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zerda_EB t1_izof7fb wrote

Reply to comment by RobusEtCeleritas in What is the difference between atomic, nuclear and hydrogen bombs? by something-stupid2134

Sorry, but quick question, why is nuclear fusion said to be 4x more powerful than nuclear fission but H bombs are 1,000x more powerful than atomic bombs? Is it because the fuel can be more compact

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annomandaris t1_izoh56y wrote

Because fission bombs tend to blow themselves apart so even with a kg of material, barely any is fissioned

So if they didnt, they’d be much closer to being 4x less powerful, instead of 1000x

Meanwhile hydrogen bombs keep together better.

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[deleted] t1_izojc94 wrote

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[deleted] t1_izorshn wrote

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[deleted] t1_izosgvz wrote

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Gmn8piTmn t1_izph6fy wrote

Moreover by design the hydrogen fusing under certain geometries enhances the fission by a large factor.

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RobusEtCeleritas t1_izopyu6 wrote

In terms of the relevant reaction Q-values (the amount of energy released by one single reaction), fission does on the order of 10 times better than fusion.

But if you divide the Q-value by the mass of the fuel particles, fusion does better than fission.

People often get that confusion, because they've heard that fusion releases more energy per unit fuel mass, and are then surprised to find out that fission releases much more energy per reaction.

And then in a weapon, the crucially important role that fusion fuel plays is that it produces fast neutrons that can induce more fission in an already supercritically-multiplying system of fission reactions. Each additional neutron produced by fission therefore has the chance to cause many more fission reactions, each of which comes with on the order of ten times more energy than the initial fusion reaction.

So thermonuclear weapons are able to much much more yield for a given total fuel mass.

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bmyst70 t1_izotmhi wrote

A related question:

What is a neutron bomb? I've heard the term and from what I recall, it was supposed to be a bomb that destroyed people but left buildings intact.

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ChemicalRain5513 t1_izoxpk0 wrote

A neutron bomb is a type of nuclear bomb designed to generate a high neutron flux, rather than a high explosive power. Since neutrons don't carry an electric charge, they can penetrate many materials more effectively than charged particles, which are slowed down by all the electrons. This means a neutron bomb produces a lot of radiation that can effectively penetrate buildings and even tank armour, killing many people from radiation poisoning while creating a relatively small explosive yield. This would make it suitable as a tactical nuclear weapon, meant to eliminate armoured divisions while limiting infrastructural damage.

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Abdiel_Kavash t1_izp2uhh wrote

If it can penetrate through buildings or armor, what makes it "stop" inside of a human?

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Ishana92 t1_izp3wmh wrote

It doesnt stop inside a human. It mostly passes through you the same as through a wall.The difference being that damage in a wall or armor is likely nothing worth writing about, while passing through living tissue neutrons break DNA and disrupt cells.

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Abdiel_Kavash t1_izp49gc wrote

Ah, I see. I interpreted "penetrating" as "passing through with relatively little interaction", not as "bullet penetrates a window".

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mfb- t1_izoy1pu wrote

It's a bomb that releases unusually many neutrons for its yield - you remove as much material around the fusion stage as possible. It's still pretty destructive to non-living things, but it's more dangerous to life than a normal bomb with the same yield.

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KauaiCat t1_izr2m74 wrote

conventional thermonuclear is like fission-fusion-fission

a neutron bomb is more like fission-fusion

Instead of using the high speed fusion neutrons to induce more fission, you just let them escape.

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mfb- t1_izooas2 wrote

You could build a giant pure fission weapon, but why would you? Fusion makes it much easier to reach megaton yields - fusion itself releases more energy per mass, and it also makes it easier to fission a lot of uranium.

For very small weapons the simplicity of a pure fission weapon can be more important.

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rootofallworlds t1_izp7nv1 wrote

A fission bomb must start with a subcritical assembly and detonates by making it become supercritical. If you want a large amount of fission fuel, making a shape that's not already supercritical is a significant design constraint.

Economics is also a factor. A pure fission or boosted fission weapon gets most of its yield from the fission of either highly-enriched uranium or weapons-grade plutonium. Those are expensive.

(Boosted fission = you put a little bit of fusion fuel in the middle. It doesn't make much yield directly but it makes lots of neutrons to cause more fission in the plutonium. Without boosting only a small percentage of the fuel actually fissions.)

A thermonuclear weapon, AKA hydrogen bomb, uses a (nowadays always boosted) fission primary to cause a larger fusion reaction. Typically, but not always, the fusion then creates additional fission in a natural or depleted uranium tamper, much cheaper per kg than highly-enriched uranium. The yield is predominantly from the fusion and the tamper fission, and a relatively small amount of expensive material is needed for the primary.

Almost all current nuclear weapons are thermonuclear. It's considered to be the best design even for relatively low-yields of a few tens of kilotons. I think North Korea and possibly Pakistan are the only states using pure or boosted fission weapons.

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