Unlike almost any other device created, Nuclear weapons must abide by the “always/never” rule. They must always work flawlessly when required, often with just a few minutes notice, they must never accidentally go off, never get stolen and never be subject to unauthorized use.
So how safe are the ultimate weapons of war from accidental detonation, deliberate sabotage, been stolen or just human error.
From 1953 the US started deploying nuclear weapons in the US and Europe in a wide variety of guises from bombs to artillery shells, missiles to handheld atomic demolition devices. This was to counter the perceived threat from the Soviet Union and also China as it was thought the west just couldn’t match the size of their armies and keep defence spending under control. Nuclear weapons would be the west’s centrepiece strategy to keep the piece and win a war if it came to it.
But this proliferation of nuclear devices raised two issues, how safe would they be from detonation due to accidents and how could the president of the United States maintain the ultimate control if weapons were scattered across not only the US but other countries in Europe which had been enemies less than a couple of decades earlier.
Considering the number of nuclear weapons that have been made since 1945, estimates vary but the number is around 70,000 by up to eight countries around the world, it’s somewhat of a miracle that up to now there hasn’t been a major nuclear accident somewhere.
Yet since their creation there have been an alarming number of incidents, with fully assembled multimegaton bombs being struck by lightning, burned to the point of melting, crushed after they hit the ground at supersonic speeds, lost in the deep oceans and blown apart by their own explosives or the missiles they were mounted on.
The term “Broken Arrow” is the name given to nuclear weapon accidents, be they accidental launching, firing, detonating, theft or loss of the weapon. The U.S. admits to having 32 broken arrows worldwide, with six nuclear weapons having been lost and never recovered. But the number of incidents from small issues like weapons being dropped or being misplaced for a while was well over 1000.
One saving grace is that to unintentionally make a nuclear weapon to explode properly and to deliver its designed yield is quite difficult compared to a conventional explosive device.
TNT, one of the most common high explosives is stable enough so that it can be melted and poured into shells and can withstand the acceleration from the gun barrel but it will explode if a detonator is used.
An atom bomb or fission bomb is different, but the basic principle of its operation is quite simple.
There are two basic types, one is a gun-type, here two sub subcritical fissionable components, usually uranium 235 are fired into each other creating a critical mass and a nuclear explosion, the Hiroshima device or “Little Boy” was a gun-type bomb.
The Nagasaki bomb code-named “Fat Man” was the second type, an implosion bomb. Here a hollow sphere of Plutonium, called a pit after the stone seed in the center of a peach is surrounded by a high explosive lens which focuses the explosion inward on to the pit. When this lens is correctly detonated a symmetrical shockwave compresses the Plutonium pit enough to initiate a chain reaction and a nuclear explosion occurs.
The timing of the explosive lens is critical to nuclear detonation occurring. In the original Trinity test, the bomb had 32 high explosive segments that surrounded the pit. Each one of these had to be triggered within 1 microsecond or less to ensure the shockwave was symmetrical all around the core. Later developments reduced this to two explosive hemispheres.
If the triggering to any one of the segments was delayed or the explosive had irregularities or air bubbles in it, the shockwave would not be symmetrical and the bomb will fizzle rather than explode.
A Thermonuclear bomb or Hydrogen Bomb is even more difficult to detonate, here a fission bomb is used to implode a capsule of fusion fuel, hydrogen in the form of Lithium-6 deuteride before igniting it, so both processes have to work correctly for the hydrogen bomb to work as designed.
Because even a small partial fission event could lead to a very large explosion, a safety protocol called “one-point safety” was introduced. A nuclear weapon is deemed to is one-point safe if, when the High Explosive inside the weapon is accidentally detonated at any single point, the probability of producing a nuclear yield exceeding the equivalent of 2kg TNT is less than 1 in one million, so safety measures were built in to the bombs to stop an accidental nuclear explosion.
In the “Little Boy” bomb, which was a gun-type weapon, 4 bags of cordite provided the propellant charge required to fire one fissile component into another. These were inserted once the bomb was in flight and on its way to the target. One problem with this type of device was that if it ended up in water like the sea, it would have a moderating effect and could cause a criticality event even if the bomb was not damaged. The gun-type weapons were soon limited to artillery shells due to their smaller size but eventually, their use was discontinued
Several methods were used in the early implosion-type bombs to stop an accidental detonation. The early bombs used a solid plutonium core which would only be placed in the bomb when it was ready for use, without it there could not be a nuclear explosion.
Later types used the hollow core design as this produced a bigger bang for less nuclear material but they were not removable, these were called sealed pit devices and possed a greater threat as they were ready to go at any time.
To make these safe, one idea was to fill the hollow pit with steel balls which would disrupt the symmetrical implosion required to reach criticality. These would be removed before use by letting them fall out into a container under the plane, they could be reinserted by rotating the bomb and filling it from the top if it returned.
Another method involved filling the hollow pit with a fine chain of a material that absorbed neutrons like cadmium. The chain would also act like the steel balls by stopping a symmetrical implosion.
