When we think of a nuclear weapon, the image that comes to mind is the vast mushroom clouds of Hiroshima, Nagasaki, or the Cold War tests in the Pacific and Russia. The destructive force of these weapons was measured in thousands or millions of tons of TNT equivalent. These were city busters, designed to flatten entire areas. But within the history of nuclear weapons development, there was another different but fascinating question, one that engineers worked on in the 1950s and 60s, this was just how small could you make a nuclear bomb and still have it work?
The idea of “small” isn’t simply about shrinking explosive yield. It touches on the physics of chain reactions, the engineering limits of implosion, the logistics of deployment, and the politics of control. It also intersects with our modern day concerns about drones, terrorism, and proliferation. So this is the story of miniature nuclear weapons, from the physics of critical mass to the Cold War’s smallest bombs and to today’s security nightmares.
Unlike conventional explosives, nuclear devices can’t simply be scaled up or down like sticks of dynamite. They rely on a chain reaction of a fissile material such as uranium-235 or plutonium-239.
Certain isotopes, such as uranium-235 and plutonium-239, are called fissile because their large nuclei are on the edge of stability. If struck by a neutron, they can split that’s the fission part, into two smaller nuclei and in the process release 2-3 neutrons as well, about 200MeV of energy per atom.
It’s these 2-3 other neutrons that can then strike other atoms of uranium or plutonium which can then split and release more neutrons and energy in what can become a runaway nuclear chain reaction.
However, for this to happen, you need enough fissile atoms packed close enough together that, on average, each fission produces at least one more fission. If the mass is too small, too many neutrons escape before hitting other nuclei, and the reaction fizzles out. This is called subcritical.
At the threshold when each fission leads to exactly one more fission, the material is critical. This is how nuclear reactors work, there is a chain reaction, but it is controlled and does not runaway.
To make an atomic bomb, each fission has to produce more than one sustained reaction. It has to multiply the results with each stage, then the reaction is known as supercritical.
But even highly refined weapons-grade uranium or plutonium won’t explode by itself. As the reaction starts, it heats up the material, which makes it expand, making it less likely for neutrons to hit other fissile atoms, and the reactions decrease. It has to be forced into the supercritical stage, and this is actually a very difficult process, not only to initiate but also to sustain long enough to make a viable device and it blows itself apart.
During the Manhattan Project, the World War 2 American project to build a nuclear weapon, it was theorised that if you could compress a piece of uranium or plutonium, the atoms in it would be closer together and thus increase the chance of neutrons hitting other atoms to create a chain reaction.
First, they came up with the gun-type assembly which used in “Little Boy” on Hiroshima. For this two subcritical pieces of uranium-235 are slammed together by a conventional explosive, forming a supercritical mass.
After they realised Plutonium was more likely to go supercritical they switched to the Implosion-type assembly as used in “Fat Man” on Nagasaki and in almost all later weapons. Here, a sphere of plutonium-239 is squeezed by precisely shaped conventional explosive charges, making it denser so that neutrons are more likely to hit other nuclei before escaping.
Both of these of devices were large, weighing in at 4.4 tons for Little Boy and about 4.9 tons for Fat Man. They were so big and heavy that only specially modified versions of the biggest bombers of the war, the B-29 bomber, could carry them.
But these were effectively prototypes, the US only had two bombs and they had used them both. Their threat to drop them on other cities forced the Japanese into surrender, but it was an empty threat, they had no more bombs to drop.
So as soon as the potential of their power had been demonstrated, there was then a race to make them on an industrial scale, and with this refinement, the size of the bombs decreased and their yield to mass increased over the following years.
As far as the actual piece of uranium or plutonium is concerned, these masses are surprisingly large. A sphere of pure uranium-235 requires around 52 kilograms to sustain a chain reaction. Plutonium-239, being more fissile, needs about 10 kilograms. That’s already the weight of a heavy suitcase before you add anything else.
But weapon designers soon learned tricks like using a tamper to surround the core which was made from beryllium, tungsten carbide, or depleted uranium. These have the property of reflecting neutrons, so those that would normally escape out of the device would be reflected back in.
These dense materials would also help the core stay together for a few microseconds longer, enough to dramatically increase the yield. If the tamper was made from depleted uranium then the extra neutron reaction it would create would add even more to the yield.
