How the Atomic Bomb Works
On August 6th, 1945, an American B-29 bomber flying low over the Japanese city of Hiroshima dropped the world’s first atomic bomb on the city’s unsuspecting inhabitants, immediately killing 80,000 innocent civilians. Three days later, a second bomb was dropped on the city of Nagasaki, killing a further 40,000 men, women, and children. In the aftermath of the bombs’ initial explosions, tens of thousands more people would die excruciatingly painful deaths due to radiation exposure.
While the world may be familiar with the tragic story of the first use of the atomic bomb, we are less familiar with exactly how it works—the atomic bomb was a devastating act of cruelty, but also a technological marvel that would forever alter the face of war. The devastating bombing of Japan was enough to deter the use of nuclear weapons for at least a few decades, but after World War 2, increasing tension between the U.S. and Russia led to the Cold War, a nuclear arms race between the two world powers that saw both sides rushing to increase their stockpile of nuclear weapons, ostensibly as a means to deter war.
People built bunkers in their backyards and stocked up on canned goods; schools ran nuclear war drills; and the world waited with bated breath for the outbreak of nuclear war and what felt like the imminent end of the world. But although the existence of nuclear weapons was common knowledge, and despite the widespread panic about nuclear war, few people truly understood just how an atomic bomb worked. To understand how the atomic bomb works, we have to take a trip back to high school physics class to revisit the concepts of atomic structure and radioactivity.
An atom is one of the smallest units of matter, and it is made up of 3 subatomic particles. The nucleus at the centre of an atom is made up of protons, which are positively charged, and neutrons, which have a neutral charge. Negatively charged electrons orbit the nucleus of an atom for atomic bomb. When the ratio of protons to neutrons is 1-to-1, the atom as a whole will have a neutral charge. But, if the number of protons in an atom is changed, an entirely different element will be created. If the number of neutrons changes, you end up with an isotope.
For example, the carbon atom has 3 naturally occurring isotopes: in its common, stable form, carbon-12 has 6 protons and 6 neutrons; carbon-13 has 6 protons but 7 neutrons, and while rare, is still a stable element. Carbon-14, with its 6 protons and 8 neutrons, is both rare and unstable—or radioactive. Radioactive nuclei emit particles called radiation through a process called radioactive decay, and it’s this process that scientists harnessed to create the powerful atomic bomb. There are a few different ways to destabilize a particle, but for understanding how nuclear bombs work, the most important processes to grasp are fission and fusion to atomic bomb.
Fission involves splitting the nucleus of an atom into 2, which scientists can do by bombarding it with free neutrons. As the nucleus splits, it ejects neutrons along with bursts of electromagnetic energy called gamma rays. Fusion, in contrast, involves bringing together the nucleus of two atoms to form a single larger one. This is actually the process by which our sun produces energy. Through endless experimentation and a process of trial and error, scientists eventually discovered that uranium was the element that was most cooperative in inducing a fission reaction to atomic bomb.
The isotope Uranium-235 is one of the few materials that can be forced to undergo fission by bombarding its nucleus with neutrons rather than waiting 700 million years for it to decay naturally. U-235’s nucleus will readily absorb the neutrons, become unstable and split, throwing off 2 or 3 new neutrons in the process. These new neutrons can then go on to collide with the nucleus of other U-235 atoms, starting a fission chain reaction. The splitting of the nucleus happens incredibly quickly-in the order of picoseconds, or 0.000000000001 seconds-yep, that’s 11 zeros! The scientific principles underlying the atomic bomb had been well known since Einstein’s days, but they wouldn’t be successfully applied and weaponized until the Second World War for atomic bomb.
In the 1930s, Italian scientist Enrico Fermi successfully bombarded elements with neutrons, transforming them into new elements, and shortly thereafter, German scientists Otto Hahn and Fritz Strassman were the first to fission uranium by bombarding it with neutrons, producing the radioactive barium isotope. These breakthroughs led the scientific community to wonder if it was possible to create a fission chain reaction that could release enormous amounts of energy that could be harnessed and weaponized—an idea that greatly intrigued the world’s governments, who were in the midst of fighting World War 2 at the time.
In an effort to be the first to weaponize fission—and beat the Nazis to the punch—the U.S. government recruited the brightest minds in physics from all over the world and launched the secretive Manhattan Project with the goal of creating the world’s first functional atomic bomb. In 1941, scientists at Columbia University tried to initiate a chain reaction using uranium-235 but failed. Shortly thereafter, Fermi, now working for the U.S. at the University of Chicago, successfully achieved the world’s first controlled nuclear chain reaction in his lab underneath the school’s squash courts.
Also in 1941, Berkley scientists discovered a new element—element 94—with nuclear fuel potential, which they named plutonium. With these discoveries, the race to develop a nuclear bomb was on in earnest, and within just a few short years, the world’s first nuclear bombs would be used in war. Understanding the concept of fission was only part of the problem—figuring out how to weaponize it and constructing devices to harness atomic power was a whole other challenge.
A critical mass is the minimum amount of material needed to sustain a chain reaction, so to harness nuclear power, the nuclear fuel has to be kept in separate subcritical masses that won’t support fission. When it’s time to detonate, the subcritical masses are brought together to form a supercritical mass, and free neutrons are introduced to jumpstart the fission process.
