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The first fission bomb was created in 1944, and the first fission reactor in 1951 (and actually productive one in 1954). This delay seems possible to explain by there being a larger amount of initial effort in building a fission bomb than a fission reactor.

But the first fusion bomb was tested in 1952, whereas we still don't have working fusion reactors, ~75 years later, and are only now starting to produce net electric energy from it.

What makes nuclear fusion reactors so much more difficult than fusion bombs, and why are fission reactors not similarly more difficult than fission bombs?

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    $\begingroup$ its hard to set up such high temperatures ($>10^{6}K$) and contain it for some viable time to allow the fusion process to happen , +expensive $\endgroup$
    – Naveen V
    Commented Mar 20, 2023 at 13:33
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    $\begingroup$ Fission reactors were a key part of the Manhattan Project to make plutonium. Starting from the Chicago pile in 1942, they were a straightforward engineering problem. So your fission reactor dates are off by a decade. $\endgroup$
    – Jon Custer
    Commented Mar 20, 2023 at 13:39
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    $\begingroup$ The first artificial fission reactor was built in 1942. en.wikipedia.org/wiki/Chicago_Pile-1 $\endgroup$
    – PM 2Ring
    Commented Mar 20, 2023 at 13:40
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    $\begingroup$ U-238+n —> U-239, which rapidly beta decays to Np-239 and again to Pu-239. Then you chemically separate the Pu from the U - no isotope separation needed. $\endgroup$
    – Jon Custer
    Commented Mar 20, 2023 at 22:33
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    $\begingroup$ @user56834 plutonium only exists in absolutely minuscule amounts naturally, creating it in fission reactors as Jon explained is necessary to get enough of it to build a bomb (which makes the fact that the Rocky Flats plant ended up misplacing 28 kg of it in the ventilation system even more astounding) $\endgroup$
    – llama
    Commented Mar 21, 2023 at 0:53

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The key difficulty in fusion power is sustaining a controlled nuclear fusion reaction.

The conditions needed for nuclear fusion here on Earth involve extremely high temperature -- on the order of $10^8$ K. The Sun can achieve fusion with "only" $1.5 \times 10^7 K$ because of its sheer bulk and intense pressure at the core.

To successfully capture the energy of nucluear fusion, we need to control the fusion process and sustain it for a much longer time. This is where the current research & development is happening. This Wikipedia page lists various methods currently being developed.

A thermonuclear weapon does indeed use nuclear fusion - at these very high temperatures - but the fusion reaction (secondary stage) only happens because a fission reaction (primary stage) precedes it to set up the conditions needed for fusion. The entire multi-stage explosive reaction happens on the order of microseconds.

In contrast, nuclear fission can be controlled (known as a moderated fission reaction), and this energy can be captured and re-distributed as electrical power.

Please note that the loss of ability to moderate nuclear fission was the cause of various high-profile nuclear accidents (e.g. Chernobyl, Fukishima, etc.). Unmoderated fission on a very short timescale (known as prompt criticality) is how the World War II-era atomic bombs worked.

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    $\begingroup$ Good answer, but I think you should add one more word: "containment." If you want to "sustain a controlled nuclear fusion reaction" and, if you want to "successfully capture the energy...," then you need to do it inside a reactor. We know several ways to fuse light nucleii, but so far, the tech needed to build a bottle within which to do it continuously has eluded us. $\endgroup$ Commented Mar 20, 2023 at 18:39
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    $\begingroup$ P.S., I'm not sure that "moderated" is the right word. The purpose of the so-called "moderator" in a nuclear fission reactor is to increase the reaction rate by slowing down fast neutrons so that they are more likely to be captured by fissionable nucleii. When you're talking about neutrons in a reactor, "slow" is not the same thing as "delayed.". (You mentioned Chernobyl. That story depended on control room operators who did not know delayed neutrons from prompt neutrons.) $\endgroup$ Commented Mar 20, 2023 at 18:44
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    $\begingroup$ "The key difficulty in fusion power is sustaining a controlled nuclear fusion reaction." But why is that difficult? $\endgroup$
    – user56834
    Commented Mar 20, 2023 at 21:26
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    $\begingroup$ @user56834 Because in Sun, due to high plasma amount and density, quantum tunneling thorough the coulomb barrier happens here, there and in many places overall. Besides core integrity is kept only by Sun gravity forces alone. Now in a fusion reactor we have to "light up" specific precise spots in plasma, what's how it's "controlled". A little bit to the left or right,- and no reaction. It's like when you fire a match not under paper, but under the wood. Besides plasma is kept by magnetic fields, which itself are unstable by nature, a little fluctuation and plasma is contaminated and "gone". $\endgroup$ Commented Mar 20, 2023 at 21:51
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    $\begingroup$ @user56834 similar to the reason why throwing a can of beans in a fireplace is a bad idea (the beans will breach containment, or in other words the can will explode). Hot stuff is hard to keep together, especially dense and charged hot stuff $\endgroup$
    – llama
    Commented Mar 21, 2023 at 1:00
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Fission is Easy

