the sphere will explode with amazing force. It works because a U-235 atom has this surprising property: If it absorbs a neutron traveling past it, the atom will split into two smaller pieces and emit three new neutrons, which fly off to trigger other U-235 atoms to do the same. The two smaller pieces plus the three neutrons weigh less than the original U-235 atom. The missing weight converts to energy at the rate E = mc². In other words, a very large amount of energy becomes available through direct mass-to-energy conversion. Hence the tremendous explosive energy seen in nuclear bombs. The absorption and splitting process happens nearly instantaneously.

To create a nuclear bomb that exploits this natural property, the engineer’s job is to figure out an efficient way to keep masses of U-235 separate until the bomb needs to explode, and then bring the masses together. The first design was almost too simple to believe. A critical mass of U-235 is separated into two parts. One part is stationary. The other part is loaded into an artillery barrel. When the artillery round fires, the two masses combine, become supercritical, and explode.

The problem with this design is inefficiency. As the bomb explodes, the supercriticality is lost, so perhaps only 1 percent of the uranium has a chance to fission before the mass shatters. Engineers therefore worked to increase the efficiency. One way is to shape the mass of U-235 (or plutonium) into a hollow, broken sphere, and then use a uniform conventional explosion around the sphere to create a solid, supercritical sphere. The initial explosive force along with momentum will keep the sphere together for just a bit longer, improving efficiency. This design was employed in the first nuclear explosion in history—the Trinity bomb, created in 1945 by scientists affiliated with the Manhattan Project, including Robert Oppenheimer, who gave the bomb its name. Other engineering techniques include neutron reflectors and tampers—strong, heavy containers that keep the exploding mass together longer. Further tests in fusion would yield a successful hydrogen bomb.

SEE ALSO Bow and Arrow (30,000 BCE), Ivy Mike Hydrogen Bomb (1952), Cluster Munition (1965), Fukushima Disaster (2011).

This graphic illustrates the two methods of assembling a fission bomb.