As you should know, there are two types of nuclear weapons. An "atomic bomb" is a weapon with a war-head powered by nuclear fission. An "H-bomb" or "hydrogen bomb" is a weapon with more powerful warhead powered by nuclear fusion. In some military documents they will refer to the nuclear warhead as the "physics package."

You can read all about the (unclassified) details of their internal construction and mechanism here.

Occasionally you will find a fusion weapon referred to as a "Solar-Phoenix" or a "Bethe-cycle" weapon. This is a reference to the nuclear scientist Hans Bethe and the Bethe-Weizsäcker or carbon-nitrogen cycle which powers the fusion reaction in the heart of stars heavier than Sol.

SECTION 9: OTHER WEAPONS

Lasers and kinetics are standard reference weapons, and for good reason. All other proposed weapons suffer from serious problems which render them ineffective compared to lasers and kinetics.

The most common alternative weapons described for space warfare are nuclear in nature. There are several myths about nuclear weapon use in space, the most common of which is that they are ineffective if not in contact with the target. The logic behind this theory is that in the atmosphere, most of the damage comes from the shockwave, which obviously cannot propagate in space. An alternative is that the damage will be inflicted by the plasma that used to be the device casing. The flaw is that the shockwave is not a property of the device itself, but instead results from the absorption by the air of the X-rays emitted by the device. The superheated air then expands and produces the shockwave. In space, the X-rays are not absorbed and instead go on to damage the target directly. They still obey the inverse square law, and are not likely to be effective against mass objects such as spacecraft beyond a few kilometers, depending on the yield of the device. This makes them essentially point-attack weapons, given the scale at which spacecraft maneuver.

However, there is another mechanism by which nuclear weapons do damage in space, namely radiation poisoning of the crew. Even a 1 kT nuclear weapon will inflict a lethal dose of radiation on an unprotected human out to about 20 km, depending on the type of weapon. Larger weapons will have greater lethal ranges, scaled with the square root of weapon yield. It is possible to armor against this radiation, reducing the lethal range by an order of magnitude or more. All spacecraft will have some radiation shielding because of the environment they operate in, although neutron radiation (probably the biggest killer) generally does not occur in nature. Civilian ships are thus likely to be far more vulnerable than military ones to nuclear weapons killing their crews, unless they themselves are nuclear-powered and manage to face their shadow shield towards the initiation.

It has been suggested that the great lethality of the radiation against the crew is likely to make enhanced-radiation weapons (commonly known as neutron bombs) the nuclear weapons of choice in space. This might well be the case, particularly as soft X-rays (such as might be produced by nuclear weapons) are significantly easier to shield against than the neutrons emitted by nuclear weapons, particularly the fusion neutrons produced by an enhanced-radiation weapon. The vulnerability of the crew to nuclear weapons is another factor that would make drones attractive, as electronics are easier to harden and generally more resistant to radiation.

The biggest disadvantages of nuclear weapons are their size and short range. Even the smallest of modern nuclear weapons are considerably larger than the SCODs described above, which makes them easy to detect and target, given that their destruction would logically take priority over that of more typical kinetics. At the same time, the nuclear weapon has to get to within a few kilometers, virtually touching the target. Given typical closing velocities, a fraction of a second is not going to significantly improve survivability vis a vis a typical kinetic. And a kinetic of the same size as the nuclear weapon (100 kg or more) is almost as lethal against a typical target. This ignores the questions of cost, which is almost certainly far higher for a nuclear weapon then an equal mass of kinetics, and of politics. Many people go into a frenzy whenever they hear the word ‘nuclear’, and would likely oppose the deployment of such weapons. Pushing said deployment through would require political and fiscal capital that might be better spent on conventional weapons.

Possibly the best use of nuclear weapons is in a defensive role. A typical kinetic will be quite vulnerable to surface and sensor damage, not to mention the relative lack of defenses against kinetics. Even then, squeamishness about nuclear weapons might well prevent their use.

The use of the X-rays from the device to pump a laser is also a common suggestion, most notably used in David Weber’s “Honor Harrington” series. The same drawbacks that apply to conventional nuclear weapons apply to these devices, though to a lesser extent. Much of the information regarding this concept is classified, which has led to conflicting views of its effectiveness. Depending on the source, the effective range is between 100 km and several thousand kilometers. Particularly at the lower end of this range, the utility is questionable. The device gains a few seconds of standoff, but still has the other disadvantages of conventional nuclear weapons. At longer ranges, particularly with low-end defenses, the idea becomes feasible.