Natural fission reactors exist on earth, at a small scale, and low energy. However, the smallest natural fusion reactor is probably the brown dwarf, which has a mass at least 10x Jupiter's. If we compare the masses of these natural reactors, it's clear that fission is at least 6 orders of magnitude easier than fusion (and probably closer to 9-12...just too lazy to do the math).

As others have pointed out, the only reason fusion bombs even exist is because we can use fission reactions to compress the fusion products. If fission bombs weren't practical, we would almost certainly not have any fusion weapons of any kind right now.

Fusion is Hard

You should be glad that fusion is hard. If it were easy, then a lot of elements would not be stable. Imagine if a car crash concentrated enough energy to transmute elements. Even a rocket launch might permanently change the exhaust products. Chemistry itself would become relatively unstable.

Unfortunately, the flip side is that to achieve fusion, you need to inject a lot of energy into a fairly small space. If you don't have 10 Jupiters of gravity to do that work for you, then the energy needs to come from somewhere else: lasers, plasma, really powerful hammers, etc. And since the fusion targets tend to be very small, it is extremely difficult to get all that input energy to drive fusion, rather than just heating up your target. This is what Mr. Doty is referring to with the square/cube scaling.

Simply heating up a fuel pellet to 1 million degrees isn't very useful. It's really an enormous waste of energy. If we could get all that energy to turn into fusion reactions, we would be golden. In reality, only a fraction of that turns into fusion reactions, so we don't get a whole lot of fusion out of each attempt.

Fusion reactors require a tremendous amount of energy just to operate. If they could produce more energy than they consume, then once started, they would be self-sustaining. But as others have noted, sustaining a fusion reaction is even harder than starting one.

ITER, one of the oldest and most mature fusion reactors, takes about half a gigawatt to operate. That's roughly a medium-sized power plant that could normally power a decent-sized city, just to warm up a single fusion reactor.

NIF, which uses lasers to trigger fusion, only converts about 10% of the laser energy into potential fusion. That's not even taking into account the thermal losses in the lasers themselves, which are some of the most powerful ever built, or all the energy spent running cooling pumps and other essential equipment.

Conclusion

Fission is efficient at human scale, but uncontrollable at planetary scale (if you had a fissile pile the size of a planet, good luck making a power plant out of that). Fusion is relatively efficient at stellar scale, but extremely difficult at human scale. The reasons have to do with the relative strength of the electromagnetic, strong, weak, and gravitational forces.

Note that thermonuclear weapons are not called "fusion bombs" because fusion is just one component of the total design. Multiple fission reactions occur and are necessary to the total effect, including fission of the fusion fuel itself! As much as half the yield of these weapons comes not from fusion, but from fission processes. The destructive power comes not from fission or fusion alone, but from fission triggering fusion, and then fusion triggering more fission.

And a bomb is always easier to design than a power plant, because no containment or energy harvesting is needed. The lack of containment is a feature for a bomb, and a massive liability for a power plant. It's just a lucky coincidence that fission is calm at small scales and explosive at larger ones, while fusion is, in some sense, the opposite (meaning, gravity-driven fusion is relatively "calm" and stable, while plasma/ICF is pretty unstable).

