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Currently most Nukes are actually thermonuclear fusion bombs ignited by a fission (uranium or plutoniom) bomb.

The fission bomb is like a lighting match that s needed to trigger the more powerful fusion reaction. The fission bomb produces lots of radioactive waste. That s why we say that nukes are "dirty".

If we manage to create a pure fusion bomb that doesn't need a fission bomb to ignite, then we would have a "clean" hydrogen bomb. The first country to gain access to such a bomb would have a tremendous tactical advantage.

Your enemy is moving his fleet close to your borders? Just throw a clean fusion bomb and sink the entire fleet.

A terrorist group is hiding in the ruins of a city? Just throw a few fusion bombs in a row to completely flatten that area.

Such a weapon might actully be cheaper than cruise missiles since they won't need to be very precise and the raw material (hydrogen isotopes) are cheaper to produce than what s needed for a fission bomb.

How do you think such a weapon would affect modern warfare?

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    $\begingroup$ Actually, the yeild is not due to fusion, so your characterization is wrong. The fusion of heavy hydrogen isotopes is to provide a neutron flux which allows more reaction from the plutonium before it’s scattered. $\endgroup$ – JDługosz Nov 29 '16 at 6:05
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    $\begingroup$ To fuse hydrogen into helium you need high temperature at least higher than the core of our Sun, OK the Sun cheated but never mind that I think people are trying to shrink LHC so... $\endgroup$ – user6760 Nov 29 '16 at 6:07
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    $\begingroup$ Perhaps you could rephrase the question, Make it purely about military doctrine in a world where "tech X" makes it possible to completely level for example a city without the long term effects of todays (thermo)nuclear bombs. Let "pure fusion bomb" simply be an example of what "tech X" might be. $\endgroup$ – Guran Nov 29 '16 at 7:14
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    $\begingroup$ Perhaps you could look at how fuel-air-explosives are used today. That is the go-to conventional weapon for target areas with nothing but enemies and "acceptable collateral losses" $\endgroup$ – Guran Nov 29 '16 at 7:19
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    $\begingroup$ An environmentally friendly nuclear bomb?! Terrorist groups tend to hide out in populated cities rather than ruins to prevent those types of attacks, which really could be carried out with today's weapons. $\endgroup$ – colmde Nov 29 '16 at 12:06

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You can already blow nuclear weapons in a "clean" manner

Nuclear weapons that are detonated as air bursts are relatively "clean" and do not produce much fallout. There will be some local "activation" but in general the trope that a nuclear detonation will create a lethally irradiating death-zone is false (in general, you can make it that way, if you "salt" the bomb and you make it a ground detonation).

Safecast map of Hiroshima

Safecast Map of Nagasaki

So your question is not hypothetical. It has always been the case that radiation damage can be made to play only a small part of an exchange of nuclear weapons. That is not why people are holding back on using them, but because of the sheer direct destructive power that they have. They are Weapons of Mass Destruction by their detonations alone. The possibility that they can also cause severe fallout is just a "bonus", but it is not the main reason for their classification as WMDs.

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Warfare is more and more moving away from having one big army in one spot where a big bomb would be effective.

Urban warfare, guerrilla tactics, hidden bases, jamming, stealth technology - those things seem to be the keywords for modern wars. Combined with cyber-attacks, espionage, propaganda, misinformation and other types of information warfare, 'diplomacy'/bullying and economic pressure.

That's why we see more development towards autonomous weapons and surveillance than weapons of mass destruction today.

Most likely, as globalization continues we will see a shift from "military" characteristics towards "police" characteristics in future warfare.

At least unless we trigger a full out world war III, in which case we are all doomed anyways.

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The only practical way to initiate a "pure" fusion explosion would be to use a small quantity of antimatter to initiate the fusion reaction (possibly by using a small amount of antimatter the centre of a sphere of Lithium Hydride and using explosive lenses to collapse the fusion fuel onto the antimatter).

While much smaller and "cleaner" than a conventional fusion weapon, it will still release a great deal of energy in the form of neutrons and x rays, so there will still be radiation effects.

As noted, there are many reasons not to use massively powerful WMDs in modern war (even strategic weapons today are thought to have yields limited to @ 300Kt). On the other hand, nuclear weapons can be used as compact, high energy sources to power other weapons systems. Atomic Rockets has a long section on this in the "Conventional Weapons" page.

A quick summary, nuclear weapons can be used to drive a "shotgun" and accelerate pellets to 100km/sec.

A nuclear shaped charge can drive a stream of liquid metal at speeds approaching 3% c

A Casaba Howitzer can use the energy to create and drive a spindle of plasma at up to 10% c

Finally, a nuclear explosion could be used to power an X-ray laser which sends a coherent beam at the speed of light at the target.