There are two possible drawbacks to the use of nuclear weapons in orbit. The first is the well-known High-Altitude ElectroMagnetic Pulse (HEMP) generated when a nuclear weapon is detonated in the upper atmosphere. This results from the interaction between the products of the bomb, and both the Earth’s atmosphere and the Earth’s magnetic field. In deep space, neither would exist, removing the HEMP. HEMP is relatively easy to protect against, adding between 5 and 10% to the price of military electronic gear. High-quality civilian surge protectors are also adequate shielding, though low-quality models have problems dealing with the rate at which the pulse occurs. Any spacecraft will almost by definition be hardened against such effects. That said, the effect does exist, and would be a consequence of orbital nuclear weapon use.

The second drawback is the lesser-known Argus Effect, in which charged particles are trapped by the Earth’s magnetic field and form artificial radiation belts, damaging or destroying satellites. These particles are mostly electrons, and tend to cluster between 1000 and 2000 km altitude. They pose a threat similar to a greatly-enhanced Van Allen Belt, and would reduce the operational lives of satellites. There is a possibility that the belts could be used as a defensive weapon, but establishing them would mean sacrificing a large portion of one’s orbital (and quite possibly planetary) infrastructure. It is also possible that an “Argus Blockade” could be implemented. This would be the intentional creation of such an effect by an attacker, intended to impair the defender’s space infrastructure and prevent him from rebuilding quickly. The effect persists for a month or so before fading back to levels that are unlikely to impair space operations.

EMP weapons have occasionally been suggested for space use. These use some non-nuclear method to generate an EMP, hopefully disabling the target’s electronics. The generation of such a pulse requires a large amount of power, which can either be generated by high explosives (most useful in a missile) or large capacitor banks, which are far better suited for shipboard use. There are two major problems with this concept, however, which will likely limit its use. The first is that any EMP will be generated using microwaves or radio waves. As discussed in Section 7, diffraction is greater for beams with longer wavelengths. This limits the range of any EMP weapon, which is hardly desirable given the ranges at which space combat is likely to occur. The second is that there are a number of natural effects encountered in spaceflight that are similar to EMPs. Solar storms in particular can produce induced currents in much the same manner, requiring spacecraft to be hardened against them. This hardening would also be effective against EMPs, requiring massive amounts of power to have any chance of working. The only really practical use for EMP weapons might be during hostile boarding missions against civilians or disabled warships. A civilian ship is likely to be somewhat less hardened then a military vessel, and the boarding ship can get very close without getting shot to pieces by the target.

by Byron Coffey (2016)

As far as warhead mass goes, Anthony Jackson says the theoretical limit on mass for a fusion warhead is about 1 kilogram per megaton. No real-world system will come anywhere close to that, The US W87 thermonuclear warhead has a density of about 500 kilograms per megaton. Presumably a futuristic warhead would have a density between 500 and 1 kg/Mt. Calculating the explosive yield of a weapon is a little tricky.

For missiles, consider the US Trident missile. Approximately a cylinder 13.41 m in length by 1.055 m in radius, which makes it about 47 cubic meters. Mass of 58,500 kg, giving it a density of 1250 kg/m3. The mass includes eight warheads of approximately 160 kg each.

Wildly extrapolating far beyond the available data, one could naively divide the missile mass by the number of warheads, and divide the result by the mass of an individual warhead. The bottom line would be that a warhead of mass X kilograms would require a missile of mass 45 * X kilograms, and a volume of 0.036 * X cubic meters (0.036 = 45 / 1250). Again futuristic technology would reduce this somewhat.

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Nuclear weapons will destroy a ship if they detonate exceedingly close to it. But if it is further away than about a kilometer, it won’t do much more than singe the paint job and blind a few sensors. And in space a kilometer is pretty close range.

Please understand: I am NOT saying that nuclear warheads are ineffective. I am saying that the amount of damage they inflict falls off very rapidly with increasing range. At least much more rapidly than with the same sized warhead detonated in an atmosphere.

But if the nuke goes off one meter from your ship, your ship will probably be vaporized. Atmosphere or no.

George William Herbert says a nuke going off on Terra has most of the x-ray emission absorbed by the atmosphere, and transformed into the first fireball and the blast wave. There ain’t no atmosphere in space so the nuclear explosion is light on blast and heavy on x-rays. In fact, almost 90% of the bomb energy will appear as x-rays behaving as if they are from a point source (specifically 80% soft X-rays and 10% gamma), and subject to the good old inverse square law (i.e., the intensity will fall off very quickly with range). The remaining 10% will be neutrons.