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    $\begingroup$ Natural fission reactors existed on Earth in the past, but almost certainly don't exist anywhere on Earth today. This is because U235 made up a greater percentage of natural uranium in the past. The fraction of U235 changes over time because U235 has a smaller half-life than U238. $\endgroup$
    – N. Virgo
    Commented Mar 21, 2023 at 6:34
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    $\begingroup$ "ITER, one of the oldest and most mature fusion reactors"? ITER's age is currently estimated at -5 years old. Also, I don't think this gives an answer to the question of "why" fusion is difficult. $\endgroup$
    – craq
    Commented Mar 22, 2023 at 2:40
  • $\begingroup$ Fusion reactors can now generate more energy than they input. sciencealert.com/… I've not got time to check the underlying paper. There are many, many caveats I'm sure, but this does somewhat go counter to your "if they could generate more power than they put in, they could be sustained" point $\endgroup$ Commented Mar 23, 2023 at 9:58
  • $\begingroup$ @ScottishTapWater that article is about NIF, which I linked to in my answer. The caveat is that they are only talking about laser energy in vs. fusion reactions triggered. Laser efficiency is not even included in this calculation, because it can be improved independently. So it's a technical milestone, but practically speaking, no energy company is going to be building NIF-style fusion reactors in the next 2 decades. Consider that there is 0 attempt to actually harvest the energy produced, which is almost as hard as getting here. $\endgroup$ Commented Mar 23, 2023 at 21:07
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Fission can be achieved using an uncharged neutron, and some isotopes, such as 235U and 239Pu, have a large cross section (high probability) for fission with a very low energy neutron.

Fusion requires combining positively charged nuclei, such as D with T, and as such the interacting nuclei must have sufficient kinetic energy to overcome coulombic repulsion to be forced sufficiently close for the nuclear force to cause fusion. It is quite difficult to create conditions here on earth where the nuclei are forced sufficiently close for fusion to occur, and this is the reason fusion reactors have not been developed as soon as fission reactors. (The sun uses its large force of gravity to force fusion.)

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    $\begingroup$ It's worth to mention that even at Sun core quantum tunneling is needed to overcome coulomb potential barrier, because even Sun's core temperatures and gravity is not enough to initiate directly fusion reaction with such small cross section. $\endgroup$ Commented Mar 20, 2023 at 15:31
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    $\begingroup$ "It is quite difficult to create conditions here on earth where the nuclei are forced sufficiently close for fusion to occur". But why is this difficult? $\endgroup$
    – user56834
    Commented Mar 20, 2023 at 21:28
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    $\begingroup$ Because you must add sufficient energy to increase the kinetic energy of the interacting nuclei to undergo fusion, while at the same time contain these highly energetic, charged particles. $\endgroup$
    – John Darby
    Commented Mar 21, 2023 at 14:36
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    $\begingroup$ @user56834: Because the stuff you want to fuse has to be really, really, really really hot or else it won't fuse. And when stuff get's that hot, it tends to melt anything you use to contain it. $\endgroup$
    – Lee Mosher
    Commented Mar 21, 2023 at 21:49
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    $\begingroup$ I have to say I was tempted to write my own answer in the exact wording of that comment, but another answer (that of @DeanMacGregor) said the same thing in a more quantitative manner which I would never have been able to do. $\endgroup$
    – Lee Mosher
    Commented Mar 23, 2023 at 16:59
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In the end, it comes down to Galileo's square-cube scaling law. Thermonuclear reactions proceed in the volume of the fuel, but lose energy and mass through the surface. To keep the reaction going, you need a large volume/surface area ratio.

In nature, the required ratio is huge, requiring the volume/surface of a star! We can make mixtures of isotopes that are considerably easier to ignite than a cosmic mixture, but there is a limit to how far we can go. Igniting a fairly large mass of thermonuclear fuel with a fission explosion is much easier than igniting the far smaller mass in a reactor.

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Let's start with fission reactors.

Their normal operating temperature is under 1000C.

Even stainless steel doesn't melt at 1000C

Now consider fusion.

It requires a steady temperature of 100,000,000K, give or take an order of magnitude. The material with the highest melting point is Hafnium carbonitride with a melting point of above 4000C. (btw I'm not bothering to keep consistent between K and C b/c the difference between them is a rounding error against the number that matters).