This is why you can never have 100% conversion of the nuclear material , like Plutonium into energy. As soon at the reaction starts to run away, the heat created blows it apart and stops the reaction, the longer you can hold it together, even for millionths of a second, the larger the yield.
However, using these methods also meant that the amount of Plutonium-239 could go down to 3-5kg making the bomb smaller and lighter.
Soon, atomic bombs were being shrunk down to fit into aircraft-mounted missiles and eventually into artillery shells
But even then, that’s still several times the mass of a modern assault rifle. Once you surround it with conventional explosives, detonators, electronics, and a casing strong enough to withstand handling, you end up with something weighing 20 to 30 kilograms, that’s not a briefcase-sized device that you can carry through the streets with ease.
There were also issues in that the smaller the device became, the yield became much more variable and unpredictable.
The W54 warhead, the smallest nuclear device ever fielded was designed in the late 1950s, it weighed just 23 kilograms, with a cylinder only 40 centimetres long and 28 centimetres across.
To shrink the warhead down to this size meant giving up the tamper and enclosing the nuclear material in a cylinder of explosive. Unlike larger devices which used many detonators and shaped charges to surround the core, this simple design used just two detonators which were placed at either end of the cylinder. When detonated the shock waves would travel through the explosive in what’s called a linear Implosion. Because of the lack of a tamper or lens to focus the shock waves, the lack of equal compression made this design inefficient, but for a small munition or artillery shells, it was good enough, but wastful in terms of mass verses yield. Because of the variability of the explosive surrounding the core the results could be highly variable, from just working to working much better than expected.
Its most famous use was in the Davy Crockett recoilless rifle. This jeep-mounted or tripod-mounted weapon fired a nuclear projectile with yields as low as 10 tons of TNT. The idea was to stop Soviet tank divisions from pouring through the Fulda Gap into West Germany. But the weapon’s maximum range was only about two kilometres—barely more than its own blast radius. In effect, firing it could be a suicide mission.
It was also used in the AIM-26A air-to-air missile in 1956. This carried a W54 with a 250 ton yield and was intended to be used against Soviet heavy bombers because the accuracy of the guidance systems at the time was too low to guarantee a high number of kills. Put a 250-ton nuclear exposition nearby and there would be a much higher kill chance. The AIM-26A was the only nuclear air-to-air missile the US produced, but due to worries with its use over friendly territories, by 1972 it was retired.
During the 1960s the US Navy and Marines collaborated on a variant of the W54 which was adapted into the Special Atomic Demolition Munition, or SADM which could be up to a 1 kiloton yield.
This was a literal backpack bomb and called for a two-man team, which would either parachute behind enemy lines or arrive by sea to plant the weapon in a harbour or some other target that was reachable by sea. They would plant the bomb, set the timer, and escape to a waiting craft nearby. The reality was grim, with short timers and the sheer weight of the device, survival was unlikely. In practice, the SADM was more a weapon of psychological warfare than practical strategy.
It also showed up the unpredictability of the small design, with yields that could anywhere from 10 to 1000 tons of TNT, not the sort of thing you would want it to be fired to just a few kilometres away as with the Davy Crockett.
Another approach was nuclear artillery. The W48 warhead was developed in the 1960s to fit inside a standard 155 mm howitzer shell. At 58 kilograms and 86 centimetres long, it looked just like any other artillery round, but inside was a miniaturised implosion system yielding about 72 tons of TNT.
In theory, this meant NATO commanders could call for nuclear fire support from ordinary artillery units. In practice, though, firing even a “small” nuke blurred the line between tactical battlefield use and strategic escalation. Although nuclear artillery shells were deployed, they were never used in combat.
Engineering these devices was never simple. Even at small scale, a nuclear weapon requires a precisely symmetrical implosion system, dozens of detonators firing within millionths of a second, arming and safety electronics, as well as rugged casings to survive drops, shocks, and transport.
In bigger bombs there was space for redundancy. In miniature devices, every cubic centimetre was precious. Cutting down components increased the risk of failure. The W54 again was prone to wildly variable yields, sometimes only a few tons, other times approaching a kiloton. That kind of unpredictability made commanders nervous and even more so the artillery crews.