The fireball and blast wave is why nuclear warheads detonating in the atmosphere will flatten buildings for tens of kilometers, but detonations in space have a damage range under one kilometer.

For an enhanced radiation weapon (AKA "Neutron Bomb") figures are harder to come by. The best guess figure I’ve managed to find was up to a maximum of 80% neutrons and 20% x-rays.

If you want to get more bang for your buck, there is a possibility of making nuclear shaped charges. Instead of wasting their blast on a spherical surface, it can be directed at the target spacecraft. This will reduce the surface area of the blast, thus increasing the value for kiloJoules per square meter.

According to John Schilling, with current technology, the smallest nuclear warhead would probably be under a kiloton, and mass about twenty kilograms. A one-megaton warhead would be about a metric ton, though that could be reduced by about half with advanced technology.

Eric Rozier has an on-line calculator for nuclear weapons. Eric Henry has a spreadsheet that does nuclear blast calculations, including shaped charges, on his website. For bomb blasts on the surface of the Earth or other planet with an atmosphere, you can use the handy-dandy Nuclear Bomb Effects Computer. But if you really want to do it in 1950’s Atomic Rocket Retro style, make your own do-it-yourself Nuclear Bomb Slide Rule!

NUCLEAR WEAPON EFFECTS IN SPACE

A. NUCLEAR WEAPON EFFECTS ON PERSONNEL

In addition to the natural radiation dangers which will confront the space traveler, we must also consider manmade perils which may exist during time of war. In particular, the use of nuclear weapons may pose a serious problem to manned military space operations. The singular emergence of man as the most vulnerable component of a space-weapon system becomes dramatically apparent when nuclear weapon effects in space are contrasted with the effects which occur within the Earth’s atmosphere.

When a nuclear weapon is detonated close to the Earth’s surface the density of the air is sufficient to attenuate nuclear radiation (neutrons and gamma rays) to such a degree that the effects of these radiations are generally less important than the effects of blast and thermal radiation. The relative magnitudes of blast, thermal and nuclear radiation effects are shown in figure 1 for a nominal fission weapon (20 kilotons) at sea level.1

The solid portions of the three curves correspond to significant levels of blast, thermal, and nuclear radiation intensities. Blast overpressures of the order of 4 to 10 pounds per square inch will destroy most structures. Thermal intensities of the order of 4 to 10 calories per square centimeter will produce severe burns to exposed persons. Nuclear radiation dosages in the range 500 to 5,000 roentgens are required to produce death or quick incapacitation in humans.

1 The Effect of Nuclear Weapons, U. S. Department of Defense, published by the Atomic Energy Commission, June 1957.

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132 ASTRONAUTICS AND ITS APPLICATIONS

Fig. 1 - Weapon effects at surface (20 KT)

If a nuclear weapon is exploded in a vacuum-i. e., in space-the complexion of weapon effects changes drastically:

First, in the absence of an atmosphere, blast disappears completely.

Second, thermal radiation, as usually defined, also disappears. There is no longer any air for the blast wave to heat and much higher frequency radiation (x-rays and gamma rays) is emitted from the weapon itself.

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ASTRONAUTICS AND ITS APPLICATIONS 133

Third, in the absence of the atmosphere, nuclear radiation will suffer no physical attenuation and the only degradation in intensity will arise from reduction with distance. As a result the range of significant dosages will be many times greater than is the case at sea level.

Figure 2 shows the dosage-distance relationship for a 20-kiloton explosion when the burst takes place at sea level and when the burst takes place in space. We see that in the range 500 to 5,000 roentgens the space radii are of the order of 8 to 17 times as large as the sea-level radii. At lower dosages the difference between the two cases becomes even larger.

Fig. 2 - Nuclear radiation intensities (20 KT)

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134 ASTRONAUTICS AND ITS APPLICATIONS

A yield of 20 kilotons has been used here as an example to show the dominance of nuclear radiation effects in space; however, it may well be that multimegaton warheads, rather than 20-kiloton warheads, will be far more representative of space defense applications. With such weapons the lethal radii (from nuclear radiation) in space may be of the order of hundreds of miles. The meaning of such huge lethal radii in possible future space warfare cannot now be assessed. It does seem clear, however, that manned space combat vehicles, unless heavy shielding is feasible, will be considerably more vulnerable to nuclear defense weapons than their unmanned counterparts.

B. POSSIBLE COMMUNICATION EFFECTS

On August 1 and 12, 1958, nuclear warheads were detonated in missiles over Johnston Island in the Pacific.2-3 These detonations were accompanied by impressive visual displays seen over wide areas, leading observers to the opinion that the detonations took place at very high altitudes.4-7 These displays were even seen on Samoa, some 2,000 miles from Johnston Island.