So even the material with the highest melting point, would easily be melted by fusion. That means to contain the plasma which is being fused, it has to be held together with magnetic fields. As we all know, magnets only work on things with a charge. Part of the fusion process is the freeing of neutrons which, as the name suggests, are neutral and hit the wall of a fusion reactor regardless of how well the magnetic field can hold the fusion fuel. That bombardment damages the wall of the reactor limiting its usefullness.

To make that more challenging is that free neutrons that have the energy of having been ejected from a fusion reactor aren't easy to come by for testing purposes. What that boils down to is essentially having to build a reactor and only when you destroy it do you see how well your material is at withstanding neutron bombardment. This is a bit of an overstatement as there is some testing that can be done short of a full build but it's imperfect testing.

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    $\begingroup$ This. My understanding is that "how well the magnetic field can hold the fusion fuel" is related to the stability of plasma flow within the containing magnetic field. As the reactor operates and fuel is injected, small instabilities cascade, causing the plasma to overcome the magnetic containment and damage or destroy the reactor. $\endgroup$
    – Blackhawk
    Commented Mar 22, 2023 at 19:41
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This is not a complete answer, as other answers a good. Note that in a thermonuclear device, a large fraction of the energy of a fission device is use to compress the fusion stage; moreover, the fusion stage contains a plutonium spark-plug at the center, which is really compressed into super-criticality (much more than high explosive compress a fission pit).

So: there is a fission bomb on the outside, and another fission device on the inside (and sometimes another U238 fast-fission jacket on the outside).

Obviously, those conditions are unacceptable for power generation.

Perhaps a better comparison for your question would the time between a pure fusion weapon (https://en.wikipedia.org/wiki/Pure_fusion_weapon), and a pure fusion reactor: neither of which exist now (as far as we know).

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    $\begingroup$ "Obviously, those conditions are unacceptable for power generation." Why? why can't we use fission as part of the reactor to start the fusion process? Is part of the problem that we might make the temperature of the fusion reactor TOO hot? Is this a matter of, the temperature has to be extremely high but has to be in a slim goldilocks temperature zone, because if it falls outside, it either overshoots and explodes or undershoots and fizzles out? $\endgroup$
    – user56834
    Commented Mar 20, 2023 at 21:33
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    $\begingroup$ @user56834 because in bombs the fusion component is triggered by a literal nuclear explosion. The sustained fission that you see in power plants is far too weak to do anything like that. The problem isn't in starting a fusion process, the problem is in sustaining it to continuously produce manageable amounts of power. Fusion isn't like fission -- in fission the problems are keeping the chain reactions manageable (once it starts it wants to continue). In fusion the whole problem is keeping the reaction going. It takes sun like amounts of material to do this naturally. $\endgroup$
    – eps
    Commented Mar 20, 2023 at 22:56
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Fission occurs naturally on earth all the time, and there's been at least one instance of a natural fission reactor which consistently put out ~100 kw of energy for >100,000 years (https://en.wikipedia.org/wiki/Oklo_Mine). So once the theory is worked out it was relatively straightforward to build a basic reactor. The Chicago Pile was literally built by high school dropouts.

Meanwhile, natural fusion on earth doesn't happen (it might rarely occur in the upper atmosphere but I can't find a source to confirm) and a natural fusion reactor is completely impossible (on earth, that is -- obviously stars are a natural example). Fusion and fission have opposite problems. All of the hard work in fission is in the moderation of the reaction; you need to continuously produce manageable amounts of energy without the thing blowing up. It's easy to get going and once started it really doesn't want to stop. Fusion, by contrast, is extremely hard to start and once started it is constantly pushing everything away (which stops the reaction). It is extremely difficult to sustain without star amounts of matter. It's often referred to as creating a star on earth, but it's even more extreme than that -- since the quantities of materials used on earth are miniscule by comparison, the fusion chamber needs to be producing an environment that is much more extreme than what you find in the core of a star. Even when you have a star, the probability of a fusion reaction occurring during a collision is on the order of $10^{-31}$, which is why average sized stars last so long. Every day the sun fuses about $5*10^{13}$ metric tons of hydrogen, which sounds like a lot until you realize that the sun has $1.5*10^{27}$ metric tons of hydrogen. Another way to look at that number is that at the current rate it takes about a billion years to convert just 1% of its total hydrogen.

