How the First Five Nuclear Powers Built Atomic and Hydrogen Bombs
Spies, Stolen Blueprints, and the Race for the Ultimate Weapon
After America and Israel struck Iran, I wanted to understand how countries actually build nuclear weapons: both the politics and the mechanics.
Making a Nuke
It starts in a laboratory in Berlin in 1938, where two chemists (Otto Hahn and Fritz Strassmann) bombarded uranium and didn’t understand their own results. They wrote to their former colleague, Lise Meitner, a Jewish physicist who had fled Nazi Germany to Sweden. Meitner and her nephew Otto Frisch analyzed the data and realized the uranium nucleus had fractured in half. They coined the term “nuclear fission.”
Within months, physicists across the West and Japan rushed to their labs. In Paris, Frédéric Joliot-Curie (Freh-deh-reek Zhoh-lee-oh Koo-ree) and his team made the next crucial breakthrough: when a uranium atom splits, it releases two or three extra neutrons, meaning a self-sustaining nuclear chain reaction was possible. Splitting atoms could be weaponized.
Joliot-Curie published his findings in Nature for the whole world to read. Every major power now had the theoretical blueprint. But theory is only half of a bomb. You also need uranium.
In 1939, Nazi Germany, which already annexed the Sudetenland a year earlier (the German-speaking part of Czechoslovakia), invaded the rest of Czechoslovakia. This mattered for many reasons, but one under-appreciated reason was that Czechoslovakia contained the Jachymov mines, Europe’s only major source of uranium ore at the time. Hitler now had both the science and the raw material.
Germany’s Program:
German physicists read Joliot-Curie's paper immediately. The Nazi regime convened a secret group of scientists, the Uranverein (Uranium Club), to begin weapons research. After Germany invaded Poland in September 1939, the military took over the project.
UK’s Program:
The UK had Jewish refugee scientists from Germany and Austria. Two of them, Otto Frisch and Rudolf Peierls, realized that if you strip out the rare isotope Uranium-235 from the more common Uranium-238, the critical mass for an explosion wasn’t tons, but rather the size of a golf ball.
An atomic bomb was not a theoretical curiosity anymore; it was an engineering problem. Britain launched the first formal nuclear weapons program, codenamed Tube Alloys.
America’s Program:
Jewish scientists who had fled Nazi Germany warned FDR that Hitler was secretly pursuing a bomb and that America needed to act. This became known as the “Einstein-Szilárd letter.” President Roosevelt created the Advisory Committee on Uranium in 1939. By 1942, it had grown into the Manhattan Project.
The Soviet Union’s Program:
A Soviet physicist named Georgy Flerov was keeping up with international physics journals. In 1942, he noticed something strange: the top American, British, and German physicists working on nuclear fission had all stopped publishing. Rather than assuming they had hit a dead end, Flerov reasoned that the three powers placed a gag order on nuclear physics because they were secretly building a bomb. He wrote to Stalin, who authorized a Soviet nuclear program. It was a genuine physics effort from the start, but Soviet espionage dramatically accelerated it. The most consequential intelligence came after 1944, when Klaus Fuchs began passing detailed blueprints from inside Los Alamos.
Japan’s Program:
Japan began nuclear weapons research in 1941 and, characteristically, ran two competing programs that refused to cooperate. Why two programs? Because the Japanese Army and Navy hated each other. The Army and Navy’s institutional hatred ran deep, rooted in the old, clannish Samurai rivalries of the Meiji Restoration: the Choshu clan had built the Army, and the Satsuma clan had built the Navy.
The Imperial Japanese Army had its Ni-Go project and the Imperial Japanese Navy had its F-Go project. The two services actively withheld intelligence from each other and undermined each other's political allies. Neither program came close to success. Japan lacked the industrial resources, sufficient uranium, and by late in the war, any respite from Allied bombing to sustain the work.
What Tipped the Outcome
By 1942, the physics was largely settled. Building a bomb was now an engineering problem, and an enormous one. To enrich uranium or breed plutonium, you need massive industrial facilities, millions of gallons of water, and staggering amounts of electricity.
