Are fusion power plants feasible for widespread use?

[Astronuc]

There are different means to create the neutrons for BNCT. Here is a company doing (p,n);
http://www.neutrontherapeutics.com/technology/

Well, yes, that is a reaction of interest for producing neutrons.

In fact, "Between 1.9– and 2.4–MeV bombarding energy, the neutrons are monoenergetic and the reaction cross section is large. Therefore the Li(p,n)Be reaction has long been used as a source of neutrons (n) at these energies [5]." Ref: https://mcnp.lanl.gov/pdf_files/la-ur-00-1067.pdf
Ref [5] from the report is J. H. GIBBONS and H.W. NEWSON, “The Li(p,n)Be Reaction,” Fast Neutron Physics, Vol.1, J. B. MARION and J. L. FOWLER, Eds., Interscience, New York (1960).

We usually refer to spallation reactions for (p,n) reactions, as opposed to fusion, but some use the term fusion. I'm not sure at what time frame is used to distinguish spallation from fusion.

When I check one of my texts on fusion for p+Li reactions, I find two 'fusion' reactions:

p + ⁶Li -> ³He + ⁴He + 4.0 MeV

p + ⁷Li -> 2 ⁴He + 17.5 MeV

Theoretically, the p + ⁷Li would form an intermediate state ⁸Be, but ⁸Be is unstable and short-lived (half-life 8.19𝑥10⁻¹⁷seconds, so the state is not even considered in the evaluation of the process.
https://www.osti.gov/servlets/purl/1773479

Similarly for ⁵He in d+t fusion, the half-life is estimated to be 7.595×10^-22 seconds. https://education.jlab.org/itselemental/iso002.html
http://atlas.physics.arizona.edu/~s...17/LectureSupplements/HeliumIsotopes_Wiki.pdf

In the case of p + ⁷Li -> ⁷Be + n, the product ⁷Be has a half-life of ~ 53.2 d and decays by electron capture back to ⁷Li, which is stable.

To be useful as a source of practical energy, the average temperature of the reactants should be as low as possible, in the keV range, as opposed to the MeV range. Note, for a nucleus or subatomic particle, 1 eV ~ 11605 K, so 1 keV ~11605000 K.

 

[PeterDonis]

we understand it's not a pet theory just because I use a different/the wrong terms, right?

Not for that reason, no.

Within a nucleus there are still electromagnetic and weak forces

Yes.

the nature of the fusion result is 'mediated' according to which force within the nucleus does the work on the resultant particles.

But the fusion reaction doesn't just involve the resultant particles. Fusion means nucleons are being rearranged, which means the strong force is involved. That will be true regardless of what non-nucleon resultant particles there are.

if you don't get helium 5 from the 'fusion' of DT, then what does 'fusion' mean?

The physics doesn't depend on the limitations of our terminology. The fact that we happen to classify the reaction D + T -> He4 + n as "fusion" does not mean that the physics has to be D + T -> He5 -> He4 + n in order to match our terminology. As far as I know (I could be mistaken) there is no experimental evidence that He5 is produced during this reaction; the only experimental evidence we have is that D + T goes in and He4 + n comes out. So I don't see any good basis for claiming that He5 must be produced. Certainly the word "fusion" is not such a basis.

 

[PeterDonis]

Similarly for ⁵He in d+t fusion

Is the D+T fusion reaction the one that is actually used to produce He5 experimentally and measure its half-life? That reaction is not mentioned in the reference you gave, unless I missed it.

 

[artis]

@PeterDonis I tried googling and found very little , but from what I found it seems that He5 should be produced, the isotope of He that hasn't been verified experimentally I think is He 2 aka "diproton" because it is extremely unstable.
That's why I read in multiple sources they say

The least stable is 5He, with a half-life of 7.6×10−22 seconds, although it is possible that 2He has
an even shorter half-life.

Including in this one .
http://atlas.physics.arizona.edu/~s...17/LectureSupplements/HeliumIsotopes_Wiki.pdf

But I can't find many other sources for He5 empirical verification, maybe others can help.
All in all @cmb I think we can just say that in fusion the strong force is what fuses the new daughter nucleus from the parent ones but that doesn't rule out that further other fundamental fields can do their work to for example "help" the nucleus undergo decay and release it's final energy, which we then for short hand might attribute to simply "fusion".

 

[Astronuc]

s the D+T fusion reaction the one that is actually used to produce He5 experimentally and measure its half-life? That reaction is not mentioned in the reference you gave, unless I missed it.

I don't know, and I cannot readily find any papers/articles on such an experiment. I'm not sure how one measures a phenomenon lasting 10^-22 s. Even traveling at the speed of light, it wouldn't go very far before disintegrating, so I don't see a time of flight experiment.

Shortest time measured - 247 zeptoseconds.
https://www.bbc.co.uk/newsround/54631056

Then again, there is mention of measuring half-life of ⁷H at 652(558) ys (6.52(5.58)×10⁻²² s). https://en.wikipedia.org/wiki/Isotopes_of_hydrogen#Hydrogen-7

 

[artis]

@Astronuc could it be that they arrive at these extremely short timescales the same way they derive the W, Z boson existence?
Although the boson lifetime seems to be 3x10^-25s which seems even less than the timescales we are talking about here

 

[cmb]

All in all @cmb I think we can just say that in fusion the strong force is what fuses the new daughter nucleus from the parent ones

Absolutely totally. There is no question whatsoever.

