Bombs Away

The Science (And History) Behind The Atomic Bomb
30

An image captured from the B29 chase plane Necessary Evil
of the actual detonation of the first atomic bomb over Hiroshima.

The law of conservation of mass states that in any closed system the ultimate mass of a system must remain constant over time. The law of conservation of energy posits that the total energy of an isolated system also must remain constant. Mass and energy are therefore said to be conserved over time. Both can change forms, but mass and energy can be neither created nor destroyed. They simply change from one state to another.

Take a match as an example. You strike a match and allow it to burn. What’s left is smaller and different from the original match. However, that excess mass didn’t just disappear. It changed forms. As the compounds in the wood degraded they gave off energy and some portion of it turned into a series of gases. That original stuff is not gone. It’s just different.

In the world of physics, this isn’t only not a good idea. It’s the law. However, there is one glaring exception. Albert Einstein codified the details within his extraordinary equation E=MC2.

Nuclear Fission

In the case of nuclear fission, a small amount of matter actually transforms directly into energy. Unlike our burning match example, some mass of fissile material consumed in a nuclear reaction no longer exists in our universe. This matter is physically transformed into energy. The ratio is driven by that equation where E is energy, M is rest mass or invariant mass, and C if the speed of light. As the speed of light is a big number and you are squaring it, the resulting amount of energy you get for a small amount of transformed matter can be truly astronomical.

As weird as all this seems, we can see the practical results clearly enough. It is this reaction that propels American aircraft carriers and nuclear submarines. It also drives nuclear power plants.

Despite their simply breathtaking size, aboard a nuclear-powered ship, a small amount of fissile material powers everything from propulsion and catapults to hot water and laundry. Producing 200,000 horsepower non-stop to power the USS Gerald Ford for a week requires less than nine pounds of enriched uranium fuel. These massive ships are expected to run for a quarter century without refueling. The tiny amount of fuel that is actually turned into energy in a nuclear reaction is even smaller yet. Now, hold that thought.

Little Boy was the first nuclear weapon fired in anger.

The Little Boy Test

The atomic bomb unleashed on Hiroshima, Japan, on August 6, 1945, was called Little Boy. This was a relatively simple gun-based device that burned enriched Uranium-235. The term enriched stems from the fact that only about one part in 140 of naturally-occurring uranium is the particular desirable U-235 isotope.

Building the bomb was fairly easy. Harvesting that specific uranium isotope was hard. That’s what the Iranians have been hell-bent on doing for the past decade.

Little Boy was little more than a stubby gun. An enriched uranium target sat at one end and a smaller uranium projectile resided at the other. At the point of detonation a chemical explosive fired the projectile down the internal barrel into the target and achieved critical mass for a spontaneous detonation.

The plutonium-based bomb, Fat Man, which dropped on Nagasaki three days later, was a more complicated implosion design. It was this mechanism that was detonated during the Trinity test in New Mexico. The first operational test of the Little Boy bomb was the Hiroshima attack. Most of the uranium used in Little Boy came from the Shinkolobwe mine in the Belgian Congo.

A Single Paperclip

Once ready to go Little Boy sported an all-up weight of 9,700 pounds. Of that mass was 141 pounds of enriched uranium. The average level of enrichment was around 80%. Upon detonation around two pounds of uranium underwent nuclear fission. Of the bit that burned, only 0.7 grams or around 0.025 ounce was actually transformed into energy. This is roughly the same mass as a dollar bill or a paperclip.

The bomb was deployed from the B29 Superfortress Enola Gay at 0815 in the morning. It fell for 44.4 seconds before its dual redundant time and barometric triggers fired the ignition charges. The weapon detonated 1,968 feet above ground, and the resulting explosion released 63 Terajoules’ worth of energy — the equivalent to 15,000 tons or 30 million pounds of TNT (Trinitrotoluene) high explosive.

The fireball was 1,200 feet in diameter with a surface temperature of 10,830 degrees Fahrenheit, roughly comparable to the surface of the sun. Every man-made structure within a mile of ground zero was instantly pulverized. The resulting firestorm was roughly two miles in diameter. Survivors reported a strong smell of ozone, as though they had been near a powerful electrical arc. This unprecedented explosion killed 66,000 people and injured another 69,000, all for the cost a single paperclip’s worth of enriched uranium.

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