How does a Neutron bomb work ?
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It’s a concept for nuclear weapons design that seeks to maximize the “prompt radiation” of the weapon, mostly in the form of fast-moving neutrons. See, nuclear fusion reactions release a lot of high-energy neutrons. These neutrons are very useful from a nuclear-weapon perspective because they can be absorbed by fissile materials, like uranium, and cause a fission reaction. So a typical ‘thermonuclear warhead’ will have a fission bomb, which ignites a fusion reaction in lithium deuteride, which in turn releases a bunch of neutrons into a uranium metal casing, which causes secondary fission reactions, which all adds up to a very big boom.
But the neutrons are also themselves very deadly to people because they are so high-energy. They are part of what is referred to as the ‘prompt radiation’ of a nuclear blast – the gamma rays and particles emitted by the nuclear reaction itself. (This is in contrast to the fallout radiation, which comes from the fission products, not the nuclear reaction itself). In an ordinary bomb the prompt radiation is basically of no consequence because everything it hits will be some combination of vaporized, flattened, or set on fire by the nuclear explosion.
But that prompt radiation has some interesting properties. It isn’t fallout, so it doesn’t linger; it’s gone once the bomb has finished going off, and those fast neutrons can penetrate most materials and still be deadly to people. So what if you just made a thermonuclear bomb with a very small explosive yield, and built its casing out of materials that let the prompt radiation escape, instead of focusing it into more fission reactions? If the bomb were quite small, and designed with this in mind, the prompt radiation would reach much farther than the fireball. So you would have a tactical-size weapon that, in theory, could be used in battlefield conditions to attack enemy troops (especially those inside armored vehicles) while not massively poisoning the area with fallout, or destroying infrastructure you want to keep in place.
That’s the concept, anyway. Whether this actually works in the tactical role described is up for debate. For one thing, the idea of a ‘clean nuclear bomb that leaves no fallout’ is pretty suspect, since most steel in the radiation radius (such as the steel in those tanks you’re trying to hit) would itself become harmfully radioactive due to neutron activation. For another, damp soil and concrete are both pretty good shielding against fast neutrons, so troops in bunkers or trenches would have good protection and it would probably have been better to just use a conventional munition.
It’s a type of hydrogen bomb. These use a ‘primary’ fission device using uranium or plutonium. In the few millionths of a second after the explosion of the primary , the fission reaction creates a massive amount of X-rays.
These X-rays are used to compress a ‘secondary’ fusion device containing deuterium and lithium. The compression is done by surrounding the secondary with something like a very light plastic. This absorbs the X rays and is heated to millions of degrees Celsius. By heating it so much and so fast, it expands almost instantly and crushes the secondary to start the fusion explosion.
This is just like a conventional H bomb so far. In a traditional H bomb, the device is surrounded by a very dense shell often made of lead or uranium. This ‘radiation case’ confines the explosion for a few millionths of a second, preventing the fusing secondary from expanding and allowing it to fuse further on increasing the size of the explosion.
The fusion reaction between deuterium and tritium generates neutrons. In some cases, they are reflected by the radiation case, but if the bomb has a natural uranium case, they will trigger fission reactions that boost the size of the explosion even further. Many early H bombs actually got much of their yield from this fission reaction – they were also incredibly dirty spilling huge amounts of fallout into the environment.
In the neutron bomb, the case is essentially transparent to the fusion neutrons. They pour out of the bomb and saturate the surrounding area with neutron radiation which can penetrate through thick walls and armour. The bomb itself would still devastate an area around the explosion, but much smaller than if it had been a traditional H bomb.
The neutrons kill living organisms within a relatively small area and don’t leave as much long lasting radioactivity which made the bombs ‘popular’ with planners as a way of using them in scenarios where areas could be occupied by troops shortly after detonating a neutron bomb.
Most nuclear bombs are optimized to get as much ‘boom’ instead of radioactivity as possible. The radiation and fallout that *does* occur from those warheads is really incidental. In fact pretty much every iteration of nuclear bomb has yielded less fallout and radiation than the ones before it.
Neutron bombs are the opposite. They sacrifice ‘boom’ in favor of more lingering radiation. Now, they’re still nuclear bombs, so they still make a massive boom. It’s only less explosive in comparison to other nuclear weapons. The idea was thrown around in the 50s 60s and 70s with the Cold War, especially as a way of countering and preventing Soviet invasions further into Europe. You can’t invade through radioactive wasteland, after all. Or, at least, it’s harder.
But the idea was really dumb in retrospect and they were phased out and decommissioned, especially with more precise weapons were made.
The weapon nowadays is completely defunct. Though, one might consider a dirty bomb the modern conceptual equivalent.
Keeping this ELI5 since nukes are complex: The main Nuclear bomb effects are radiation, blast, and heat. You can juggle the design to promote or demote these effects. So a neutron bomb has been designed to maintain the radiation effect without producing as much blast and heat.
For example, the initial W70 warheads went up to 100 kilotons explosive force. The third “neutron bomb” version was 1 kiloton. Obviously this means “Kill only the people and leave infrastructure intact” is an overstatement since a 1 kiloton blast will still do an awful lot of damage.
Explaining exactly how is way beyond ELI5.
These are called enhanced radiation weapons.
Neutrons are attenuated very rapidly in the atmosphere, so they make the blast effect as small as possible and allow as many neutrons to escape as they can. One method is to use certain fuel (deuterium/tritium) because it emits a lot of neutrons and it’s easy to fuse, so the blast energy produced is lower.
The point is mainly to kill tank crews (Russia had way more tanks than the west). Tanks are very good at protecting people from nuclear blasts, but radiation can still get through the armour. It also can make the armour itself radioactive, potentially harming crew who aren’t killed by the initial dose from the bomb.
Not much is known publicly about how they achieve this because warhead design is highly classified. It’s still just a nuclear bomb, they just maximize the neutron radiation and minimize the blast.
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