Are fusion power plants feasible for widespread use?

[PeterDonis]

I have a chapter from an electronic book

What book?

 

[PeterDonis]

This is discussing the astrophysical factor (the value that goes into an equation to describe fusion cross-sections) of different fusion reactions.

My concern is not the cross sections or equations but the basis for terms like "electromagnetic nuclear interactions", which to me seems like an oxymoron. Any nuclear reaction is going to involve the strong interaction, and given the respective interaction strengths I would expect the strong interaction to dominate the energy change involved. It's been a long time since I studied nuclear physics in college, but I don't recall any classification of nuclear reactions as "mediated by the electromagnetic interaction" or "mediated by the weak interaction" (except for the case of radioactive beta decay, which was typically described as a conversion of one type of quark to another mediated by the weak interaction).

 

[cmb]

What book?

"from a degree course but the attribution isn't on the document", I just said I don't know and I will go looking.

What's your problem here?

I'm explaining these things and saying I'll find it for you, but independent of any referencee, it's frankly flat-out plainly obvious there are 3 distinct types of fusion reaction, independent of any 'book'. It doesn't need someone to actually put their name to it, the physics is obvious;

A) fusion where stuff ends up as kinetically energetic nuclear parts only
B) fusion where only one nuclear part remains and has no energy, but
(i) releases the excess energy as photons, and/or
(ii) releases the excess energy as electrons or positrons, and nutrinos.

The cross-section for all 'A' type reactions are orders of magnitude higher than B(i), which are further orders of magnitude higher than B(ii). Whether you decide that is my own categorisation or not, the physics is a fact.

 

[cmb]

My concern is not the cross sections or equations but the basis for terms like "electromagnetic nuclear interactions", which to me seems like an oxymoron. Any nuclear reaction is going to involve the strong interaction, and given the respective interaction strengths I would expect the strong interaction to dominate the energy change involved. It's been a long time since I studied nuclear physics in college, but I don't recall any classification of nuclear reactions as "mediated by the electromagnetic interaction" or "mediated by the weak interaction" (except for the case of radioactive beta decay, which was typically described as a conversion of one type of quark to another mediated by the weak interaction).

Well, if in the course of a few decades I have fallen into an imprecise and poorly remembered definition of classifications which has diverged from what is accepted now, I stand guilty of a gross misdemeanour and apologise. But give me a chance to find the texts first before passing sentence.

Meanwhile, the physics of the three distinct categories of fusion reactions remains clear, and therefore is not in any way 'a pet theory'. It is physics fact.

 

[PeterDonis]

it's frankly flat-out plainly obvious there are 3 distinct types of fusion reaction, independent of any 'book'. It doesn't need someone to actually put their name to it, the physics is obvious;

A) fusion where stuff ends up as kinetically energetic nuclear parts only
B) fusion where only one nuclear part remains and has no energy, but
(i) releases the excess energy as photons, and/or
(ii) releases the excess energy as electrons or positrons, and nutrinos.

As far as what's left after the reaction and where the kinetic energy ends up, I have no problem with that. But I don't see how any of this maps to terms like "mediated by the electromagnetic interaction" or "mediated by the weak interaction". If those terms are supposed to refer to your B(i) and B(ii), then they don't seem to me to be describing what "mediates" the reaction, but just what carries away the kinetic energy.

 

[PeterDonis]

if in the course of a few decades I have fallen into an imprecise and poorly remembered definition of classifications which has diverged from what is accepted now

I'm not very current in terminology or classification of nuclear reactions, so I don't know what is accepted now. As I have remarked, I don't recall being taught the classification you're describing when I studied nuclear physics in college (which was in the mid 1980s).

 

[cmb]

As far as what's left after the reaction and where the kinetic energy ends up, I have no problem with that. But I don't see how any of this maps to terms like "mediated by the electromagnetic interaction" or "mediated by the weak interaction". If those terms are supposed to refer to your B(i) and B(ii), then they don't seem to me to be describing what "mediates" the reaction, but just what carries away the kinetic energy.

As said, if my descriptions of those three forms of reactions is at odds with someone else's then they can have a problem with it if they really want to make it a problem. But the physics stands so I don't see really what the issue is.

 

[PeterDonis]

the physics stands so I don't see really what the issue is.

The issue before you described the physics in post #93 was that I had no idea what physics the terms you were using referred to. As I said in my response in post #95, I'm not sure the terms "mediated by the electromagnetic interaction" and "mediated by the weak interaction" are very good terms, but the physics you described in post #93 is clear and, as I said in post #95, I have no problem with it.

 

[cmb]

The issue before you described the physics in post #93 was that I had no idea what physics the terms you were using referred to.

That's fine, we can figure out any lack of clarity on terminology no problem, but we understand it's not a pet theory just because I use a different/the wrong terms, right?

Within a nucleus there are still electromagnetic and weak forces, the nature of the fusion result is 'mediated' according to which force within the nucleus does the work on the resultant particles.

It HAS to be an electromagnetic force to create an energetic photon, and as there are no nucleons released then the strong nuclear force does no work on any such energy outputs.

Likewise for weak force.

In answer to your question about a reference for the fission of an intermediate helium 5 from DT fusion, well, if you don't get helium 5 from the 'fusion' of DT, then what does 'fusion' mean? I believe the common view is simply that there is a helium 5 that forms and spontaneously fissions, if you have a different explanation for the formation of helium 4 and a neutron, please provide a reference.