All of this to say, sustained fusion is orders of magnitude more difficult than fission. Even given the right ingredients, it's hard to start, hard to sustain, hard to produce more energy than you put in. Fission, given the right ingredients, is child's play by comparison, the only difficulty is stopping the thing once it starts.

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  • $\begingroup$ just realized a lot of this was in Lawnmower Man's answer, which wasn't yet posted while I started formulating this answer. dang $\endgroup$
    – eps
    Commented Mar 21, 2023 at 0:17
  • $\begingroup$ "once started it is constantly pushing everything away". What do you mean by "push everything away"? do you just mean that the temperature and therefore pressure is so high that the plasma disperses? $\endgroup$
    – user56834
    Commented Mar 22, 2023 at 11:52
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Fission (splitting atoms) is a chain reaction. The particles produced when one atom splits will excite other nearby atoms enough to cause them to split as well, etc etc. You don't need super special conditions to make this happen, just knock over the first domino and everything proceeds naturally from there. The key part of a fission reactor is slowing down the chain reaction to ensure it can still be controlled and won't generate heat faster than they can remove it.

Fusion (smashing atoms together) is not a chain reaction. Something external provides a force that compresses the fuel together, causing atomic nucleii to fuse and release energy. Without that force, the reaction halts. A fusion reactor doesn't just contain the fuel and reaction, it has to actively keep the reaction going. That makes it much more complex. It takes a certain amount of kinetic energy to counteract the electrostatic forces that make fuel nucleii repel each other. Stars use their immense internal gravity to force nucleii together. We can't do that on Earth, so we have to supply that energy using things like lasers or electromagnets.

The problem with fusion reactors isn't that they're difficult to build. Many working reactors exist, using a variety of designs. The hard part is building one that's efficient enough to be worth the hassle. Fission reactors have the benefit of being more like traditional power plants, where you have fuel going in and energy coming out. Fusion requires both fuel and energy as inputs since the reaction isn't naturally self-sustaining at normal Earth conditions. The hard part about fusion is that the ratio of output energy to input energy has to be high enough before you can say that it's useful as a power source. For many of our current designs, that ratio is less than one, meaning they consume more than they generate. It was only a few months ago (Dec. 2022) that a fusion reactor reported passing the "break-even" point for energy production, but is still far short of what would be required for commercialization.

Don't let the timeline of nuclear weapon development confuse your understanding here, as that's a completely different scenario than nuclear power. Bombs don't have to be energy efficient, they just have to destroy the target. Nuclear reactors have a completely different - if not polar opposite - set of design criteria.

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One intuitive answer is that the "natural size of a reactor" is much larger for fusion than for fission. If you get a couple kg of uranium or plutonium together at standard temperature and pressure it will release a bunch of energy through fission. The same happens for fusion if you put different hydrogen isotopes together. The only difference is that for fusion this size is somewhere between the mass of Jupiter and that of the sun. In order to get a fusion reaction to happen with a reasonable amount of material (less than the size of a building) you need really large temperatures and pressures. The way a fusion bomb achieves these temperatures and pressures is by detonating a fission bomb to compress the fuel. This is not a feasible powerplant design (to see why, detonate a nuke next to your neighborhood powerplant).

Fundamentally, the answer basically boils down to the fact that fusion reactions really want to be big balls of plasma, and buildings really don't want to be big balls of plasma.

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    $\begingroup$ I think my HOA will object to your proposed experiment... but it isn't listed in the bylaws anywhere. $\endgroup$ Commented Mar 21, 2023 at 18:11
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When humans do Fission, we do it with unstable atoms. These atoms where created by nuclear processes, and in the long term would evaporate themselves. Sometimes we modify existing atoms to be more unstable, then use them for Fission.

When you take a modest pile of such unstable atom and give it a bit of help, you can get it to start a chain reaction and make the atoms even more unstable. This chain reaction is both how Fission weapons and Fission reactors work.