Britain could not do this. It was fighting for its life, getting bombed by the Luftwaffe nightly, and was virtually bankrupt.
America was untouched by bombing, had vast empty deserts for testing, and a massive electrical grid. So Churchill agreed to merge Tube Alloys into the Manhattan Project. In August 1943, the two countries signed the Quebec Agreement, and Britain shipped its best physicists to Los Alamos to work under Oppenheimer.
Canada and French scientists also contributed to the Manhattan Project. Joliot-Curie's colleagues in Paris, Hans Halban and Lew Kowarski, had identified heavy water as an ideal moderator for nuclear chain reactions. When Germany invaded France in 1940, the two scientists fled to England carrying 185 kg of heavy water, nearly the world's entire supply. Their research eventually migrated to the Montreal laboratory in Canada, which became the Allied center for heavy water reactor research.
Most of the Manhattan Project's uranium came from the Shinkolobwe mine in the Belgian Congo (now the Democratic Republic of Congo), with additional sources from the Eldorado Mine in Canada and the Colorado Plateau in the United States.
Two Paths to a Bomb
Building a nuclear weapon is hard. It requires building infrastructure at the scale of a small city. There are two paths, and each produces a different type of atomic bomb.
Uranium Path: Natural uranium is overwhelmingly Uranium-238, which does not sustain a chain reaction. Only about 0.7% of natural uranium is the fissile isotope, Uranium-235. To build a bomb, you need to "enrich" the uranium by separating U-235 from U-238. Because the two isotopes are chemically identical, this is an agonizingly difficult process. Today, scientists convert uranium into a gas (uranium hexafluoride) and spin it at supersonic speeds inside centrifuges. The slightly lighter U-235 drifts toward the center while the heavier U-238 moves outward. It takes thousands of centrifuges running in sequence (called a "cascade") for months to produce enough weapons-grade material (90%+ U-235).
BUT, during WWII, centrifuges kept breaking under the stress. Instead, America relied on massive, brute-force industrial methods at Oak Ridge, Tennessee: Gaseous Diffusion (an insanely energy-inefficient process of filtering the gas through microscopic barriers) and Electromagnetic Separation (using giant magnetic machines). It took months of running these facilities just to get enough weapons-grade material (90%+ U-235) for a single bomb.
Below you will see a picture showing different levels of uranium enrichment.
Plutonium Path: Plutonium doesn’t exist in nature, you need to cook it up in a nuclear reactor. When Uranium-238 absorbs a neutron in a controlled reactor, it gradually transforms into Plutonium-239, which is fissile. The reactor needs a moderator to slow the neutrons and sustain the reaction. The Americans used graphite at their Hanford reactors; the Canadians and French scientists used heavy water. After the reactor runs long enough, you extract the highly radioactive spent fuel rods and chemically separate the plutonium from the remaining uranium and waste products. This reprocessing step is one of the most dangerous parts of the entire process.
In 1945, America used both paths. The Hiroshima bomb ("Little Boy") was a uranium atom bomb (gun type design). The Nagasaki bomb ("Fat Man") was a plutonium atom bomb (implosion design).
Slamming the Door Shut
After Japan surrendered, American President Truman passed the Atomic Energy Act of 1946, which ended all nuclear information sharing with the UK, Canada, and every other nation.
The law was extraordinarily restrictive. Sharing “restricted data” about nuclear weapons was punishable by death or life in prison. No exceptions were made for allies. The Act also stipulated that any information concerning the design, manufacture, or utilization of atomic weapons was automatically classified the moment it was created. If an independent physicist working at a private university figured out a piece of the puzzle entirely on their own, without any access to government files, their thoughts, calculations, and equations were legally considered government secrets from the exact moment the pen hit the paper.
Little Aside: Relating Nuclear and AI Policy
According to Venture Capitalists Marc Andreessen and Ben Horowitz, the Biden administration told them that it planned to use Truman’s nuclear blueprint for AI.
