How Do Nuclear Bombs Function?

The first nuclear bomb, designed to cause massive human casualties, exploded over Hiroshima, Japan, on August 6, 1945. Three days later, the second bomb detonated over Nagasaki. The combined death toll from these two blasts, estimated at 214,000 lives, and the unparalleled destruction they caused marked a historic shift in the nature of warfare [source: Icanw.org]

At the conclusion of World War II, the United States stood as the sole superpower possessing nuclear weapons. However, this dominance was short-lived. In 1949, the Soviet Union, aided by a network of spies who had infiltrated U.S. nuclear secrets, successfully tested their own atomic bomb [sources: Icanw.org, Holmes]

As tensions between the U.S. and the Soviet Union deepened into a prolonged period of hostility known as the Cold War, both superpowers developed an even more destructive nuclear weapon — the hydrogen bomb — and amassed large arsenals of warheads. Both nations enhanced their fleets of strategic bombers and deployed land-based intercontinental ballistic missiles capable of striking each other's cities from thousands of miles away. Submarines, too, were equipped with nuclear missiles, further increasing the potential for a devastating attack [sources: Locker, Dillin].

By the late 1960s, several other nations, including the United Kingdom, France, China, and Israel, had also acquired nuclear weapons [source: Icanw.org].

The threat of nuclear bombs cast a shadow over everyone and everything. Schools practiced air raid drills in preparation for a potential attack. Governments constructed fallout shelters, and homeowners dug bunkers in their backyards. Eventually, the nuclear-armed countries reached an impasse, each relying on a strategy of mutual assured destruction. This meant that even if one country successfully launched a surprise attack that killed millions and caused widespread devastation, the other country would still have enough weapons to retaliate and deliver an equally devastating counterstrike.

The grim reality of this threat kept the two sides from using nukes against each other, but the fear of a catastrophic nuclear war persisted. In the 1970s and 1980s, tensions remained high. Under President Ronald Reagan, the U.S. embarked on a program to develop anti-missile defense systems — a project skeptics dubbed "Star Wars" — which was designed to protect the U.S. from attack but could also allow the U.S. to strike first with little risk of retaliation. By the end of the 1980s, as the Soviet Union's economy began to crumble, Reagan and Soviet leader Mikhail Gorbachev began serious negotiations aimed at reducing nuclear arms.

In 1991, Reagan's successor, George H.W. Bush, and Gorbachev signed the landmark START I treaty, agreeing to major cuts in their nuclear arsenals. After the Soviet Union dissolved in 1991, Bush and Boris Yeltsin, president of the newly formed Russian Federation, signed another agreement, START II, in 1992, which further reduced the number of warheads and missiles [source: U.S. State Department].

Despite efforts to move beyond the threat of nuclear weapons, the shadow of the bomb still loomed. In the early 2000s, the U.S. invaded Iraq and deposed its dictator, Saddam Hussein, partly out of concern that he was pursuing a nuclear weapon program. However, it was later revealed that he had abandoned these covert efforts [source: Zoroya]. Meanwhile, Pakistan had already conducted its first successful nuclear test in 1998 [source: armscontrolcenter.org].

North Korea, another authoritarian regime, succeeded where Saddam had faltered. In 2009, the North Koreans detonated a nuclear weapon as powerful as the bomb that obliterated Hiroshima. The underground explosion was so powerful that it registered as a 4.5 magnitude earthquake [source: McCurry]. By the 2020s, the escalating tensions between Russia and the West, combined with the rise of a new generation of hypersonic missiles capable of bypassing early-warning systems to deliver nuclear warheads, brought about the specter of a dangerous new nuclear arms race [source: Bluth].

Although the political dynamics of nuclear warfare have evolved dramatically over the years, the fundamental science behind nuclear weapons — the atomic reactions that release such overwhelming energy — has been understood since the era of Einstein. This article will explore the mechanics of nuclear bombs, including their construction and deployment. We'll begin with a brief refresher on atomic structure and radioactivity.