The visual displays were accompanied by disruptive effects on radio communications. Specifically, most commercial communication systems operating on the high-frequency (about 5 to 25 megacycles) bands in the Pacific noted substantial disturbances. Most links within a few hundred miles of Johnston Island experienced "outages" for as long as several hours, at various times over a period of about a day. In general, the effects on high-frequency communication links appear to have been quite similar to the effects produced by giant solar flares.

2 Note to Editors and Correspondents, U. S. Atomic Energy Commission, Department of Defense, Joint Office of Test Information, August 1, 1958 3 Note to Editors and Correspondents, U. S. Atomic Energy Commission, Department of Defense, Joint Office of Test Information, August 12, 1958. 4 Atomic-Like Flash Seen Here-Nuclear Rocket Test Indicated, The Honolulu Advertiser, August 1, 1958. 5 Samoa Bulletin, August 1, 1958. 6 Samoa Bulletin August 15. 1958. 7 Cullington, A Man-Made or Artificial Aurora, Nature, vol. 182, No. 4646, November 15, 1958, p. 1365.

KILOTONS PER KILOGRAM

(ed note: this is a historical look at the kiloton per kilogram alphas of actual nuclear weapons. Also see his interactive Yield To Weight explorer)

What makes nuclear weapons impressive and terrible is that their default yield-to-weight ratio — that is, the amount of bang per mass, usually expressed in terms of kilotons per kilogram (kt/kg) — is much, much higher than conventional explosives. Take TNT for example. A ton of TNT weighs, well, a ton. By definition. So that’s 0.001 kilotons per 1,000 kilograms; or 0.000001 kt/kg. By comparison, even a crude weapon like the Little Boy bomb that was dropped on Hiroshima was about 15 kilotons in a 4,400 kg package: 0.003 kt/kg. That means that the Little Boy bomb had an energy density three orders of magnitude higher than a regular TNT bomb would. Now, TNT isn’t the be-all and end-all of conventional explosives, but no conventional explosive gets *that *much boom for its buck compared to a nuke.

The Little Boy yield is much lower than the hypothetical energy density of uranium-235. For every kilogram of uranium-235 that completely fissions, it releases about 17 kt/kg. That means that less than a kilogram of uranium-235 fissioned in the Little Boy bomb to release its 15 kilotons of energy. Knowing that there was 64 kg of uranium in the bomb, that means that something like 1.3% of the uranium in the weapon actually underwent fission. So right off the bat, one could intuit that this is something that could probably be improved upon.

The Fat Man bomb had a much better use of fissile material than Little Boy. Its yield wasn’t that much better (around 20 kilotons), but it managed to squeeze that (literally) out of only 6.2 kilograms of plutonium-239. Pu-239 releases around 19 kilotons per kilogram that completely fissions, so that means that around 15% of the Fat Man core (a little under 1 kg of plutonium) underwent fission. But the bomb itself still weighed 4,700 kg, making its yield-to-weight ratio a mere 0.004 kt/kg. Why, despite the improve efficiency and more advanced design of Fat Man, was the yield ratio almost identical to Little Boy? Because in order to get that 1 kg of fissioning, it required a very heavy apparatus. The explosive lenses weighed something like 2,400 kilograms just by themselves. The depleted uranium tamper that held the core together and reflected neutrons added another 120 kilograms. The aluminum sphere that held the whole apparatus together weighed 520 kilograms. The ballistic case (a necessary thing for any actual weapon!) weighed another 1,400 kg or so. All of these things were necessary to make the bomb either work, or be a droppable bomb.

So it’s unsurprising to learn that improving yield-to-weight ratios was a high order of business in the postwar nuclear program. Thermonuclear fusion ups the ante quite a bit. Lithium-deuteride (LiD), the most common and usable fusion fuel, yields 50 kilotons for every kilogram that undergoes fusion — so fusion is nearly 3 times more energetic per weight than fission. So the more fusion you add to a weapon, the better the yield-to-weight ratio, excepting for the fact that all fusion weapons require a fission primary and usually also have very heavy tampers.

I took all of the reported American nuclear weapon weights and yields from Carey Sublette’s always-useful website, put them into the statistical analysis program R, and created this semi-crazy-looking graph of American yield-to-weight ratios:

• Click for larger image Online interactive version here

The horizontal (x) axis is the yield in kilotons (on a logarithmic scale), the vertical (y) axis is the weight in kilograms (also on a log scale). In choosing which of the weights and yields to use, I’ve always picked the lowest listed weights and the highest listed yields — because I’m interested in the optimal state of the art. The individual scatter points represent models of weapons. The size of each point represents how many of them were produced; the color of them represents when they were first deployed. Those with crosses over them are still in the stockpile. The diagonal lines indicate specific yield-to-weight ratio regions.