But the military was more worried about the reliability of the bombs to explode when required than the 1 in a million chance of an accidental explosion and so additional safety measures were resisted for a long time.
Something that didn’t help was a safety feature developed by the Livermore lab for their W-47 warhead in 1958 which was found not to be one point safe and was used in all the Polaris submarine missiles. Because of a moratorium of testing at the time, the design could not the furthered by nuclear tests. So to make the bomb safer, a variation of the cadmium chain, a cadmium and boron wire was inserted into the pit, this would absorb the neutrons and stop a reaction.
During the arming process, this wire would be pulled out a by a small motor to make the bomb live. However, it was found that over time that the wire corroded inside the pit and got stuck or became brittle and broke off inside. The motor was too small and weak to pull it out if it got stuck so the bomb would not work when required. Eventually, about 75% of the warheads that were part of the guaranteed retaliation in case of a Soviet first strike were found to be duds and all the warheads had to be replaced with a new warhead design which incensed the Navy.
Fire was also an issue, although 23 weapons had been involved in fires and none had exploded that didn’t mean that number 24 wouldn’t be the one ending in a mushroom cloud. One of the designers at Sandia National lab, Bob Peurifoy found out about an explosive that had been around since 1888 called 1,3,5-triamino- 2,4,6 trinitrobenzene or TATB.
This was so hard to detonate it wasn’t even classified as an explosive but as a flammable solid. It could be hammered, dropped, set on fire, crashed at high speed and not detonate. It was ten times less sensitive to shock compared to the existing high explosives but with the right type of detonator, it could produce a shockwave almost as comparable. Eventually, this would replace the conventional explosives used and make an accidental detonation even more unlikely
In Some weapons that had been involved in fires when planes had crashed, for example, the plutonium-oralloy core had literally melted as it has a relatively low melting point of 640 C and jet fuel burns at 980 C. Although this didn’t pose an explosion risk it did pose a radiation risk if the highly radioactive plutonium got into the air as dust and smoke.
So Later warheads were fitted with a Fire Resistant Pit or FRP, a high-melting-point metal shell that can withstand prolonged exposure to a jet fuel fire and the corrosive action of molten plutonium. So whilst the plutonium itself may melt, it would remain contained within the encasing shell and not be dispersed into the environment.
To avoid a detonation of a bomb that might have been accidentally armed but might still be on the ground, environmental sensing devices or ESDs were fitted. One of these is the trajectory switch, this detects when the warhead of falling along a trajectory as it would if dropped from a plane or launched in a missile. Others include barometric sensors to detect its altitude, piezoelectric sensors in the nose to detect it had hit the ground or radar to measure its distance from the ground. All of these would have to have operated in the correct sequence for the bomb to detonate
So whilst a detonation in an accident would very difficult to achieve that doesn’t mean an accidental detonation couldn’t occur if all the correct arming and firing procedures happened but in an unintended way.
Early bombs were armed with a single high energy electrical pulse generator when a weapon is released from its delivery vehicle but this wasn’t safe as it was first thought.
Just to prove that a simple random event could have far-reaching consequences, a crew removing four Mark 28 thermonuclear bombs from a B-47 bomber found that they were all armed. After a seven-month investigation, it turned out that a tiny metal nut had come off a screw somewhere and lodged against an unused radar heating circuit and the arming circuit, creating a new unintended circuit and accidentally armed the bombs.
So the designers came up with the idea that instead of a single pulse to arm the weapons, a series of on/off pulses in a code that couldn’t be replicated by a random event like shorted wire should be used instead.
There was also another reason for a coded system. Although most of the nuclear warheads were owned by the US, many were in other countries in the NATO alliance. Because it was easy to arm the early nuclear devices, the power to use them could reside much lower down the chain of command and in other countries effectively on trust alone. A disgruntled commander or pilot or even a government could use them for their own purposes without the knowledge of the US. By making bombs code enabled the authorization and control of their use would come back to the US President.
This coding system was called a Permissive Action Link or PAL and would make it extremely difficult to arm a lost or stolen weapon without the appropriate code being entered. It would take intimate knowledge of the design of the bomb to try and hotwire it, something which was a closely guarded secret.
The early PAL’s were mechanical and didn’t always work reliably, their batteries had a tendency to run flat with little notice and the mechanics were noisy enough that a skilled technician could work out the code by listening to the clicks it made.
Eventually, these would be replaced with computerized systems and buried deep within the bomb itself with extra security collectively called the “strong/weak” link. If any attempt to arm the device was not made in the exact sequence either during an accident or deliberately then the firing mechanism would be permanently disabled.
So as we come to today the problems of safety and security have been overcome and as a result, there have been no unwanted nuclear detonations. As an extra layer of safety In modern missile system, the targets are not preloaded, so in the unlikely event of an unauthorized launch, the default target would be somewhere in the nearest ocean. Only when an authorized launch command is sent is the target set.
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