Atomic Structure and Radioactivity

Before diving into the workings of bombs, we need to start at the most basic level — atomic. An atom consists of three key subatomic particles: protons, neutrons, and electrons. The core of an atom, called the nucleus, is made up of protons and neutrons. Protons have a positive charge, neutrons are neutral, and electrons carry a negative charge. The number of protons equals the number of electrons, giving the atom a neutral charge. For instance, a carbon atom has six protons and six electrons.

But it’s not as simple as it seems. The characteristics of an atom can drastically change depending on the number of each type of particle it contains. Altering the number of protons transforms the element entirely. Changing the number of neutrons results in an isotope.

For instance, carbon has three isotopes:

As illustrated by carbon, most atomic nuclei are stable, but some are inherently unstable. These unstable nuclei release particles, a phenomenon scientists call radiation. A nucleus that emits radiation is considered radioactive, and the process of emitting particles is referred to as radioactive decay. There are three primary forms of radioactive decay:

It's important to particularly note the fission process, as it will be central to understanding the mechanics of nuclear bombs.

Nuclear Fission

Nuclear bombs rely on the strong and weak nuclear forces that bind an atom's nucleus, particularly in atoms with unstable nuclei. There are two main ways nuclear energy is released from an atom.

Both fission and fusion processes release vast amounts of heat energy and radiation.

The discovery of nuclear fission is credited to Italian physicist Enrico Fermi. In the 1930s, Fermi showed that elements subjected to neutron bombardment could be transformed into new elements. His work led to the discovery of slow neutrons and new elements not found on the periodic table.

Not long after Fermi's breakthrough, German scientists Otto Hahn and Fritz Strassman bombarded uranium with neutrons, which resulted in the formation of a radioactive barium isotope. Hahn and Strassman determined that the uranium nucleus had undergone fission, splitting into two smaller fragments due to the impact of slow-moving neutrons.

Their discovery ignited a wave of research in laboratories worldwide. At Princeton University, Niels Bohr joined forces with John Wheeler to create a theoretical model of the fission process. Bohr and Wheeler theorized that it was the uranium isotope uranium-235, not uranium-238, undergoing fission.

Around the same time, other researchers observed that the fission process produced even more neutrons. This led Bohr and Wheeler to raise a pivotal question: Could the free neutrons released during fission initiate a chain reaction that would unleash an immense amount of energy? If so, it could be possible to develop a weapon of unprecedented power.

And indeed, it was.

Nuclear Fuel

In March 1940, scientists at Columbia University in New York validated Bohr and Wheeler’s hypothesis that the isotope uranium-235, or U-235, was responsible for nuclear fission. Although the Columbia team tried to induce a chain reaction using U-235 in the fall of 1941, they were unsuccessful. The research was subsequently moved to the University of Chicago, where Enrico Fermi achieved the world’s first controlled nuclear chain reaction beneath Stagg Field on a squash court. The rapid development of a nuclear bomb using U-235 as fuel followed.

Given its importance in nuclear bomb design, let’s examine U-235 more closely. U-235 is one of the few substances capable of undergoing induced fission. Instead of waiting for uranium to decay naturally over 700 million years, U-235 can be broken down much more quickly when a neutron collides with its nucleus. Upon impact, the nucleus absorbs the neutron, becomes unstable, and splits immediately.

Once the nucleus captures a neutron, it splits into two lighter atoms and releases two or three additional neutrons (the exact number depends on the specific way the U-235 atom splits). The lighter atoms then emit gamma radiation as they stabilize. Several intriguing aspects of this induced fission process make it particularly significant:

In 1941, scientists from the University of California, Berkeley, uncovered a new element, element 94, that showed potential as a nuclear fuel. They named it plutonium and produced enough of it the following year for experimentation. Over time, they confirmed plutonium's fission properties and recognized it as a second viable fuel for nuclear weaponry.

Fission Bomb Design

In a fission bomb, the fuel must remain in separate subcritical masses to prevent a premature detonation. A subcritical mass is one that is too small to sustain a fission chain reaction. Critical mass, on the other hand, refers to the smallest amount of fissionable material necessary to maintain a continuous fission reaction.