At a first approximation, once you have put enough of the right kind of atom together (with some moderators - nuclear catalysts - to help), the amount of Fission that occurs diverges. The hard part is:

a) Gathering enough of the unstable atoms, and b) Not killing yourself when the math of divergence goes boom

The Fission bomb lets the divergent mathematics run. To maximize yield, work is done to pass the point where divergence in the amount of Fission happens as fast as possible, so the reaction can proceed further along before the entire apparatus blows itself apart.

A Fission reactor stays on the other side of this divergent cliff. We arrange it so that as the reaction rate grows, something slows it down and pulls it away from the cliff. We then harvest the energy output. There is lots of engineering to get this right.

In both cases, the unstable atoms and the divergent reaction means that we are just speeding up the energy that wants to be let loose a bit faster.

Fusion is different.

If you place a bunch of Fusion-capable atoms together, they will start to emit energy. But the amount of atoms required is more than the mass of Jupiter!

What more, this process is self-moderating over a huge mass scale.

We are lucky that this process is self-moderating. If it wasn't, only stars of a very narrow mass range would be stable; larger ones would explode basically instantly.

Our "Fusion" enhanced bombs don't try to replicate this process of "first take a larger than Jupiter pile of hydrogen". Instead, we use Fission to induce Fusion, then use the Fusion byproducts to boost the Fission yield, and back and forth.

This is a very different process than Fission bombs. A Fission reaction could occur naturally (and has!) on Earth. We are just speeding it up a bit when we make a bomb, and we are making a kind of controlled slower version when we make a Fission reactor.

Meanwhile, there is no known way to make a Fusion reaction happen on Earth in any non-trivial amount.

If we made Fusion bombs and reactors like we did Fission ones, we'd build a white dwarf and feed it some Jupiter-sized gas giants to make a Fusion bomb - it would look like a Nova. To make a Fusion reactor, we'd smash together a bunch of Jupiter-sized planets and ignite a star, possibly with custom mixtures of elements to get the right output and lifetime.

Instead, our Fusion enhanced bombs are hacking physics at a lower level. And that approach didn't work if we tried to slow it down; the energy density of an actual Fission explosion is key to making the Fusion reaction occur at all! And we can't contain a Fission explosion in a reactor.

The approach we are using for Fusion reactors is thus completely different than how we do Fusion bombs. We are building containment devices that generate conditions more extreme than inside stars in order to pull off the yields we want, while containing the reaction safely.

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    $\begingroup$ @PM2Ring I'd like to have some words with the scientists who decided to make those two words so incredibly similar... $\endgroup$
    – bta
    Commented Mar 21, 2023 at 18:09
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The analogy between fission and fusion is closer than is usually discussed:

To make fission work, the rate at which slow neutrons are liberated in the fission substrate (proportional to volume) must be greater than or equal to the rate at which slow neutrons escape from the fission substrate (proportional to surface area). Since surface-area-to-volume-ratio decreases with increasing size, this means that the piece of fission substrate must be larger than a certain size to make the reaction work. That's what "critical mass" is. One can manipulate the critical mass in two ways: either by increasing the rate of liberation of slow neutrons, which is achieved by including a "moderator" material that scatters and slows down fast neutrons in the fission substrate; or by reducing the rate of escape of slow neutrons, which is achieved (I think only in explosive contexts) by surrounding the fission substrate with a neutron-reflective "tamper". These manipulations can bring the critical mass down to a human scale, for either civilian or military applications.

Similarly, to make fusion work, the rate at which energy is liberated in the fusion substrate (proportional to volume) must be greater than or equal to the rate at which energy (along with the matter that carries it) escapes from the fusion substrate (proportional to surface area). Since surface-area-to-volume-ratio decreases with increasing size, this means that the piece of fusion substrate must be larger than a certain size to make the reaction work. Just like with fission, there is a "critical mass". One can manipulate the critical mass in two ways: either by increasing the rate of liberation of energy, which is achieved (only in explosive contexts) by detonating a fission explosive inside the fusion substrate (note that it's all about rate of liberation of energy, so it has to be explosive, a steady, safe fission reactor won't do); or by reducing the rate of escape of energy, which can been achieved in three ways: firstly (I think only in explosive contexts) by surrounding the fission substrate with an X-ray-reflective "tamper"; secondly, by holding matter and the energy it carries in under its own gravitational pull (that's what a star does); thirdly, by holding matter and the energy it carries in using complicated shaped electric and magnetic fields (this method has not yet been successfully made to generate more energy than it consumes). To date, only the explosive variant has got the critical mass down to anything like a human technological scale.

