Hydrogen Bombs

[Carl Fredrik Ahl]

Hi,

I have heard that both fission and fusion take place in a hydrogen bomb. Is it true that the fission part is to make the fusion part very hot so that fusion can take place? Is it hydrogen atoms that will fuse together to helium atoms and this will make the big explosion?

[StringJunky]

Yes.

[mathematic]

https://en.wikipedia.org/wiki/Thermonuclear_weapon

 

Above is a good description.   note that the primary fuel for the fusion us Li6 which generates H3, which fuses.

[Carl Fredrik Ahl]

14 hours ago, StringJunky said:

Yes.

Why will hydrogen fuse together to form helium? How do they get attracted to each other just because it gets very hot and how (in what form) is the energy released when they fuse?

[Sensei]

51 minutes ago, Carl Fredrik Ahl said:

Why will hydrogen fuse together to form helium? How do they get attracted to each other just because it gets very hot

Tritium and Deuterium fusion reaction:

[math]_1^3H + _1^2H \rightarrow _2^4He + n^0 + 17.6 MeV[/math]

alternative form:

[math]T + D \rightarrow _2^4He + n^0 + 17.6 MeV[/math]

Tritium can fuse with other Tritium and release 10.446 MeV:

[math]_1^3H + _1^3H \rightarrow _2^5He + n^0 + 10.446 MeV[/math]

Helium-5 is unstable isotope of Helium, and has only one decay mode via neutron emission:

[math] _2^5He \rightarrow _2^4He + n^0 + 887 keV[/math]

 

Free neutrons will be captured by Uranium-235, Lithium-6 or other fissile element.

[math]_3^6Li + n^0 \rightarrow _1^3H + _2^4He + 4.8 MeV[/math]

alternative form:

[math]_3^6Li + n^0 \rightarrow T + _2^4He + 4.8 MeV[/math]

51 minutes ago, Carl Fredrik Ahl said:

and how (in what form) is the energy released when they fuse?

As always, kinetic energy of newly created particles (the lighter particle takes more energy), and gamma photons.

Edited November 7, 2018 by Sensei

[Carl Fredrik Ahl]

1 hour ago, Sensei said:

Tritium and Deuterium fusion reaction:

31H+21H→42He+n0+17.6MeV

alternative form:

T+D→42He+n0+17.6MeV

Tritium can fuse with other Tritium and release 10.446 MeV:

31H+31H→52He+n0+10.446MeV

Helium-5 is unstable isotope of Helium, and has only one decay mode via neutron emission:

52He→42He+n0+887keV

 

Free neutrons will be captured by Uranium-235, Lithium-6 or other fissile element.

63Li+n0→31H+42He+4.8MeV

alternative form:

63Li+n0→T+42He+4.8MeV

As always, kinetic energy of newly created particles (the lighter particle takes more energy), and gamma photons.

Thx for the answer. Can you please explain more basic why the hydrogen atoms wanna fuse together to be helium atoms instead of giving the formulas?

So kinetic energy is released, does this mean that the helium atom willl get that kinetic energy and move very fast, or will the surrounding atoms move fast or both? I'm don't know much about this, but I'm curious.

[Sensei]

Sum of rest-masses of Helium-4 and free neutron is smaller than sum of rest-masses of Tritium and Deuterium. So they can fuse together. They just need to overcome Coulomb's Barrier.

18 minutes ago, Carl Fredrik Ahl said:

does this mean that the helium atom will get that kinetic energy and move very fast, or will the surrounding atoms move fast or both?

Yes. Very fast moving particle will hit surrounding it medium, decelerate, and give away part of its kinetic energy to particles which it hit on its path (which means they will also being accelerated after collision) (and eventually ionization, disintegration, or pair-production of matter-antimatter etc. etc. can happen). That's what we see in Cloud Chambers - the more particle has kinetic energy, the longer is particle trace. Particles which weakly interact with matter, such as neutrinos or antineutrinos, don't have many collisions with matter, so don't leave traces. They require more sophisticated methods of detection.

Edited November 7, 2018 by Sensei

[swansont]

By virtue of being hot, the particles have enough energy to overcome the Coulomb barrier, i.e. electrostatic repulsion (or get close enough for quantum tunneling to be likely), and once that happens, the nuclear force of attraction is present, allowing the particles to fuse.