Driving ultra high speed devices makes targeting much simpler (very little lead time), and provides massive amounts of kinetic and thermal energy on the target on impact. These weapons would be most useful in space warfare, but could also be used from orbit to hammer targets on the ground (a single shot from a nuclear shaped charge would destroy a missile silo, sink an aircraft carrier or render an airfield unusable, for example). Conversely, assuming there is a way to contain the damage from firing the driving unit, these weapons could also be used from the ground to hit targets in LEO as well.

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The biggest change is you get SMALLER weapons. There is no minimum yield for a pure fusion device; the National Ignition Facility is looking for fusion yields on the order of 1-5 lbs of TNT. The holy grail of advanced weapons research is the ability to build fractional kiloton weapons, say 10 - 100 tons. These would allow a single device to destroy targets that now take take multiple weapons or even multiple strike missions, battle damage assessments, mop up operations and unexploded ordnance.

Your examples were interesting but think smaller; a precision 10 ton cruise missile instead a bomber with 20 500 lb bombs.

A single missile instantly destroys the capital ship and damages 4 close escorts. Destroy the terrorist headquarters (a block), clearing the hideouts and rubble, but leaving the inhabited neighborhood intact. One cruise missile that takes out an attack column but leaving the village it is attacking without target coordination, damage assessment and mob up strikes.

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Actually, you will not create 100% pure (i.e. no fallout) thermonuclear (fusion) bomb. Fusion process itself generates stray neutrons (among other radiation types) that can, and will cause secondary radioactive isotopes form from matter around the bomb, thru neutron activation process. While it's not nearly as large amount of fallout compared to same yield of fission, it's still significant. Gamma rays and xrays are also inherently produced during fusion reactions and are both ionising (though these are considered prompt radiation source, not fallout).

You can limit the radiation output of a classic teller-ullam design (together with its yield) by changing second stage pusher/tamper layer (which is part of the weapon that is both used as a - sort of - "hammer" / casing to the fusion fuel, that compresses it into the "anvil" - the U235 "spark plug" in the center of secondary stage) from usually used Uranium-238 to Lead (as i.e. Russians did when they limited the Tsar Bomba from its design yield of 100 megatons to ~50 megatons). As lead does not fission (and indeed, it will actually prevent neutrons from irradiating the area, by a little bit), as uranium does, the yield, and release neutrons/ionising photons/fission byproducts is smaller.

Another thing to consider is, fusion weapons are also considered "clean" because any promptly dangerous radiation is present so close to the hypocenter, that survival is very unlikely anyway, especially for very high yield weapons, range for very high radiation emission might be well inside the fireball.

As for costs, it will be expensive anyway. You cannot just use regular hydrogen in a fusion weapon. Sure, a star can use H-H fusion process, but it is dependent on extreme pressure, that a bomb cannot generate. Instead, thermonuclear weapon provides enough pressures/energy needed for another kind of fuel, namely a heavier isotopes of hydrogen - deuterium (mostly in form of lithium deuteride solid fuel) or tritium (rarely in form of either tritium gas, in fusion boosted fission weapons, or cryogenic tritium). Both are very expensive to manufacture large-scale, and in case of tritium, decay is so fast that weapons using it need their fusion fuel changed every so often in order to remain viable (this, and the need for cryostorage, is one of reasons that modern systems use Li6D fuel. About the only western system using tritium were non-weapon test systems.).

Now, as to why it would not change anything - WMDs are not feared because radiation (which is very overrated and mostly a psychological scare, but if one knows of it, precautions can be taken to limit its effectiveness (aside from prompt neutron/gamma/xray pulse)). They are feared for combined issue of easy delivery (nowadays, with a ballistic missile, you can obliterate any place on earth within 20-30 minutes of launch, with little to none chance of someone stopping the warhead) and sheer destructive power of blast and thermal pulse. While Hiroshima's Little Boy was very powerful compared to classical explosives, it pales in comparison to really heavy warheads (though almost all of really heavy stuff was removed from stockpiles due to international ban treaties).

For example - take the Russian R36M (or NATO name SS-18 Satan) ICBM. One of its warhead wariants is using single 20-25 megaton warhead. That yield is able to cause enough prompt thermal pulse to cause lethal 3rd degree burns at the range of 40 (forty) kilometers from the hypocenter, provided there is line of sight to the fireball at the moment of detonation. Literally everyone that sees the explosion moment (or more fitting, is "seen" by the bomb), will be burned. Badly.

Go closer than those 40 km, and thermal pulse effects are worse. Up to and including your body flashing into steam (and, effectively, leaving a less bleached patch of ground below, as a shadow - as those preserved in Hiroshima) a few kilometers from the fireballs surface (and that fireball alone is 3-4 kilometers in diameter).