 

[artis]

In answer to your question about a reference for the fission of an intermediate helium 5 from DT fusion, well, if you don't get helium 5 from the 'fusion' of DT, then what does 'fusion' mean? I believe the common view is simply that there is a helium 5 that forms and spontaneously fissions

I think you might be right that indeed in DT fusion for example the free neutron isn't produced before the reaction of D+T is finished and the nucleus of He 5 is assembled which then decays to He 4 + a free neutron.
Wikipedia says the lifetime of He 5 is

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

I think the extremely short half life of 5 He is what makes people just disregard it as an intermediary step and write the reaction in it's simple form.

Hopefully @Astronuc or anyone else for that matter can correct this question here , but I do think that not all fusion reaction that are below Fe56 release energy (EM or particle KE) due to the process of the decay of the created daughter nucleus after fusion of two lighter parent ones?
Also it is said that fusion of nuclei lighter than Fe56 is exothermic but if we take the example of DT then the fusion itself is actually endothermic and only becomes exothermic after the daughter nucleus of He 5 undergoes it's fast decay releasing one neutron while the remaining He 4 or Alpha particle gains it's kinetic energy from the original KE's of D and T ?
So in theory if He 5 was stable then D+T would actually consume energy not release energy?

As for fusion on Earth , even if we could catch the gammas the limiting factor is we can't reach the temperatures required for PP or CNO cycle and sustain them in any long term, not to mention the low fusion output for PP for example.
As for particles I think we would actually benefit greatly if DT for example produced no neutrons but just alpha's because our goal is to maximize plasma self heating and the neutron carries away lots of energy while Alpha's would be trapped even in our "thin" plasmas.

 

[cmb]

I think you might be right that indeed in DT fusion for example the free neutron isn't produced before the reaction of D+T is finished and the nucleus of He 5 is assembled which then decays to He 4 + a free neutron.
Wikipedia says the lifetime of He 5 is
I think the extremely short half life of 5 He is what makes people just disregard it as an intermediary step and write the reaction in it's simple form.

Hopefully @Astronuc or anyone else for that matter can correct this question here , but I do think that not all fusion reaction that are below Fe56 release energy (EM or particle KE)

For sure there are a multitude of fusion reactions that are endothermic, in fact probably most of all 'possible' fusion reactions are endothermic.

One which is regularly used is the proton lithium (p,n) reaction which consumes 1.8MeV (or so, I am recalling from memory) and produces a neutron. This endothermic fusion is being used for neutron therapeutics as the neutron source for neutron boron capture therapy, and is also a reaction used in the 'Unicorn' test facility in France (I don't recall the French name) it actually fires lithium ions into the hydrogen gas (rather than protons into lithium) which enhances the directionality of the output neutrons. These reactions aim for collision energies above 2MeV, which provides the energy to excite the resultant fused nucleus into splitting (fissioning) with a neutron output. The neutron is accelerated by the strong nuclear force, the energy for which comes from the excitation state of that fused product.

 

[artis]

One which is regularly used is the proton lithium (p,n) reaction which consumes 1.8MeV (or so, I am recalling from memory) and produces a neutron. This endothermic fusion is being used for neutron therapeutics as the neutron source for neutron boron capture therapy

I do think that for BNCT they use Beryllium as the target due to Beryllium being more chemically stable and safer to work with than Lithium, which among other things burns in contact with water a far as I'm aware.

 

[cmb]

I do think that for BNCT they use Beryllium as the target due to Beryllium being more chemically stable and safer to work with than Lithium, which among other things burns in contact with water a far as I'm aware.

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

Here is the French neutron source;
https://www.researchgate.net/publication/273421998_LICORNE_A_new_and_unique_facility_for_producing_intense_kinematically_focused_neutron_beams_at_the_IPN_Orsay

"LICORNE is a new neutron source recently installed at the tandem accelerator of the Institut de Physique Nucleaire d'Orsay, where a Li7-beam is used to bombard a hydrogen-containing target to produce an intense forward-directed neutron beam."Here is an academic paper;
Appl Radiat Isot
. 2004 Nov;61(5):817-21.
doi: 10.1016/j.apradiso.2004.05.032.

Lithium neutron producing target for BINP accelerator-based neutron source​

B Bayanov 1, V Belov, V Kindyuk, E Oparin, S Taskaev
Affiliations expand

• PMID: 15308150
• DOI: 10.1016/j.apradiso.2004.05.032

Abstract​

Pilot innovative accelerator-based neutron source for neutron capture therapy is under construction now at the Budker Institute of Nuclear Physics, Novosibirsk, Russia. One of the main elements of the facility is lithium target, that produces neutrons via threshold (7)Li(p,n)(7)Be reaction at 25 kW proton beam with energies 1.915 or 2.5 MeV.These are utilising the endothermic neutron producing proton-lithium fusion route.

 

[Alex A]

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

 

[Astronuc]

I do think that for BNCT they use Beryllium as the target due to Beryllium being more chemically stable and safer to work with than Lithium, which among other things burns in contact with water a far as I'm aware.

⁹Be is used as a target for gamma rays that induce a photon,neutron reaction, since the energy threshold is about the lowest. Be is highly toxic to life forms, so has to used with care. The photoneutron threshold for ⁶Li and ⁷Li is considerably greater.

 

[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.