Let's revisit the marble analogy. If the marbles are spaced too far apart — representing subcritical mass — only a small chain reaction occurs when the "neutron marble" hits the center. But when the marbles are placed closer together — representing critical mass — there's a much higher chance of triggering a large chain reaction.

The challenge in keeping the fuel in subcritical masses is in bringing them together to form a supercritical mass, which provides enough neutrons to sustain a fission reaction during detonation. This is one of the key obstacles in fission bomb design, and it requires two solutions, which we will explore in the next section.

To initiate the fission in the supercritical mass, free neutrons must be introduced. This is accomplished with a neutron generator, which consists of a small pellet of polonium and beryllium, separated by foil within the fissionable fuel core. Here's how the process works:

To maximize the amount of material that undergoes fission before the bomb detonates, the design incorporates a dense material known as a tamper, typically made of uranium-238. As the fission core heats up, the tamper expands. This expansion creates pressure that slows the core's expansion, while also reflecting neutrons back into the core, thereby enhancing the fission reaction's efficiency.

Fission Bomb Triggers

The simplest method for bringing the subcritical masses together is by constructing a gun that fires one mass into the other. A sphere of U-235 surrounds the neutron generator, and a small bullet of U-235 is placed at one end of a long tube. Explosives are placed behind it, while the U-235 sphere is at the opposite end. A barometric-pressure sensor detects the correct altitude for detonation and triggers the following sequence of actions:

Little Boy, the bomb dropped on Hiroshima, was a gun-type bomb with a 20-kiloton yield (equivalent to 20,000 tons of TNT) and an efficiency of roughly 1.5 percent. This means that only 1.5 percent of the material underwent fission before the explosion dispersed it.

The alternative method for achieving a supercritical mass involves compressing the subcritical masses into a sphere via implosion. Fat Man, the bomb dropped on Nagasaki, was an example of this implosion-triggered bomb design, and constructing it was no easy feat.

The early designers of these bombs encountered various challenges, particularly in controlling and directing the shock wave uniformly across the sphere. To solve this, they designed an implosion system featuring a sphere of U-235 as the tamper, surrounding a plutonium-239 core, and encased in high explosives. When detonated, this bomb produced a 23-kiloton yield with an efficiency of 17 percent. The process unfolded as follows:

In 1943, Edward Teller, an American physicist, introduced the concept of boosting, which enhanced the basic implosion-triggered design. Boosting is a method where fusion reactions generate neutrons, which in turn trigger fission reactions at a much higher rate. It took an additional eight years of testing to prove the concept's effectiveness, but once it was validated, it quickly became a favored design. By the following years, nearly 90% of the nuclear bombs produced in America incorporated this boosting technique.

Fusion reactions can also serve as the primary energy source in a nuclear weapon. In the next section, we will explore the detailed workings of fusion bombs.

Fusion Bombs

Fission bombs were effective, but their efficiency left much to be desired. It didn't take long for scientists to question whether the opposite nuclear process, fusion, could prove more efficient. Fusion occurs when the nuclei of two atoms combine to form a heavier atom. Under extreme temperatures, hydrogen isotopes such as deuterium and tritium fuse easily, releasing vast amounts of energy in the process. These types of weapons are known as fusion bombs, thermonuclear bombs, or hydrogen bombs.

Fusion bombs surpass fission bombs in terms of kiloton yield and efficiency, but they come with their own set of challenges that need to be addressed:

To tackle the first challenge, scientists use lithium-deuterate, a stable solid compound that doesn't decay at normal temperatures, as the primary thermonuclear material. To solve the tritium shortage, bomb designers rely on a fission reaction to produce tritium from lithium. This fission process also addresses the final problem.

Most of the radiation emitted in a fission reaction consists of X-rays, which provide the immense heat and pressure required to initiate fusion. As a result, a fusion bomb incorporates a two-stage design: a primary fission or boosted-fission stage and a secondary fusion stage.