Deployment was equally problematic. The Davy Crockett was actually fielded in Europe in the early 1960s. But the fact that soldiers were expected to be within the fallout zone of their own weapon made the system almost unusable. It was retired within a decade.
The SADMs lasted longer, into the 1980s. Their main value was forcing Soviet planners to worry about hidden nukes sabotaging key infrastructure, but as time went by, conventional weapons increased in accuracy, and there was no fallout effect if it were to be used on an area that was expected to be captured by friendly troops.
This is the reason why the Soviets made massive bombs like the 50 megaton Tsar Bomba. Their missiles weren’t that accurate but it doesn’t matter if it’s a kilometre or two off target when the total annihilation radius is 35km.
So if the SADM was the Cold War’s nuclear backpack, then drones are its modern equivalent. A 25-kilogram payload is well within the capabilities of larger unmanned aerial vehicles, and even some commercial models. In theory, a compact nuclear charge could be flown into a target by remote control. And be almost undetectable at very low levels
The physics and engineering problems remain: fissile material degrades, explosives require expert handling, and arming systems are tightly controlled. Non-state groups lack the capability to build such a weapon. But the spread of drone technology has raised alarm. Unlike missiles or bombers, drones are cheap, ubiquitous, and difficult to stop. A device once requiring a 2 man team could be delivered by a remote-controlled aircraft or drone.
This leads to the nightmare scenario: a non-state actor, a terrorist group obtaining a compact nuclear device. In the Cold War, delivery meant bombers, artillery, or special forces, all requiring state infrastructure. Today, drones are cheap, GPS-guided, and globally available.
A stolen SADM-sized warhead paired with a large drone could devastate several city blocks or a major port or seat of government. Even a 10-ton yield, the lowest ever deployed, could kill hundreds and cause global panic.
This is why governments place such emphasis on securing fissile material and dismantling small warheads. By the mid-1970s, both the US and the Soviets had come to the conclusion that having a large number of these backpack-sized nukes around was just too much of a risk of them falling into the wrong hands.
The smaller a nuclear weapon is, the harder it is to secure. In the 1960s, the U.S. began fitting weapons with Permissive Action Links, electronic locks requiring codes to arm them. But these early systems were primitive compared to today’s safeguards.
Storage and handling posed constant risks. A 30-kilogram backpack bomb is easier to smuggle, easier to lose, and harder to guard than a 10-ton missile warhead. By the late Cold War, both the U.S. and the Soviet Union recognised the danger and withdrew their smallest weapons.
But could we go smaller still, could you make, say a nuclear hand grenade? That really would be a suicide mission, but no, the Physics just won’t work at that size. Even with advanced reflectors and compression, you still need several kilograms of fissile core, and you still need kilograms of high explosives to compress it.
That sets a minimum size, about the size of a backpack. Below that, the weapon won’t function, or it becomes so unstable that it’s more dangerous to the user than the target.
Today, nuclear development has shifted focus. Rather than physically smaller packages, designers have pursued lower-yield but highly efficient warheads mounted on precision missiles. These are still larger than a backpack device but allow more controlled escalation in theory. Concerns about “briefcase nukes” persist, fuelled by rumours of Soviet portable devices in the 1990s.
This came about when in 1997, General Aleksandr Lebed claimed that out of 132 portable devices, only 48 were found, implying 84 were missing. Later, he revised estimates to a broader range 100 to 500 total, but “probably closer to 100”. That’s a pretty wide margin of error.
Claims of dozens or even hundreds of missing devices stem from uncorroborated statements by certain political or scientific figures. These have been strongly denied by official Russian sources and we don’t know if there was an ulterior motive behind them.
The terminology of a “suitcase” or “briefcase” bomb might well be misleading and could have been taken to mean a small nuclear device to be used by special forces. After all, a suitcase can be a lot bigger than a briefcase.
But there is just no evidence that anything approaching the size of the briefcase nuke had ever been produced by the US or the Soviets, although it has been a staple of spy fiction for decades, which keeps the idea alive, even if it can’t be made with current technology.
The story of miniature nuclear weapons is really a story of limits, of physics setting a floor on size, of engineering struggling to make them safe, of deployment doctrines that bordered on suicidal, and of politics that finally judged them too dangerous to keep.
They remain a fascinating Cold War footnote, and a reminder that in nuclear weapons, smaller doesn’t necessarily mean better.