And the effects arrive, as I said - promptly. Not within seconds. You are hit by the thermal pulse at the moment you see the explosion (as it's not heated air coming to you, its thermal photons arriving together with the ones in visual range of em spectrum). Buildings will be heavily damaged at about half that range, construction strength dependent.

It's the extent, and literal inevitability of damage, that is being feared, not radiation itself.

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Even if the device would not procude the kind of nuclear waste as our current weapons, the shear destructive power of the "explosion" would render the weapon in the same category with current nuclear weapons. Essentially, a few countries posses big enough stockpile and a reliable enough delivery system to completely "flatten" an aggressor that first uses nuclear weapons, thus rendering using such weapon impossible in the first place.

Your enemy is moving his fleet close to your borders? Just throw a clean fusion bomb and sink the entire fleet.

The enemy has similar fusion bombs or conventional thermonuclear weapons safely hidden in submarines that cannot be destroyed.

A terrorist group is hiding in the ruins of a city? Just throw a few fusion bombs in a row to completely flatten that area.

Loss of civilian life, possible international response, and risking the triggering of a nuclear response from other countries (see above) would prevent using this, in the same way as we do not nuke ISIS today.

If we manage to create a pure fusion bomb that doesn't need a fission bomb to ignite, then we would have a "clean" hydrogen bomb. The first country to gain access to such a bomb would have a tremendous tactical advantage.

Nuclear weapons are almost always strategic weapons. This would be especially true for weapons that "completely flatten a city"

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One key impact of a pure fusion weapon is that it would break the nuclear fuel cycle -- which gives us our best means of gaining intel about who is building/attempting to build nukes. The nuclear powers watch every aspect of fissile materials (U235, Plutonium) and isotope enrichment of Uranium, because right now, fissile material is the essential ingredient for both atomic and thermonuclear bombs (ditto reactors) and generally starting a full-up nuclear industry. A pure fusion bomb would be one heck of a nasty surprise, if the bad guys (or aliens) had these to use.

A second aspect is that the minimum explosion power from such a weapon might be much, much lower than the minimum imposed (by our physics and materials) via the minimum critical mass of fissionable materials they use for their primary detonation. Pure fusion weapons could conceivably be much smaller (both weapon and blast radius) and might also be much cheaper to make in quantity. Talk about a weapons-proliferation nightmare! Glad this is only fictional.

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Too much collateral damage

There is nothing on earth that is worth destroying with a megaton explosion that will not also cause massive collateral damage to something/someone innocent along the way. There are people everywhere. If there aren't people there are polar bears. No one wants to nuke polar bears. And if there are no people or polar bears or other cuddly creatures, then you are talking about nuking Antarctica. And that just seems like a waste of a nuke.

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What about civilians? and other living organisms?

Currently we need the accurate missiles to reduce the causalities!

regarding to both of your scenarios:

  • I imagine this bombs to be costly! and also when your defending your coast you might have your troops down there too! so if you use this kind of bomb, your gonna waste a lot of money to kill some of your own troops too.
  • I don't think any terrorist group is that stupid to live in a city without any hostage!
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It's not possible to make a pure fusion bomb because the weight of the equipment to achieve fusion temperatures, and the electric power station to put in that energy, would be a lot more than the weight of the biggest planes.

If you were to concede the Pure constraint to using a dense energy input, it still wouldn't detract from us needing to use fission as an energy input.

That already exists, and the result is that, whereas previously you could destroy the world, now you can destroy the world... even more? Same as you being dead multiple times. logical?

The power of the explosives is already high enough and practical enough, the only difficulty in practical warfare is interception, surprise and large numbers.

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We already have "tactical" nuclear weapons with a relatively low yield, including nuclear artillery shells, mines, torpedoes, etc. So the limitation on using them isn't technical or due to battlefield contamination, it is social due to a negative public reaction (at least in the case of a open democratic country) or fear of escalating reprisal (for other countries) leading to the M.A.D. concept. A "clean" explosion might reduce public distaste, but there will still be lots of toxic byproducts of vaporizing modern war machines loaded with depleted uranium and all sorts of toxic fluids and chemicals. I've been to military war games in the 90's where use of tactical nukes were considered acceptable options. Unlikely that authorization would be granted, but if we get into another Total War situation like WW2 I can easily see it happening. We've just been lucky that all the nuclear capable combatants haven't really been on the ropes since WW2, all the fighting has been relatively localized and low key (from a casualty perspective, if not a resource one). Once one side starts using tac nukes the other side will probably retaliate, we can only hope that some level of rationality keeps their use confined to battlefields (which are increasingly urban, unfortunately).

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