To understand how this bomb is designed, picture a bomb casing containing an implosion fission bomb, along with a uranium-238 cylinder casing (which acts as a tamper). Inside the tamper, there's lithium deuteride (serving as fuel), with a hollow rod of plutonium-239 positioned centrally within the cylinder.

The space between the cylinder and the implosion bomb is filled with a shield made of uranium-238 and plastic foam, which occupies the remaining areas inside the bomb casing. Upon detonation, the sequence of events unfolds as follows:

All of these events take place within a span of 600 billionths of a second (550 billionths for the implosion of the fission bomb, 50 billionths for the fusion processes). The result is an enormous explosion with a yield of 10,000 kilotons—700 times more powerful than the Little Boy explosion.

Nuclear Bomb Delivery

While constructing a nuclear bomb is one challenge, successfully delivering it to its intended target and detonating it is a completely different one. This was particularly true for the first bombs created by scientists at the conclusion of World War II. In a 1995 edition of Scientific American, Philip Morrison, a member of the Manhattan Project, commented on these early bombs: "All three bombs of 1945 — the test bomb and the two bombs dropped on Japan — were more akin to improvised complex laboratory equipment than reliable weaponry."

The transportation of these bombs to their intended locations was just as improvised as their design and construction. The USS Indianapolis carried the components and enriched uranium fuel of the Little Boy bomb to the Pacific Island of Tinian on July 28, 1945. The components for the Fat Man bomb, delivered by three modified B-29 bombers, reached their destination on August 2, 1945.

A team of 60 scientists traveled from Los Alamos, New Mexico, to Tinian to assist with assembly. The Little Boy bomb — weighing 9,700 pounds (4,400 kilograms) and measuring 10 feet (3 meters) in length — was the first to be ready. On August 6, the team loaded Little Boy onto the Enola Gay, a B-29 flown by Col. Paul Tibbets. The plane completed the 750-mile (1,200-kilometer) journey to Japan and dropped the bomb over Hiroshima, where it detonated precisely at 8:12 a.m.

On August 9, the nearly 11,000-pound (5,000-kilogram) Fat Man bomb made its way aboard the Bockscar, a second B-29 piloted by Maj. Charles Sweeney. Its destructive payload detonated over Nagasaki just before noon.

The delivery method used during World War II against Japan — gravity bombs carried by aircraft — remains a practical option for delivering nuclear weapons. However, as warheads have become smaller over time, other delivery methods have emerged. Many nations now possess numerous ballistic and cruise missiles equipped with nuclear warheads.

Most ballistic missiles are launched from either land-based silos or submarines. These missiles exit the Earth's atmosphere, travel thousands of miles to their designated targets, and reenter the atmosphere to deploy their warheads. Cruise missiles, on the other hand, have shorter ranges and smaller warheads compared to ballistic missiles, but they are more difficult to detect and intercept. These missiles can be launched from the air, from mobile launchers on the ground, or from naval ships.

Tactical nuclear weapons (TNWs) became increasingly popular during the Cold War. These weapons, designed for targeting smaller areas, include short-range missiles, artillery shells, land mines, and depth charges.

Consequences and Health Risks of Nuclear Bombs

The explosion of a nuclear weapon brings about immense devastation, and the debris will contain microscopic traces from the bomb's components. When a nuclear bomb detonates over a densely populated area, it results in colossal damage. The extent of the damage is determined by the proximity to the bomb's ground zero, also known as the hypocenter. The closer one is to the hypocenter, the more catastrophic the damage. Several factors contribute to the destruction:

At the hypocenter, everything is instantly vaporized by the extreme heat (reaching temperatures of up to 500 million degrees Fahrenheit or 300 million degrees Celsius). Moving outward from the hypocenter, the majority of casualties are due to burns caused by the heat, injuries from flying debris propelled by the shockwave, and severe radiation exposure.

Beyond the immediate vicinity of the explosion, people suffer from the intense heat, radiation, and fires sparked by the heatwave. Over time, radioactive fallout spreads across a larger area, driven by the wind. These radioactive particles contaminate water supplies and are inhaled or ingested by those far from the blast zone.