[Carl Fredrik Ahl]

17 minutes ago, swansont said:

By virtue of being hot, the particles have enough energy to overcome the Coulomb barrier, i.e. electrostatic repulsion (or get close enough for quantum tunneling to be likely), and once that happens, the nuclear force of attraction is present, allowing the particles to fuse.

Thx for the answer. But I still wonder why they want to fuse, what benefits are there?

[swansont]

5 minutes ago, Carl Fredrik Ahl said:

Thx for the answer. But I still wonder why they want to fuse, what benefits are there?

Benefits?

They "want' to fuse because it puts them in a lower energy state, which is the tendency of all systems, and the concept behind what Sensei has posted.

Classically this manifests itself in forces; the force is the negative gradient of the potential. Nucleons in close proximity feel a net force of attraction. There are quantum mechanical complications and restrictions to this, so not all states are the same, but it is the general tendency for light nuclei, where the nuclear attraction far outweighs the electrostatic repulsion. Heavier nuclei do not release energy by fusing, so it is not a favored reaction for them.

[mistermack]

Without researching the matter, I'm suggesting that pressure comes into it as well as temperature. 

Inside the Sun, pressures are phenomenal due to the huge gravitational forces. When the nuclear fission trigger goes off in an H bomb, the pressures are presumably enormous too.  

It's very difficult to reproduce those kind of conditions in a fusion reactor, especially the pressures, so I'm guessing that they have to get the plasma to higher temperatures than in the Sun or an H bomb to compensate for lower pressures. 

4 hours ago, Carl Fredrik Ahl said:

Why will hydrogen fuse together to form helium? How do they get attracted to each other just because it gets very hot and how (in what form) is the energy released when they fuse?

They actually repel each other with electrostatic force, and need the high temperatures and pressures to overcome that. Once they get past that barrier, the nuclear force, which acts over shorter distances, takes over and they then want to fuse. When they fuse, the resultant mass is slightly less than originally, the missing mass is converted to heat and light in huge quantities. 

[swansont]

2 hours ago, mistermack said:

Without researching the matter, I'm suggesting that pressure comes into it as well as temperature. . 

Pressure and temperature will be correlated (consider an ideal gas, PV=nRT) so these are more like two sides of the same coin, rather than independent effects.

[mistermack]

48 minutes ago, swansont said:

Pressure and temperature will be correlated (consider an ideal gas, PV=nRT) so these are more like two sides of the same coin, rather than independent effects.

They are for the same amount of gas. But if you have two reactors running at the same temperature, then the one that achieves a higher pressure would be more effective, because the plasma would be denser, meaning more frequent collisions, and a higher probability of a suitable collision. 

I just read that the critical factors are called the "triple product" of density of plasma, temperature and time, for a fusion reactor.

https://en.wikipedia.org/wiki/Lawson_criterion   

[swansont]

2 hours ago, mistermack said:

 I just read that the critical factors are called the "triple product" of density of plasma, temperature and time, for a fusion reactor.

https://en.wikipedia.org/wiki/Lawson_criterion   

I notice that pressure is not included. Perhaps because it's redundant once you specify the other three?

[John Cuthber]

7 hours ago, mistermack said:

It's very difficult to reproduce those kind of conditions in a fusion reactor, especially the pressures, so I'm guessing that they have to get the plasma to higher temperatures than in the Sun or an H bomb to compensate for lower pressures. 

Fusion at low pressures (well below atmospheric) is perfectly possible.
The trick is to ensure that the particles are moving fast and they are contained for a relatively long time.
https://en.wikipedia.org/wiki/Fusor
It's also important to recognise that stars like the Sun do a very slow job of fusion. They take billions of years to do it.
On a weight for weigh basis, I emit more heat than the Sun.

[swansont]

22 minutes ago, John Cuthber said:

Fusion at low pressures (well below atmospheric) is perfectly possible.

We're talking about self-sustained fusion, though.

[mistermack]

2 hours ago, swansont said:

I notice that pressure is not included. Perhaps because it's redundant once you specify the other three?

I think for a given temperature, density is directly related to pressure, so if you want increased density, you have to increase pressure. 

In the Sun, the pressure is presumably pretty constant, so the density goes up and down with temperature, which stabilises the fusion rate. 