A few points of interest here. You can see Little Boy (Mk-1), Fat Man (Mk-3), and the postwar Fat Man improvements (Mk-4 — same weight, bigger yield) at the upper left, between 0.01 kt/kg and 0.001 kt/kg. This is a nice benchmark for fairly inefficient fission weapons. At upper right, you can see the cluster of the first H-bomb designs (TX-16, EC-17, Mk-17, EC-24, Mk-24) — high yield (hence far to the right), but very heavy (hence very high). Again, a good benchmark for first generation high-yield thermonuclear weapons.

What a chart like this lets you do, then, is start to think in a really visual and somewhat quantitative way about the sophistication of late nuclear weapon designs. You can see quite readily, for example, that radical reductions in weight, like the sort required to make small tactical nuclear weapons, generally results in a real decrease in efficiency. Those are the weapons in the lower left corner, pretty much the only weapons in the Little Boy/Fat Man efficiency range (or worse). One can also see that there are a few general trends in design development over time if one looks at how the colors trend.

First there is a movement down and to the right (less weight, more yield — improved fission bombs); there is also a movement sharply up and to the right (high weight, very high yield — thermonuclear weapons) which then moves down and to the left again (high yield, lower weight — improved thermonuclear weapons). There is also the splinter of low-weight, low-yield tactical weapons as well that jots off to the lower left. In the middle-right is what appears to be a sophisticated “sweet spot,” the place where all US weapons currently in the stockpile end up, in the 0.1-3 kt/kg range, especially the 2-3 kt/kg range:

• Click for larger image

These are the bombs like the W-76 or the B-61 — bombs with “medium” yield warheads (100s rather than 1,000s of kilotons) in relatively low weight packages (100s rather than 1000s of kilograms). These are the weapons take advantage of the fact that they are expected to be relatively accurate (and thus don’t need to be in the multi-megaton range to have strategic implications), along with what are apparently sophisticated thermonuclear design tricks (like spherical secondaries) to squeeze a lot of energy out of what is a relatively small amount of material. Take the W-76 for example: its manages to get 100 kilotons of yield out of 164 kilograms. If we assume that it is a 50/50 fission to fusion ratio, that means that it manages to fully fission about 5 kilograms of fissionable material, and to fully fuse about 2 kilograms of fusionable material. And it takes just 157 kg of other apparatus (and unfissioned or unfused material) to produce that result — which is just a little more than Shaquille O’Neal weighs.** **

Such weapons aren’t the most efficient. Weapon designer Theodore Taylor wrote in 1987 that 6 kiloton/kilogram had been pretty much the upper limit of what had even been achieved. Only a handful of weapons got close to that. The most efficient weapon in the US stockpile was the Mk-41, a ridiculously high yield weapon (25 megatons) that made up for its weight with a lot of fusion energy.

But given that high efficiency is tied to high yields — and relatively high weights — it’s clear that the innovations that allowed for the placing of warheads on MIRVed, submarine-launched platforms are still pretty impressive. The really magical range seems to be for weapons that in the hundred kiloton range (more than 100 kilotons but under a megaton), yet under 1,000 kilograms. Every one of those dates from after 1962, and probably involves the real breakthroughs in warhead design that were first used with the Operation Dominic test series (1962). This is the kind of strategic miniaturization that makes war planners happy.

What’s the payoff of thinking about these kinds of numbers? One is that it allows you to see where innovations have been made, even if you know nothing about how the weapon works. In other words, yield-to-weight ratios can provide a heuristic for making sense of nuclear design sophistication, comparing developments over time without caring about the guts of the weapon itself. It also allows you to make cross-national comparisons in the same fashion. The French nuclear arsenal apparently developed weapons in that same miniaturized yield-to-weight range of the United States by the 1970s — apparently with some help from the United States — and so we can probably assume that they know whatever the United States figured out about miniaturized H-bomb design in the 1960s.