A small pellet made of the elements polonium and beryllium serves as the neutron generator, and the entire reaction is confined within a dense material called a tamper, usually made of another uranium isotope, U-238, to reflect the neutrons back into the core and to slow the core’s expansion to ensure that as much fission as possible happens before the bomb explodes. Scientists developed two different trigger systems for the first atomic bombs.
Little Boy, the bomb dropped on Hiroshima, was a gun-triggered bomb with a 14.5 kiloton yield, equal in power to 14,500 tons of TNT. Little Boy was 1.5% efficient, meaning that 1.5% of the material fissioned before the bomb exploded. In a gun-fired nuclear weapon, a bullet of U-235 is placed at one end of a long tube packed with explosives, which will fire the bullet down the tube where it collides with the neutron generator, initiating fission and starting the chain reaction that will lead to the bomb’s explosion. In contrast, Fat Man, the atomic bomb that was dropped on Nagasaki, was an implosion device with a 23 kiloton yield and 17% efficiency-much more effective, but also much more complicated to make than Little Boy.
Implosion bombs feature a sphere of radioactive U-235 as the tamper around a plutonium-239 core. The entire sphere is surrounded by high explosives which, when detonated, create a shockwave that compresses the core and initiates the fission chain reaction. In the wake of World War 2, scientists recognized that fission bombs were wildly inefficient and turned their attention to fusion next. Fusion bombs, also called thermonuclear or hydrogen bombs, rely on the hydrogen isotopes deuterium and tritium as fuel and can yield up to 10,000 kilotons, making them up to 700 times more powerful than the Little Boy fission bomb a atomic bomb.
Hydrogen bombs combine fission and fusion to achieve a more powerful and more efficient explosion. Within the bomb’s casing is a tamper made of U-238, which is packed with hydrogen isotope fuel and surrounds a hollow rod of plutonium-239 at the core. An implosion fission device detonates first, compressing the fuel and causing the plutonium core to fission. The fissioning rod in turn gives off heat and pressure, which initiates fusion in the hydrogen isotopes and causes the bomb to explode. The entire process takes just 600 billionths of a second.
Not only have the bombs themselves improved drastically, but the delivery methods have come a long way since World War 2. Philip Morrison, a former member of the Manhattan project, told Scientific American in 1995 that “All three bombs of 1945—the [Trinity] test bomb and the two bombs dropped on Japan—were more nearly improvised pieces of complex laboratory equipment than they were reliable weaponry.”
Today, nuclear weapons come in many forms, from ballistic missiles that can exit the atmosphere and travel thousands of miles before reentering and detonating; to cruise missiles, shorter-range missiles with smaller warheads that are harder to detect and intercept; and to a range of tactical nuclear weapons like artillery shells and land mines that can target a smaller area. Nuclear weapons are terrifying because of their immense destructive power relative to their size of atomic bomb.
The most severe damage happens at the blast’s hypocenter, or ground zero, where everything is immediately vaporized. Outward from the centre, most of the damage is the result of flying debris, intense heat, a powerful shockwave and acute exposure to high radiation. Beyond the immediate blast area, death and injury can result from heat and resulting fires, as well as radiation. The physical destruction caused by a nuclear bomb is no doubt catastrophic, but the most dangerous part of a nuclear bomb is the radiation and radioactive fallout.
After the initial explosion, clouds of fine dust made of radioactive particles are carried away by the wind and fall back to the ground, poisoning the water supply and getting ingested and inhaled by people even miles away from the blast. We now know that radiation affects the cells in our body that readily divide, like hair and gut cells, bone marrow, and reproductive organs, leading to nausea, vomiting, and diarrhea, and long-term health consequences like cataracts, hair loss, loss of blood cells, and an increased risk of leukaemia, cancer, infertility, and birth defects.
At the height of the cold war in the 1980s, scientists warned about the danger of a nuclear winter. In a worst-case scenario, so many nuclear bombs could explode that great clouds of radioactive dust could travel high into the atmosphere, blocking out sunlight and lowering surface temperatures. This could lead to major disruptions in the food chain and mass extinctions of species, including humans. The Cold War may be over, but the threat of nuclear war is by no means gone.
Countries around the world have signed treaties agreeing to limit their stockpile of nuclear weapons and prohibiting them from using them against other countries, but still, the number of nuclear weapons around the world continues to grow—and not all countries have agreed to use them responsibly. At least 9 countries currently have ballistic nuclear weapons, and 3 of those countries—the U.S., Russia, and China—have weapons powerful enough to hit any target anywhere in the world.
Then there’s North Korea-in 2009, they tested a nuclear bomb as powerful as the atomic bomb dropped on Hiroshima, and the underground test explosion caused a magnitude 4.5 earthquake. There’s no doubt that nuclear warfare still presents a huge threat to world peace-not to mention the continuation of our species! If you thought this blog was fascinating, be sure and check out our other blogs, like this one called “5 Reasons Why an Embedded System is Better Than a PC: A Blog about Why Embedded Systems are Better Than PCs and the 5 Reasons Why,” or perhaps you’ll like this other blog.
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