As they recounted it, one official said: “We classified whole entire areas of physics in the nuclear era and made them state secrets and that research vanished. And we’re absolutely capable of doing that again for AI—we will classify any area of math that we think is leading in a bad direction, and it will end.” Biden’s executive order on AI (EO 14110), signed October 2023, was perceived as a step in that direction by these folks.
But, Trump revoked Biden’s executive order on his first day in office.
America Enraging Allies
Truman’s Atomic Energy Act enraged the British, and for good reason. Roosevelt and Churchill had signed the Quebec Agreement (1943) and the Hyde Park Memorandum (1944), secret treaties that explicitly promised full US-UK collaboration on the bomb and continued sharing of nuclear technology for both commercial and military purposes after the war. Truman either was never briefed on these secret agreements by Roosevelt, who famously kept critical information close to the chest, or he knew and chose to break them in the name of national security. Either way, America's closest nuclear ally was cut off overnight.
The Monopoly Breaks
America’s monopoly on the atomic bomb did not last long. A handful of scientists and technicians inside the Manhattan Project had been Soviet spies all along: Klaus Fuchs, Theodore Hall, David Greenglass, George Koval. This is not a defense of McCarthyism, but you would not have any legitimacy of an anti-communist Red Scare if American communists had not literally handed the blueprints of the ultimate weapon to America’s greatest enemy.
The Soviets were paranoid that their spies were feeding them disinformation, so they largely conducted their own research in parallel. They built a secret city in the Ural Mountains using forced labor from the Gulag system. Before the Soviets found their own uranium in Central Asia, their uranium came from Eastern Europe: the Jáchymov mines in allied Czechoslovakia and the Ore Mountains in Soviet-occupied East Germany. In 1949, they tested their atomic bomb in Soviet Kazakhstan.
The next country to get the bomb was the United Kingdom, which detonated its own weapon in 1952.
The British did not need to steal blueprints; they had helped draw them for the Manhattan Project. After Truman cut off access, Prime Minister Clement Attlee authorized an independent British program. William Penney, one of the lead scientists who had worked at Los Alamos, rebuilt the plutonium bomb from the knowledge and notes his team retained. Britain sourced its uranium from the Belgian Congo (secured during wartime), as well as from Commonwealth allies Australia, Canada, and South Africa.
That same year, America leapfrogged everyone. In November 1952, the U.S. detonated the first hydrogen bomb, completely vaporizing the island of Elugelab. Instead of splitting atoms (fission) like an atomic bomb, a hydrogen bomb smashes atoms together (fusion). It was roughly 700 times more powerful than the bomb dropped on Hiroshima.
The Soviets, still benefitting from their espionage inside Los Alamos, tested a boosted atomic bomb, also known as a “layer cake” bomb in 1953.
By 1955, the Soviets tested their first true hydrogen bomb, achieving nuclear technological parity with the United States.
In 1957, the United Kingdom followed with its own hydrogen bomb (Operation Grapple).
Opening the Door Again
By 1953, Eisenhower realized that Truman’s strategy of “kicking away the ladder” from the 1946 Atomic Energy Act was failing. In a speech at the United Nations called “Atoms for Peace”, he proposed that nuclear technology should be shared internationally for civilian purposes like electricity and medicine.
Eisenhower then signed the 1954 Atomic Energy Act so America could sell civilian nuclear research reactors, nuclear fuel, and training to allied countries. To prevent weapons programs, he helped create the International Atomic Energy Agency (IAEA) in 1957, which is the UN watchdog.
As great as the intention was for this, “Atoms for Peace” had a fatal flaw: nuclear technology is inherently dual-use. The same science required to split atoms to boil water for electricity is the same science used to build a bomb. By handing out research reactors, uranium, and world-class training, the United States inadvertently laid the foundational groundwork for several future nuclear weapons programs.
Between 1955 and 1962, America provided research reactors to Austria, Denmark, Portugal, the Netherlands, Sweden, Turkey, Israel, Brazil, Spain, Italy, Switzerland, Japan, the Philippines, Argentina, South Korea, Taiwan, Thailand, Colombia, Iran, Greece, South Africa, Zaire (Congo), South Vietnam, Indonesia, and Pakistan. (Every linked country either attempted, explored, or eventually acquired nuclear weapons capabilities.)