Even in the weak mediated fusion (take this as my terminology to assist the discussion for now, until such time as I find you a reference you are happy with) of, say, pp, the protons fuse and are held momentarily by the strong force but their state of excitation (work done by the strong force during the fusion on the excitation state of the 2He) is too great for them to stay together for very long. In that instant, if one proton decays to a neutron, the excitation energy (from the strong force as it binds) is released as work done by the weak force on positrons/neutrinos.

Likewise, when I refer to electromagnetically mediated fusion what I mean is the excitation energy gained by work done by the strong force (as always) during the fusion is then released by work done by electromagnetic force.

The fusion we need to make work on Earth is where the strong force excites the product nucleus which is energy then released by work done by, again, the strong force. Only this type of fusion can result in viable terrestrial fusion as it results in kinetically energetic (thermal) matter. Fortunately for prospects for terrestrial fusion, this type of reaction is distinctly orders of magnitude hither reactivity than the other two types. This observation is the reason for my original post, that we have to make 'that' sort of fusion work, here, but that the Sun actually does a different sort of fusion. It'd be like comparing a redox reaction with an acid-alkali reaction, they both swap electron around and make new molecules, but they have fundamental differences.

 

[cmb]

No reasonable person would call (p,n) fusion.

That would really depend on whether an intermediate product nucleus is formed during the process, i.e. whether the strong nuclear force does work on a product nucleus and form an excited product that then releases a neutron.

If the proton simply knocks off a neutron kinematically, I would agree.

[edit .. I am just looking for an exothermic example, maybe there isn't one]

 

[Astronuc]

If the proton simply knocks off a neutron kinematically, I would agree.

[edit .. I am just looking for an exothermic example, maybe there isn't one]

From K. H. Beckurts, K. Wirtz, Neutron Sources, in Neutron Physics,
https://link.springer.com/chapter/10.1007/978-3-642-87614-1_2

There is a variety of reactions which lead to neutron production. In such reactions compound nuclei excited* with the sum of the binding energy and the kinetic energy (in the center-of-mass system) of the projectiles first are formed by bombardment of target nuclei with α-particles, protons, deuterons, or γ-rays. If the excitation energy is larger than the binding energy of the "last neutron" in the compound nucleus, then a neutron is very likely to be emitted. The remaining ex-citation energy is distributed as kinetic energy between the neutron and the residual nucleus. The residual nucleus can remain excited and later return to the ground state by γ-emission.

* Many neutron-producing reactions proceed directly, i.e., without the formation of a compound nucleus. One important example of such a reaction is deuteron stripping.

See the statement by Beckurts and Wirtz, "All (p,n) reactions on stable nuclei are thus endothermic, . . . ."

The text is from 1964, and perhaps perspectives have changed on what constitutes a compound nucleus (i.e., half-life threshold).

A consideration from 1956 on "The formation of the compound nucleus."
https://www.sciencedirect.com/science/article/abs/pii/S0031891456900520More recently, http://websites.umich.edu/~ners311/CourseLibrary/bookchapter17.pdf

In the table of nuclear reactions, compound reactions a + X -> Y^m -> fragments, where t_1/2 >> 10^-23 s. Not sure where >> gets one.

In the same table, is a transfer reaction and resonance reaction. Perhaps the way to discern is the angular distribution of reactants, i.e., more forward directed of the proton in (n,p) or neutron in (p,n) would indicate a lack of a compound nucleus.

 

[cmb]

From K. H. Beckurts, K. Wirtz, Neutron Sources, in Neutron Physics,
https://link.springer.com/chapter/10.1007/978-3-642-87614-1_2

* Many neutron-producing reactions proceed directly, i.e., without the formation of a compound nucleus. One important example of such a reaction is deuteron stripping.

See the statement by Beckurts and Wirtz, "All (p,n) reactions on stable nuclei are thus endothermic, . . . ."

The text is from 1964, and perhaps perspectives have changed on what constitutes a compound nucleus (i.e., half-life threshold).

A consideration from 1956 on "The formation of the compound nucleus."
https://www.sciencedirect.com/science/article/abs/pii/S0031891456900520More recently, http://websites.umich.edu/~ners311/CourseLibrary/bookchapter17.pdf

In the table of nuclear reactions, compound reactions a + X -> Y^m -> fragments, where t_1/2 >> 10^-23 s. Not sure where >> gets one.

In the same table, is a transfer reaction and resonance reaction. Perhaps the way to discern is the angular distribution of reactants, i.e., more forward directed of the proton in (n,p) or neutron in (p,n) would indicate a lack of a compound nucleus.

Thanks for the links I am working through those.

I'm unclear on your inference and how it relates to (p,n) fusion as opposed to excitation of a nucleus by a projectile proton.

Not only have I already described the difference, I would also wholly confirm and agree with you all that (p,n) fusions are exceptional and very much not the norm. I'm looking now and only one 'exotic' looks to exist so far (yes, with unstable nuclei). So, by and large yes, I'd agree (p,n) is spallation and I am not particularly bothered about cases even if a product nucleus forms momentarily.

What I am interested in is exothermic (p,n) reactions, because this means the excitation energy for the nucleus to fission with a neutron can come purely from the work done as the proton is pulled into the nucleus by the strong force. I would regard this as 'fusion'. But exothermic (p,n) is very exceptional and not normal.

 

[cmb]

No reasonable person would call (p,n) fusion.

I've made a post here for your further consideration;
https://www.physicsforums.com/threads/are-there-any-p-n-fusion-reactions.1006603/

 

[PeterDonis]

I've made a post here for your further consideration;
https://www.physicsforums.com/threads/are-there-any-p-n-fusion-reactions.1006603/

And with that, this thread is closed.