Or, to take another tack, and returning to the initial impetus for me looking at this topic, we know that the famous “Tsar Bomba” of the Soviet Union weighed 27,000 kilograms and had a maximum yield of 100 Mt, giving it a yield-to-weight ratio of “only” 3.43 kilotons/kilograms. That’s pretty high, but not for a weapon that used so much fusion energy. It was clear to the Atomic Energy Commission that the Soviets had just scaled up a traditional H-bomb design and had not developed any new tricks. By contrast, the US was confident in 1961 that they could make a 100 Mt weapon that weighed around 13,600 kg (30,000 lb) — an impressive 7.35 kiloton/kilogram ratio, something well above the 6 kt/kg achieved maximum. By 1962, after the Dominic series, they thought they might be able to pull off 50 Mt in only a 4,500 kg (10,000 lb) package — a kind of ridiculous 11 kt/kg ratio. (In this estimate, they noted that the weapon might have an impractically large diameter as a result, perhaps because the secondary was spherical as opposed to cylindrical.) So we can see, without really knowing much about the US had in mind, that it was planning something very, very different from what the Soviets set off.

From KILOTONS PER KILOGRAM by Alex Wellerstein (2013)

Nuclear Nullification

The nuclear bomb is essentially a machine that assemble - smash together - a sub-critical mass of fissionable material into a supercritical mass, so the chain reaction would run in it. The trick is to do it fast: to smash the fissionable material together before the energy, released from nuclear fission, would vaporize it and throw it away.

If the assembly is not fast enough, then the chain reaction (with the massive release of energy) would start before the fissionable material mass is properly assembled. The result would be a pre-detonation: a weak thermal explosion, that destroy the bomb and threw the bulk of fissionable material around before it have a chance to get involved into chain reaction. Such situation during nuclear tests is often called “fizzle”.

Making nuclear bomb to fizzle is quite a good way to “disarm” it. Sure, we would still have an explosion – and quite dirty one, spreading a lot of radioactive dust around – but much weaker than the bomb was supposed to do. It would still have a power from sub-kiloton to low-kiloton level, thought. Still, dealing with consequences of low-level detonation is generally much more preferable than with full-power blast.

How exactly we could do it? Obviously, we could not slow down the assembly – at least, without physical access to the enemy bomb (which the enemy would not be eager to grant us, of course). But we could affect the other factor – the neutron breeding rate in fissionable material inside the bomb.

If we subject the enemy nuclear bomb to a powerful neutron flux – for example, by small nuclear detonation of our own nearby – then external neutrons, penetrating the fissionable materials of enemy bomb, would induce a fission reaction in it. Since the mass of fissionable materials is still subcritical, it would not cause the proper chain reaction. But it would create a lot of short-lived isotopes in bomb’s fissionable materials, pre-heating or “poisoning” it with a lot of neutrons.

And when such pre-heated/poisoned bomb would attempt to detonate? There would be a lot more free neutrons flying around inside its fissionable fuel than it was designed to dealt with. These additional neutrons would ignite the chain reaction long before the assembly is complete. The bomb would fizzle: it would destroy itself in low-level explosion.

The concept of pre-heating enemy bombs to make them fizzle was one of the main reasons behind putting nuclear warheads on surface-to-air missiles and air-to-air missiles (and at least one air-to-air unguided rocket) in 1950s and 1960s. They were supposed to not only knock down enemy planes and cruise missiles, but also “poison” their nuclear payload, so it would not detonate at full power. The same idea was behind early anti-ballistic missile defense systems as well.

So why nuclear warheads aren’t widely used on SAM’s and AAM’s nowadays? Well, the general reason is that nuclear technology marched on. “Poisoning” the fissionable fuel worked good enough against pure fission bombs, and fusion bombs with fission triggers – but not against boosted fission weapons. Which is the backbone of the modern nuclear stockpiles.

The boosted fission weapon is, essentially, a fission bomb, with a small amount of fusion fuel (deuterium-tritium mix) placed inside its hollow pit. When chain reaction starts in fission material, the heat and pressure cause the D-T mix inside to undergo fusion. The energy release from such fusion is negligible. But during fusion, a lot of high energy neutrons got released. They shoot into fissionable material around, causing a lot of additional fission, and boosting up the chain reaction. Essentially, the fusion worked as a source of additional neutrons to “afterburn” the fissionable material, ensuring that much more of it would undergo fission.

The boosted fission weapon is immune to neutron “poisoning”. The fusion of D-T mix starts long before the fission material assembly is complete. And high energy neutrons, released from such fusion, would “afterburn” the fissionable material of the bomb by themselves. So even if the boosted fission bomb is “poisoned”, it would still be able to ignite a fusion, and the fusion would boost it to full power.

So, do we have a “nuclear nullifier” here? Yes and no. The neutron poisoning could prevent enemy nuclear bombs from detonating at full power, but they would still explode, albeit at much reduced power. And the effect is not “blanketing”; you still need to go after each enemy bomb personally, with either a neutron bomb of your own, or a fictional neutron beam.