Wikipedia is interesting on that :

"The fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the density and hence the fusion rate and correcting the perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the density and increasing the fusion rate and again reverting it to its present rate."^[88][89]  

Another interesting fact is that the Sun's core only produces as much heat per unit volume as the average reptile, or compost pit. 

On pressure and density, wiki says this about confinement :

Confinement[edit]

The key problem in achieving thermonuclear fusion is how to confine the hot plasma. Due to the high temperature, the plasma can not be in direct contact with any solid material, so it has to be located in a vacuum. Also, high temperatures imply high pressures. The plasma tends to expand immediately and some force is necessary to act against it. This force can take one of three forms: gravitation in stars, magnetic forces in magnetic confinement fusion reactors, or inertial as the fusion reaction may occur before the plasma starts to expand, so the plasma's inertia is keeping the material together.

Edited November 7, 2018 by mistermack

[swansont]

14 minutes ago, mistermack said:

I think for a given temperature, density is directly related to pressure, so if you want increased density, you have to increase pressure. 

Same idea.  PV = nRT (or P = pRT, where p is density (rho))

If you arbitrarily hold a variable constant, there will be a correlation of the remaining two. They aren't independent variables

 

[mistermack]

Yes, the way I see it is that for two systems at the same temperature, the one with the higher pressure has more collisions per second than the other, because the density is higher. There's less space to travel to hit another particle. More collisions means more pressure so the two have to go hand in hand. So pressure is a vital factor in fusion, because density is. You could say that it's really density that's important, but to raise the density, you have to raise the pressure. 

When the talk about the triple product of density, temperature and time, they are really saying that density and temperature are the critical factors, and you need to be able to hold it long enough for fusion to happen. Going by what happens in the Sun's core, where a drop in temperature actually speeds up the fusion, because it raises the density, it appears that density, and hence pressure, has more effect than temperature, at fusion conditions.

The pressure in reactors is provided by the confinement method. It's either magnetic as in a Tokamak, or inertial, as in Laser systems. On the TV last night there was a brief bit about the US laser experiments. They said that the power used on the "target" was something like sixty times all of the power being used in the US. But only for a matter of milliseconds. The pressure for the reaction is provided by the inertia of the target. It can't expand fast enough to lower the pressure. Presumably, to design a constantly running system, you would constantly replace the target, and keep zapping them, maybe using the heat from the previous target to help the next one to fuse. 

In the extreme conditions of fusion, it would be difficult to find materials that could survive long enough to keep replacing the target accurately, I would have thought. 

[swansont]

2 hours ago, mistermack said:

Yes, the way I see it is that for two systems at the same temperature, the one with the higher pressure has more collisions per second than the other, because the density is higher. There's less space to travel to hit another particle. More collisions means more pressure so the two have to go hand in hand. So pressure is a vital factor in fusion, because density is. You could say that it's really density that's important, but to raise the density, you have to raise the pressure. 

But you don't, since you could raise the temperature (and thus pressure) and keep the density the same. There are three variables here (if we are using density), so it's not the case that there is only one way to achieve some result, and since two variables are correlated if the third is constant, you can't say which one is the driving factor. They both are. Is the density higher because the pressure is, or is the pressure higher because the density is? The answer is yes. You can explain it either way.

 

 

 

 

[mistermack]

 

3 hours ago, swansont said:

But you don't, since you could raise the temperature (and thus pressure) and keep the density the same. There are three variables here (if we are using density), so it's not the case that there is only one way to achieve some result, and since two variables are correlated if the third is constant, you can't say which one is the driving factor. They both are. Is the density higher because the pressure is, or is the pressure higher because the density is? The answer is yes. You can explain it either way.

What the core of the Sun shows though, is that if you want to increase the reaction rate, raising the temperature can be counter-productive, if you can't contain the expansion by upping the pressure. The density drops and the reaction slows. 

It's not easy pressurising plasma without physically touching it, due to the extreme temperatures. 

Surprisingly, the record plasma pressure so far is only two atmospheres, according to this :   https://www.theguardian.com/environment/2016/oct/17/mit-nuclear-fusion-record-marks-latest-step-towards-unlimited-clean-energy  

Presumably, pressure is where the H bomb has a big advantage. You can confine the explosion physically, for a tiny bit of time, whereas you can't in a Tokamak reactor.