It took just a few years to develop the first atomic weapon, and only weeks between the first test and its use in war. Seventy-five years later, humanity is still coming to grips with the results.
The atomic bomb that the United States dropped on Hiroshima on Aug. 6, 1945, killed tens of thousands and flattened the Japanese city in an instant.
“Little Boy,” as it was known, was the endpoint of years of research, wrangling a physics theory into a mechanism to release the energy that binds together atoms. An eyeblink, by modern standards. And it was tested only once—an event so momentous that one of the bomb’s chief scientists, Robert Oppenheimer, said it brought to mind words from Hindu scripture: “Now I am become death, destroyer of worlds.”
The concept was simple: driving together enough uranium or plutonium at high enough speeds will create a “critical mass” so quickly that it will start an uncontrolled, nearly instantaneous chain reaction of neutrons knocking apart atomic nuclei.
Each atom’s lost mass is converted to energy at a staggering exchange rate. Only 1.09 kg of the 64 kg of uranium in Little Boy became energy.
It was the equivalent of detonating 15,000 tons of TNT, according to Los Alamos National Laboratory calculations.
About one square mile of Hiroshima was flattened, crushed by the hammer blow of Little Boy detonating about 580 metres (1,900 ft) overhead. Nearly everyone in that area died instantly. Farther away, the bomb’s heat ignited buildings and people, and deadly radiation bloomed.
Fires raged through the city. Doctors saw more cases of acute radiation sickness than at any other point in history. In the end, as many as 100,000 people were dead and more than half of the city’s buildings lay in ruins.
The birth of Little Boy
The work to turn nuclear fission, a process German scientists discovered in the late 1930s, into a weapon was dubbed the Manhattan Project in the United States. Led by the U.S. Army, it involved at its peak more than 100,000 personnel, ranging from scientists to construction workers, according to the U.S. Department of Energy. Scientists first built a working nuclear reactor. The site: beneath the bleachers of the University of Chicago football stadium. The reactor, called the “Chicago Pile,” proved in 1942 that a controlled fission reaction was possible – the heart of modern nuclear power plants.
It began the race to enrich enough of the isotope of uranium needed – uranium 235, whose structure makes a chain reaction possible – and plutonium, another fissionable element isolated by a team at the University of California in 1940. Scientists, meanwhile, struggled on how to best create the instantaneous critical mass needed for an explosion.
They decided on two methods: firing a small piece of uranium into a larger piece in a sort of “gun” arrangement, and creating a hollow sphere of plutonium that explosives would implode, or collapse, into a critical mass. The first was Little Boy. The second was Fat Man, the implosion-style bomb that destroyed Nagasaki on Aug. 9, 1945 and killed as many as 75,000 people.
In modern times, novel weapons are developed not just over years, but sometimes decades. The “Trinity” test evaluating the implosion-type atomic bomb was on July 16, 1945, less than two years after the Chicago Pile.
It was first used in war less than a month later.
Since World War II, no country has attacked another with a nuclear weapon. But at least eight have developed them, and as scientists theorised new designs – including the vastly more powerful fusion weapons, so-called “hydrogen bombs” - testing began all over the world. More than 2,000 nuclear weapons have been detonated in experiments since Oppenheimer watched the Trinity test fireball scour the New Mexico desert.
Nevada test site
The Sedan crater is the result of a 104-kiloton thermonuclear explosion in July 1962. The crater is located within the Nevada Test Site at Yucca Flat. It is 390 m (1,280 ft) wide and 100 m (328 ft) deep.
For decades, many of these tests were atmospheric, meaning the weapons were detonated above ground, and sometimes even in space. Others were underground, detonated in vaults deep below the surface, meant to contain the blast and prevent fallout while instruments measured how well the new designs worked.
The world’s two biggest nuclear powers, the United States and Russia, have not tested any nuclear weapons since 1992. Other countries trying to develop their own arsenals have carried out tests more recently.
“Nuclear technology is only getting easier,” said Melissa Hanham, deputy director of the Open Nuclear Network. “It’s not new tech anymore. Other countries and even non-state actors could choose to build covert nuclear programs.”
On the firing line
Testing has human consequences. Even when things went as planned, early atmospheric tests threw fallout into the atmosphere that could wind up hundreds of miles away or more.
When they went badly, the results could be catastrophic. America’s Castle Bravo test in 1954 was meant to evaluate the design of a 5 megaton weapon - the equivalent of 5 million tons of conventional high explosives. Instead, the device exploded with a yield of 15 megatons, vaporising many of the test instruments and throwing fallout high into the atmosphere. Several hours later, it blanketed a Japanese fishing vessel called the Daigo Fukuryū Maru. All of the 23 crew members suffered from radiation sickness, and one died.
Since 1945, more than 2,000 nuclear explosive tests have been carried out around the world.
Hundreds of native people were moved from their homes on and around the South Pacific atolls where the United States did most of its atmospheric testing.
“Uranium mining, waste and testing are often done on indigenous land, and those performing the work and locals suffer health, environmental and economic damage.”
Melissa Hanham, deputy director of the Open Nuclear Network.
“Nearly everywhere in the world nuclear weapons are tested, indigenous people are affected disproportionately,” Hanham said.
Seventy-five years after the atomic flash set fire to Hiroshima, thousands of nuclear weapons sit in arsenals around the world, ready to deploy by aircraft or missile. The Arms Control Association estimates that there are nearly 14,000 such weapons, and that the United States and Russia account for the most by far: 6,185 for the United States and 6,490 for Russia, although of these only a third or so could be immediately used in a war.
The number of such “deployed” nuclear weapons is limited by the New START treaty, which Russia and the United States ratified in 2011. At the height of the Cold War, the total number of warheads was several times greater.
World’s nuclear warhead stockpile
Treaties have lowered the number of nuclear warheads since the end of the Cold War.
The other states with nuclear weapons are Britain, China, France, India, Israel and Pakistan. South Africa developed nuclear weapons in the 1980s, but by the end of the decade decided to dismantle them. In 1994, the International Atomic Energy Agency confirmed all of the weapons had been destroyed.
In 2018, Russia announced it had developed a nuclear-armed underwater autonomous vehicle dubbed Poseidon. The vehicle, Russian officials said, could quietly carry a nuclear warhead with a yield of tens of megatons to a point just offshore an enemy city.
The United States spends nearly $50 billion a year on its nuclear weapons. In 2020, media said the Trump administration was considering ways to restart testing.
Nuclear-armed neighbors China and India have seen border disputes escalate to bloodshed, and North Korea is building a nuclear-armed submarine.
Even so, 75 years have passed without a nuclear attack.
“I worry that people have become complacent and think that nuclear devastation only happens in black-and-white photographs,” said Jeffrey Lewis, head of the East Asia Nonproliferation Project at the Middlebury Institute of International Studies. “I am hopeful that we can stretch the streak for decades more - but the real question is whether nuclear deterrence will work forever. I am not so sure about that. And that means, sooner or later, our luck will run out.”
A previous version of this story referred to the 1963 test ban treaty eliminating all atmospheric testing. This has been corrected to specify the ban prohibited the United States, the Soviet Union, and the United Kingdom, from testing nuclear weapons in the atmosphere, underwater or in outer space.
Nuclear Explosion DataBase (NEDB). Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization. United States Strategic Bombing Survey, 1946. Bulletin of the Atomic Scientists. Sentinel-2, European Space Agency - ESA. Reuters Research. UNESCO.