The Soviet Union provided research reactors to China, Poland, Hungary, Romania, Czechoslovakia, Yugoslavia, Bulgaria, Egypt, East Germany, North Korea, Iraq, Libya, Vietnam, and Cuba.
Also, once UK achieved thermonuclear parity, Eisenhower decided it was absurd to keep secrets from a country that already knew them. He saw no point in forcing the British to duplicate American research when the real threat was the Soviet Union. In 1958, Eisenhower amended the Atomic Energy Act to allow the sharing of nuclear weapons design information with allied nations that had made “substantial progress” on their own. That same year, the U.S. and UK signed the Mutual Defense Agreement, restoring the partnership Truman had severed.
France
This infuriated Charles de Gaulle. France did not yet have an atomic bomb, let alone a hydrogen bomb, and now the Anglo-Americans were sharing secrets exclusively with each other. The humiliation of the 1956 Suez Crisis/2nd Arab-Israeli War, where America sided with the Soviets and Egypt to force France, the UK, and Israel to withdraw, convinced de Gaulle that relying on American security guarantees was dangerous.
France accelerated its independent nuclear program and detonated its first atomic bomb in the Sahara desert (French Algeria) in 1960.
France already had premier nuclear scientists. After all, Joliot-Curie proved nuclear chain reactions were possible back in 1939. Also, some French scientists had worked in the Manhattan Project. For raw materials, France had domestic uranium from the Limousin region, and it sourced ore from its colonial holding in Madagascar (Gabon would follow in 1961 and Niger by 1971, both after independence).
China
The 5th country to build an atomic bomb was China in 1964.
China's motivation was straightforward: America had threatened to nuke China four times in the 1950s.
Nuclear Threat #1: Truman threatened to nuke China during the Korean War (when China backed North Korea), phrasing it “diplomatically” as using “every weapon that we have.”
Nuclear Threat #2: In 1953, Eisenhower told China (through India since America did not recognize Mao’s China at the time) that if China didn’t agree to an armistice in the Korean war, then Eisenhower would nuke them.
Nuclear Threat #3: In 1954-1955, during the first Taiwan Strait Crisis, China was shelling Taiwanese controlled islands (Quemoy and Matsu). Eisenhower said he would use tactical nuclear weapons if China attempted to incorporate Taiwan island.
Nuclear Threat #4: In 1958, when China resumed shelling islands in the 2nd Taiwan Strait Crisis, Eisenhower once again moved nuclear-capable bombers and artillery into the region to force Beijing to back down.
If America threatens you with nuclear annihilation four times in a decade, then you needed your own deterrent. The 1950 Sino-Soviet alliance theoretically obligated Moscow to defend China, but Mao knew the Soviets would not risk World War III on China's behalf, and any military help they offered came with demands that violated Chinese sovereignty (like making Manchuria a quasi-Soviet colony from 1950 to 1954). Outsourcing China's survival to a foreign power was a death sentence. Mao needed his own bomb.
China's program started with Soviet help. In 1954, the Soviet Union agreed to give full assistance to China in nuclear physics and atomic energy; in return China would sell uranium to them. Soviet engineers helped China discover and mine uranium deposits in their nation, and built China a research reactor in Beijing. In 1957, the Soviets signed an agreement to provide China with a working prototype of the atomic bomb.
But cracks were already forming. Mao wanted a Soviet nuclear powered submarine, but Khrushchev declined. Khrushchev unilaterally denounced Stalin without consulting the rest of the communist bloc, undermining the global socialist movement. He also pursued détente with Washington while America was still threatening to annihilate China, and treated China as a junior partner rather than a sovereign ally. The Sino-Soviet split ended Soviet nuclear assistance. The Soviets tore up their agreement to provide the bomb prototype and pulled their equipment and engineers out of China.