Most disappointing, though, is the fact, that this method is… obsolete. It did not work against boosted fission weapons. And pure fusion bombs (without fission trigger at all) would also be completely immune. So in modern, or futuristic warfare setting, this “nuclear nullifier” is nearly useless.

On the other hands, in retro warfare setting neutron poisoning could work just fine. From late 1940s to early 1970s it was a perfectly viable way of dealing with enemy nuclear weapons. So if your setting involves “Fat Man” type fission bombs dropped from piston-engine bombers (1940s), or megaton-scale thermonuclear bombs dropped from jet bombers (1950s), or “Orion” nuclear pulse spaceship clashing around the Moon (1960s), you actually could turn the enemy warheads into duds.

P.S. Also, it means that “Failsafe” movie is, sadly, not doing it right. If the “Vindicator” bombers were subjected to doses of radiation, fatal for their crews, then their 1960s bombs would be turned into nearly-harmless dud. So much for the drama…

by Alexey Shiro (2023)

A "neutron bomb" is a nuclear warhead design that has been tweaked so it is much better at killing soldiers and civilians while doing much less damage to military vehicles and civilian buildings. It makes it easier to kill off the enemy soldiers so you can steal their stuff. Neutron bombs are also good to use if the enemy is invading your country. No sense in blowing huge holes in your own cities when all you want to do is exterminate enemy soldiers.

This weapons is what you call an "enhanced radiation bomb". They are specially constructed so more of the bomb’s energy is emitted as neutrons instead of x-rays. This means there is far less blast to damage the buildings, but far more lethal neutron radiation to kill the enemy troops. Conventional nuclear warheads typically release 5% of the energy as neutrons, but in neutron bombs it is a whopping 40%. Neutron energy is higher as well: 14 MeV instead of the conventional 1 to 2 MeV.

A 1 kiloton neutron bomb will irradiate anybody unfortunate enough to be at a range of 900 meters with 80 Grays of neutrons. According to dosages set by the US military, this is high enough to instantly send the victim into a coma, with certain death to follow within 24 hours due to damage to the central nervous system. The LD50 dose is at a range of between 1350 and 1400 meters (almost a mile).

Problems include:

• Neutron activation of the steel girders of buildings would render them unsafe. Which was one of the selling points of neutron bombs: the buildings could be immediately used by an advancing army, once you removed all the dead enemy soliders.
• Armored fighting vehicles provide enemy soldiers with a surprisingly high protection of neutron radiation, and can be easily increased. Since all spacecraft include radiation shielding from solar storms and galactic cosmic rays, this will drastically reduce the effect of neutron bombs used as anti-spacecraft weapons. Spacecraft with nuclear propulsion will try to aim their shadow shields at the neutron bomb for added protection.
• Enemy ground soldiers can also find high amounts of protection by sheltering inside buildings with 12 inch concrete walls and ceiling, or in a cellar under 24 inches of damp soil. Both will reduce the radiation exposure by a factor of 10.
• Neutron bomb ordinance requires maintenance, since one of the components is Tritium with its annoyingly short half-life of 12.32 years. This means that every few years the neutron bombs will have to be opened up and have their tritium replaced.

NEUTRON BOMB

Energy type	Proportion of total energy (%)	Fission	Enhanced
Blast	50	40 to minimum 30
Thermal energy	35	25 to minimum 20
Prompt radiation	5	45 to minimum 30
Residual radiation	10	5

A neutron bomb, officially defined as a type of enhanced radiation weapon (ERW), is a low-yield thermonuclear weapon designed to maximize lethal neutron radiation in the immediate vicinity of the blast while minimizing the physical power of the blast itself. The neutron release generated by a nuclear fusion reaction is intentionally allowed to escape the weapon, rather than being absorbed by its other components. The neutron burst, which is used as the primary destructive action of the warhead, is able to penetrate enemy armor more effectively than a conventional warhead, thus making it more lethal as a tactical weapon.

The concept was originally developed by the US in the late 1950s and early 1960s. It was seen as a "cleaner" bomb for use against massed Soviet armored divisions. As these would be used over allied nations, notably West Germany, the reduced blast damage was seen as an important advantage.

ERWs were first operationally deployed for anti-ballistic missiles (ABM). In this role the burst of neutrons would cause nearby warheads to undergo partial fission, preventing them from exploding properly. For this to work, the ABM would have to explode within approximately 100 metres (300 ft) of its target. The first example of such a system was the W66, used on the Sprint missile used in the US’s Nike-X system. It is believed the Soviet equivalent, the A-135’s 53T6 missile, uses a similar design.

