An atomic bomb is built around a core of rare, highly refined radioactive material, either uranium-235 or plutonium-239, surrounded by precisely shaped conventional explosives, electronic triggers, and a metal shell. The concept is straightforward: force enough of that radioactive material together, fast enough, to start an uncontrolled chain reaction that releases an enormous burst of energy. But every component has to work in concert, and each one is engineered to extreme tolerances.
The Fissile Core
The heart of any atomic bomb is its fissile material, the substance that actually splits apart and releases energy. Only two materials have been used in nuclear weapons: uranium-235 and plutonium-239.
Uranium-235 makes up just 0.7% of natural uranium ore. The rest is uranium-238, which doesn’t sustain the kind of rapid chain reaction a bomb needs. To be useful in a weapon, uranium must be “enriched” so that the concentration of U-235 climbs from that 0.7% to over 90%. Anything above 20% is classified as highly enriched uranium, but weapons-grade material sits much higher. The bare critical mass of pure U-235, the minimum amount needed to sustain a chain reaction with no surrounding reflector, is roughly 50 kilograms, or about 110 pounds.
Plutonium-239 doesn’t exist in nature in useful quantities. It’s produced artificially by bombarding uranium-238 with neutrons inside a nuclear reactor. Weapons-grade plutonium needs to be extremely pure, above 93% Pu-239, because contamination with the isotope Pu-240 causes problems. Pu-240 fissions spontaneously, releasing stray neutrons that can trigger a premature chain reaction before the bomb is fully assembled. That risk is why plutonium weapons require a fundamentally different design than uranium ones. Plutonium’s critical mass is smaller than uranium’s, so a plutonium core is physically more compact.
Two Assembly Designs
Getting fissile material to critical mass is the central engineering challenge. The two original atomic bombs, dropped on Japan in 1945, used completely different approaches.
Little Boy, the uranium bomb used over Hiroshima, used a gun-type design. It’s the simpler of the two concepts. A conventional explosive propellant fires one sub-critical piece of uranium-235 down a barrel into another piece, like a bullet into a target. When the two pieces slam together, their combined mass exceeds the critical threshold and a chain reaction begins. The weapon was 10 feet long, just over 2 feet in diameter, and weighed 9,700 pounds. Los Alamos scientists were so confident in this design that it was never tested before combat use.
Fat Man, the plutonium bomb used over Nagasaki, required the more complex implosion design. Because plutonium’s stray-neutron problem makes the gun-type approach too slow (the material would start reacting and blow itself apart before full assembly), engineers had to compress a plutonium core inward from all sides, simultaneously, in a fraction of a millisecond. Fat Man was wider and heavier: nearly 11 feet long, 5 feet in diameter, and 10,800 pounds.
Conventional Explosives and Lenses
In an implosion bomb, the plutonium core is surrounded by a shell of carefully shaped conventional explosives. These aren’t simple blocks of TNT. They’re “explosive lenses,” so named because they focus the blast wave inward the way a glass lens focuses light.
The original Fat Man design used a combination of fast and slow explosives arranged in precise geometric shapes. Composition B, a mixture of RDX and TNT, served as the fast explosive. Baratol, a blend of barium nitrate and TNT, acted as the slow component. When detonated together, the fast explosive’s shock wave hits the slower material and gets redirected, creating a perfectly symmetrical inward-squeezing pressure wave. Any asymmetry, even slight, and the core squirts out sideways instead of compressing evenly, and the bomb fizzles.
The Neutron Initiator
Compressing the core to supercritical density isn’t enough on its own. The chain reaction needs a precisely timed burst of neutrons to get started at exactly the right instant. Early bombs used an internal device combining polonium-210, a powerful emitter of alpha particles, with beryllium, a metal that releases neutrons when struck by alpha particles. In normal configuration the two materials were kept separate. When the implosion shock wave crushed them together, the polonium’s alpha particles hit the beryllium and kicked out a spray of neutrons, igniting the chain reaction at peak compression.
Electronic Firing Systems
An implosion bomb has dozens of detonation points spread across its explosive shell, and every single one must fire within nanoseconds of the others. This requires specialized high-speed electronic switches. The most well-known type is the krytron, a small gas-filled tube containing a trace of radioactive krypton-85 that helps ionize the gas almost instantly. Krytrons can switch on in under 5 nanoseconds, fast enough to ensure all the explosive lenses fire simultaneously. These switches are connected to capacitor banks that store the large electrical charge needed to set off the detonators. The export of krytrons is tightly controlled internationally because they have very few uses outside of nuclear weapons.
The Reflector and Tamper
Surrounding the fissile core, between it and the explosives, sits a heavy metal shell that serves two purposes. As a “neutron reflector,” it bounces escaping neutrons back into the core, which means less fissile material is needed to reach critical mass. As a “tamper,” its sheer weight holds the core together for a few extra microseconds as the chain reaction builds, giving more atoms time to split before the whole assembly blows itself apart. Natural uranium (U-238) and beryllium have both been used in this role. The reflector is one reason real weapons use significantly less fissile material than the bare critical mass numbers suggest.
Boosting With Hydrogen Isotopes
Modern fission weapons often include a small amount of gas at their center: a mixture of deuterium and tritium, two heavy isotopes of hydrogen. This isn’t a hydrogen bomb, but it borrows a principle from one. At the extreme temperatures generated by the beginning of the fission chain reaction, the deuterium and tritium atoms fuse together. That fusion reaction produces a flood of additional high-energy neutrons, which split far more of the remaining fissile material than the chain reaction would have reached on its own. The result is a dramatically higher explosive yield from the same amount of plutonium or uranium.
Tritium is radioactive with a half-life of about 12 years, so it decays and must be periodically replenished. Los Alamos National Laboratory operates a dedicated facility for handling and loading this boost gas into sealed metal reservoirs. In a deployed weapon, the gas would be injected into the core at the precise moment of detonation.
How Fissile Material Is Produced
The reason only a handful of nations have built nuclear weapons has less to do with bomb design, which is well understood, and more to do with producing the fissile material itself.
Enriching uranium requires converting uranium ore into a gas, uranium hexafluoride, and then separating the slightly lighter U-235 molecules from the heavier U-238 ones. The dominant method today is the gas centrifuge: the gas is spun at extremely high speeds inside cylinders, and centrifugal force pushes the heavier molecules outward while the lighter ones drift toward the center. The enriched stream is drawn off and fed into the next centrifuge in a long cascade. Going from natural uranium at 0.7% U-235 to weapons-grade material above 90% requires thousands of centrifuges running in sequence over months. An older method, gaseous diffusion, forced the gas through porous barriers and was used during the Manhattan Project, but it consumed enormous amounts of electricity and has largely been retired.
Producing plutonium-239 requires a nuclear reactor. Uranium-238 fuel rods are irradiated with neutrons, converting some U-238 atoms into Pu-239. The irradiated fuel is then chemically reprocessed to extract the plutonium. Keeping the plutonium weapons-grade means removing the fuel rods relatively early, before too much Pu-240 accumulates.