Mao pressed ahead anyway. Even as tens of millions starved during the Great Leap Forward/Three Years of Natural Disasters, China poured staggering amounts of GDP and electricity into the nuclear program in Xinjiang and other regions. To Beijing, the bomb was not a vanity project but a way to prevent annihilation by America or the Soviet Union.
China was dirt poor, but its human capital was far above what other developing countries had at the time. Many of its top physicists had been educated at Caltech, Purdue, University of Michigan, or my alma mater, MIT. (Ironically, the paranoia of the McCarthy-era Red Scare had deported several of these brilliant minds, like Qian Xuesen, right back to Mao.) Doing the math entirely on their own, China refined uranium at a processing plant in Inner Mongolia and enriched it at a gaseous diffusion plant in Lanzhou. Three years after detonating its first atomic bomb, China tested a hydrogen bomb in 1967, a year before France tested its own hydrogen bomb in August 1968.
At this point, all five winners of WWII had nuclear weapons: the United States, the Soviet Union, the United Kingdom, France, and China. Things were getting out of hand, especially since America and the Soviet Union almost ended human civilization in October 1962 in the Cuban Missile Crisis. That event terrified both Kennedy and Khrushchev so deeply that they immediately installed the Washington-Moscow hotline, signed the Limited Test Ban Treaty in 1963 (with UK as well), and desperately rushed to draft the NPT to ensure no rogue states could ever trigger a global apocalypse. So America, the Soviet Union, and Britain worked on creating the Nuclear Non-Proliferation Treaty (NPT).
The Non-Proliferation Treaty
In 1968, the Nuclear Non-Proliferation Treaty (NPT) opened for signature.
The basic bargain was this: the five countries that already had nuclear weapons would be recognized as nuclear-weapon states, and everyone else would agree not to develop them in exchange for access to civilian nuclear technology and a vague promise that the nuclear powers would eventually disarm. Article IV in NPT guarantees the inalienable right for all countries to use nuclear energy for civilian purposes.
France and China initially refused to sign, holding out until 1992. While they delayed signing, China and France helped other nations obtain nuclear reactors. (i.e. Israel, Iraq, South Africa, Pakistan, and Algeria). We will talk about other nations’ secret nuclear programs in a future post (including Switzerland, Argentina, Taiwan, Libya, Iran, and more). Fun fact, before the 1980s, France had nearly zero-safeguards or inspections for nuclear weapons programs when they sold nuclear reactors. France even sold weapons-grade uranium to some clients!
Concluding Thoughts
After researching this, I came away with a few thoughts:
The dual-use dilemma: While "Atoms for Peace" tried to temper clandestine weapons programs by offering a legal civilian route, it accidentally handed out the starter kits for global proliferation. Once the fundamental technology was out there, commercially driven nations like France stepped in to fill the gaps, willing to sell nuclear infrastructure with far fewer strings attached than the Americans or Soviets ever intended.
You can’t hide math: The ultimate lesson from the first five nuclear powers is that you cannot contain math forever. The math always leaks. The true barrier to a nuclear weapon is not the blueprint; it is the staggering, multi-billion-dollar industrial capacity required to spin uranium in thousands of centrifuges or breed plutonium in heavy-water reactors. That is why the NPT focuses on tracking uranium, reactors, and fuel cycles rather than trying to control equations. This is probably why Biden’s executive orders focused on GPU exports and tracking compute more than regulating the actual algorithm. If Democrats take power in 2028, I can imagine the administration targeting the monitoring of AI infrastructure rather than regulating math.
Great article as usual.
The bit about the Japanese navy/army was really interesting - I always knew the Navy and Army hated eachother to an almost unreasonable degree, but never understood why it was so much more severe than other nations. The clan association explains a lot.
Good and informative read thanks. A sad state of affairs. The end of the cold war and Mandela giving up the SA program gave some hope. But now that you have North Korea, Russia, China routinely making nuclear threats to their non-nuclear neighbours, the US not defending its allies and attacking all non-nuclear enemies, non proliferation is dead.
Only silver lining is that the guy at the top also dies in nuclear war, which has been quite effective in calming their urges.