Researchers have examined the survivors of the Hiroshima and Nagasaki bombings to gain insight into both the immediate and long-term health impacts of nuclear explosions. The radiation and fallout primarily harm the body's cells that are actively dividing, such as those in the hair, intestines, bone marrow, and reproductive organs. The resulting health issues include:

These health conditions significantly raise the likelihood of developing leukemia, cancer, infertility, and birth defects.

Scientists and medical experts continue to study the long-term effects on survivors of the bombings in Japan, with expectations for more findings to emerge in the coming years.

In the 1980s, researchers investigated the potential consequences of widespread nuclear warfare (involving numerous nuclear bombs detonating across different regions of the world) and proposed the idea of a 'nuclear winter.' In this scenario, the detonation of multiple bombs would release massive clouds of dust and radioactive particles that would travel high into the atmosphere, blocking sunlight from reaching the Earth.

This reduction in sunlight would cause Earth's surface temperature to drop, diminishing photosynthesis in plants and bacteria. As photosynthesis declines, the food chain would be disrupted, potentially leading to the mass extinction of life, including humans. This hypothesis shares similarities with the asteroid impact theory, which suggests that an asteroid contributed to the extinction of the dinosaurs. Supporters of the nuclear-winter theory pointed to the dust and debris clouds that spread across the globe following the eruptions of Mount St. Helens and Mount Pinatubo.

Nuclear weapons possess immense, long-lasting destructive power that extends far beyond their initial targets. This is why governments worldwide are focused on limiting the spread of nuclear bomb-making technologies and materials, as well as reducing the nuclear stockpiles left over from the Cold War. It also explains the strong international reactions to nuclear tests conducted by countries like North Korea. While decades have passed since the bombings of Hiroshima and Nagasaki, the tragic memories of that fateful August day remain as vivid and unsettling as ever.

A Perilous Future

In the more than 75 years since the nuclear bombings of Hiroshima and Nagasaki, no other nuclear weapons have been used in warfare, and the global nuclear stockpile has drastically reduced from a peak of 70,300 in 1986 to an estimated 12,700 by early 2022. The two largest nuclear powers today are the United States, with slightly over 5,400 warheads, and Russia, holding nearly 6,000. However, the U.S. has a marginally larger number of deployed strategic warheads—1,644 compared to Russia's 1,588 [source: Federation of American Scientists].

Unfortunately, this reduction is largely the result of arms control measures taken in the 1990s. While the U.S. continues its gradual nuclear disarmament, other countries—China, India, North Korea, Pakistan, the U.K., and possibly Russia—are thought to be increasing their nuclear capabilities [source: Federation of American Scientists].

Additionally, technological progress threatens to make nuclear weapons even more devastating than in the past. For instance, U.S. ballistic missiles now feature advanced electronic sensors in their warhead tips, allowing for precise detonation above the target at the optimal moment for maximum destruction. These innovations could allow a nuclear warhead to penetrate and destroy even deeply buried structures like underground missile silos.

While these advanced weapons could deter adversaries from aggressive actions that might provoke a nuclear response, experts in nuclear strategy express concern that enemies might decide to strike preemptively to avoid the risk of their weapons being destroyed in a surprise attack [source: Smith].

An additional factor that could lead to instability is the emergence of hypersonic missiles, which are capable of traveling at significantly higher speeds and being more agile than traditional missiles. This makes it harder for a target to effectively respond, thereby raising the likelihood of an adversary initiating a first strike [source: Zutt and Onderco].

Another pressing issue is the increasingly volatile behavior of global leaders, particularly as traditional norms continue to erode. For instance, when Vladimir Putin, the Russian president, tried to deter other nations from interfering with his 2022 invasion of Ukraine, he issued a warning of "such consequences that you have never encountered in your history," which some interpreted as a potential threat of nuclear warfare. In reply, Jean-Yves Le Drian, France's foreign minister, stated, "I think that Vladimir Putin must also understand that the Atlantic alliance is a nuclear alliance" [source Reuters].