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What makes nuclear fusion reactors so much more difficult than fusion bombs ?

Controlled nuclear fusion is more difficult than a fusion bomb in the same way as making an internal combustion engine is more difficult than making a Molotov cocktail.

Why are fission reactors not similarly more difficult than fission bombs?

For the differences between fission and fusion, see the other excellent answers.

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    $\begingroup$ I think this could indeed be a useful analogy, but for it to provide an answer to the question you also need to cover the fission side and why it looks different there. $\endgroup$ Commented Mar 23, 2023 at 17:09
  • $\begingroup$ @leftaroundabout Maybe, but other answers cover the differences between fission and fusion in copious detail, and I have nothing to add. $\endgroup$
    – gandalf61
    Commented Mar 23, 2023 at 18:10
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There are a number of challenges to build a successful fusion reactor. The main reason is that the fuel has to be hot. The optimum temperature is 200 million degrees! 10x hotter than the centre of the sun!! That's because we're trying to get two positively charged nuclei to stick together, but positive charges repel. They have to be moving very fast, otherwise they bounce off before they get close enough to stick.

Such high temperatures bring a whole series of challenges:

  1. you can't put it in an ordinary container. You can use magnetic fields to keep the hottest parts in the centre and only the cooler parts (a few thousand degrees) touch the walls. Or you can use small pellets of fuel a long way from any walls (inertial fusion).
  2. even if the fuel doesn't touch any walls, it is constantly losing heat by convection and radiation (e.g. Bremsstrahlung, synchrotron and line radiation). If it loses heat faster than fusion reactions generate heat, then it will cool down and the reaction stops. So reactors want a long energy confinement time. There are many strategies for improving the confinement time. The main ones are increasing the magnetic field strength or simply building a bigger machine. Both of those options are expensive, and at the high end the sheer size and forces between magnets create engineering challenges of their own.
  3. at that temperature, everything is a plasma and plasma physics is one of the most complex branches of physics around. It has all the chaos of fluid dynamics, with electromagnetism thrown in. That leads to a whole host of instabilities, any one of which can lead to a decrease or even a critical breakdown in confinement. (Examples are Rayleigh-Taylor for inertial fusion and the Greenwald density limit or Electron Temperature Gradient mode for tokamaks.) Poor understanding of plasma physics also led to expending research effort on things like mirror machines, or to extremely optimistic predictions of confinement scaling for tokamaks and stellarators.
  4. heating systems and measurement systems for something that hot are challenging. Systems have been developed, but they are not cheap, and they may need further development to improve radiation hardness and reliability for a commercial reactor.

There are a couple of other unsolved challenges too, like breeding Tritium from Lithium, managing Helium "exhaust" in the reactor, and the longevity of materials in an environment with high energy neutrons.

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Fusion is easy if you use nuclear fission bombs to reach the necessary pressure and temperature. The main problems arising from this technology are:

  • How do you actually harvest the released energy for purposes other than fancy crematoriums?
  • Can you produce atom bombs at a rate sufficient to sustain fusion energy production on a large scale? Are the finite natural resources of fission material sufficient?
  • People became antsy already with the few fusion experiments on the Bikini atoll; what do you think they'd say about such a fusion plant on Three Mile Island?

Fusion by different means is a hard problem. Try it! This seems to be a good moment in time to allocate venture capital for fusion experiments if you are able to produce a good YouTube video and put some CNC machines in a factory hall. Maybe you can get Henry Kissinger onto the board of advisors, that would help.

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Fission releases neutrons. Neutrons can cause fission in surrounding fuel. If you can get one atom to undergo fission, you can build that into a working reactor.

Fusion also releases neutrons. Neutrons do not cause fusion in the surrounding fuel. If you get one atom to fuse, you have bupkis. To build a working reactor, you have to entice each and every atom in it to undergo fusion, and doing so requires enormous amounts of energy.

That is the reason fusion is harder.

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