The weapon was once again proposed for tactical use by the US in the 1970s and 1980s, and production of the W70 began for the MGM-52 Lance in 1981. This time it experienced a firestorm of protest as the growing anti-nuclear movement gained strength through this period. Opposition was so intense that European leaders refused to accept it on their territory. President Ronald Reagan built examples of the W70-3 which remained stockpiled in the US until they were retired in 1992. The last W70 was dismantled in 2011.

Basic concept

In a standard thermonuclear design, a small fission bomb is placed close to a larger mass of thermonuclear fuel. The two components are then placed within a thick radiation case, usually made from uranium, lead or steel. The case traps the energy from the fission bomb for a brief period, allowing it to heat and compress the main thermonuclear fuel. The case is normally made of depleted uranium or natural uranium metal, because the thermonuclear reactions give off massive numbers of high-energy neutrons that can cause fission reactions in the casing material. These can add considerable energy to the reaction; in a typical design as much as 50% of the total energy comes from fission events in the casing. For this reason, these weapons are technically known as fission-fusion-fission designs.

In a neutron bomb, the casing material is selected either to be transparent to neutrons or to actively enhance their production. The burst of neutrons created in the thermonuclear reaction is then free to escape the bomb, outpacing the physical explosion. By designing the thermonuclear stage of the weapon carefully, the neutron burst can be maximized while minimizing the blast itself. This makes the lethal radius of the neutron burst greater than that of the explosion itself. Since the neutrons disappear from the environment rapidly, such a burst over an enemy column would kill the crews and leave the area able to be quickly reoccupied.

Compared to a pure fission bomb with an identical explosive yield, a neutron bomb would emit about ten times the amount of neutron radiation. In a fission bomb, at sea level, the total radiation pulse energy which is composed of both gamma rays and neutrons is approximately 5% of the entire energy released; in neutron bombs it would be closer to 40%, with the percentage increase coming from the higher production of neutrons. Furthermore, the neutrons emitted by a neutron bomb have a much higher average energy level (close to 14 MeV) than those released during a fission reaction (1–2 MeV).

Technically speaking, every low yield nuclear weapon is a radiation weapon, including non-enhanced variants. All nuclear weapons up to about 10 kilotons in yield have prompt neutron radiation as their furthest-reaching lethal component. For standard weapons above about 10 kilotons of yield, the lethal blast and thermal effects radius begins to exceed the lethal ionizing radiation radius. Enhanced radiation weapons also fall into this same yield range and simply enhance the intensity and range of the neutron dose for a given yield.

History and deployment to present

The conception of neutron bombs is generally credited to Samuel T. Cohen of the Lawrence Livermore National Laboratory, who developed the concept in 1958. Initial development was carried out as part of projects Dove and Starling, and an early device was tested underground in early 1962. Designs of a "weaponized" version were carried out in 1963.

Development of two production designs for the army’s MGM-52 Lance short-range missile began in July 1964, the W63 at Livermore and the W64 at Los Alamos. Both entered phase three testing in July 1964, and the W64 was cancelled in favor of the W63 in September 1964. The W63 was in turn cancelled in November 1965 in favor of the W70 (Mod 0), a conventional design. By this time, the same concepts were being used to develop warheads for the Sprint missile, an anti-ballistic missile (ABM), with Livermore designing the W65 and Los Alamos the W66. Both entered phase three testing in October 1965, but the W65 was cancelled in favor of the W66 in November 1968. Testing of the W66 was carried out in the late 1960s, and it entered production in June 1974, the first neutron bomb to do so. Approximately 120 were built, with about 70 of these being on active duty during 1975 and 1976 as part of the Safeguard Program. When that program was shut down they were placed in storage, and eventually decommissioned in the early 1980s.

Development of ER warheads for Lance continued, but in the early 1970s attention had turned to using modified versions of the W70, the W70 Mod 3. Development was subsequently postponed by President Jimmy Carter in 1978 following protests against his administration’s plans to deploy neutron warheads to ground forces in Europe. On November 17, 1978, in a test the USSR detonated its first similar-type bomb. President Ronald Reagan restarted production in 1981. The Soviet Union renewed a propaganda campaign against the US’s neutron bomb in 1981 following Reagan’s announcement. In 1983 Reagan then announced the Strategic Defense Initiative, which surpassed neutron bomb production in ambition and vision and with that, neutron bombs quickly faded from the center of the pub